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How to identify, inspect, install, repair, or service heating boiler expansion tanks: here we explain the function of expansion tanks on hot water (hydronic) heating systems.
We discuss what happens if the expansion tank becomes waterlogged, how to drain a waterlogged expansion tank, and what sorts of expansion tanks, like the one shown at page top, should never become waterlogged.
We describe where the expansion tank may be found (sometimes in the attic!) and we illustrate different sorts of expansion tanks used over the history of hydronic heating in buildings. We provide a heating system expansion tank Troubleshooting & Repair Guide.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
What is a Heating Boiler Expansion Tank & How Do Expansion Tanks Work
Heating boiler expansion tanks are metal tanks of varying sizes that are installed to absorb the initial pressure increase that occurs when the heating boiler system warms up.
Air molecules entrained in water inside the heating boiler itself as well as in the heating system piping, baseboards, or radiators, expands and thus cause an initial pressure increase in the heating system.
[Click to enlarge any image]
Article Series Contents
Inside of a working expansion tank is a reserve space of air. In fact, when the heating boiler and system are cool, the expansion tank will contain mostly air.
As the heating system warms up and as air entrained in the water raises system pressure, the increased pressure forces some of the heating system water into the expansion tank, thus permitting the tank to absorb the initial increase in system pressure.
Technical note: the water and air inside the expansion tank will always share the same pressure, but since water is not (very) compressible and air is, the increase in pressure will squeeze the air down to a smaller volume. That's why some heating system water can pass into the expansion tank.
If heating system pressures and/or temperatures exceed normal (for a residential heating boiler that would be a pressure above 30 psi or a temperature much above 200 °F) then the expansion tank is dismissed from duty and the pressure/temperature relief valve on the boiler will open to provide emergency relief.
In older expansion tanks that do not have an internal bladder to keep air and water separate, over time air in the expansion tank may become absorbed into the heating water. Air may also find its way via the heating water to other air bleeders or vents in the system.
As air is lost and water just stays in the expansion tank, the space and pressure cushion provided by air in the expansion tank gets too small, or is lost entirely. There is no more air cushion to absorb initial pressure increases in the heating system during each heating boiler on-cycle.
In a modern internal-bladder type expansion tank use on heating systems it is not usually necessary to add air nor to change the tank pressure. But occasionally the internal bladder of these tanks an rupture, leading to a water-logged tank. In that case the heating system pressure relief valve may spill or water may be found leaking out at the expansion tank's air valve.
What Happens if an Expansion Tank Becomes Waterlogged?
When the expansion tank is no longer able to absorb this initial pressure increase, it is possible that the heating system's internal pressure would exceed 30 psi - the typical point at which a heating boiler pressure/temperature relief valve will open to spill excess pressure. If the relief valve is forced open in this manner the heating system will first lose water each time a heating cycle starts by heating up the boiler.
Then the heating system will take in makeup water (through the automatic water feed valve) each time the system cools down. The result would be recurrent loss and then inflow of water through the boiler, increasing the risk of system corrosion as well as wasting water and possibly causing other damage or operating problems.
Watch out: spillage at the pressure temperature relief valve is potentially dangerous: eventually minerals in the water can clog a leaky relief valve, causing it to stop leaking - which might look ok but this means that the relief valve has become clogged - the boiler is operating without this critical safety device and an explosion could occur.
Expansion tanks on hot water heating systems can be divided roughly into two groups: older type bladderless heating system expansion tanks and newer type bladder-type expansion tanks such as those sold by Amtrol Inc.
A modern internal bladder type expansion tank like this Amtrol Extrol tank (photo at left) is generally trouble free, and it's unusual for these tanks to fail. The Ex-Trol® is properly referred to as a pressurized diaphragm-type expansion tank.
Both older non-bladder type expansion tanks and even modern (and much smaller) internal bladder expansion tanks can get in trouble.
But on occasion the internal bladder in one of these tanks could rupture or develop a leak. If that happens, because the tank is not field-repairable, the "fix" is to install a new expansion tank.
But far more often, it is older non-bladder type expansion tanks on boilers, like the one below) that become waterlogged.
Trouble signs that could be traced to a water-logged boiler expansion tank include:
How Else We Know the Boiler Expansion Tank Needs Draining
Newer type heating system expansion tanks that use an internal bladder keep their water and air separated. These tanks should not need service. As we explained above, when a bladder-type expansion tank has become waterlogged it's because the bladder has ruptured and the tank needs replacement.
How to Try Some Neat Diagnostic Tests for a Waterlogged Heating Boiler Expansion Tank
The following tips are from Bell & Gossett's installation manual for pressure reducing valves and from their Airtrol Compression Tank [expansion tank] System Installation Manual . Our illustration at left shows the typical location of a traditional conventional, non-bladder-type heating boiler expansion tank, and is adapted from that B&G manual.
[Click to enlarge any image]
Periodic draining of non-bladder boiler expansion tanks:
Older heating system expansion tanks such as the one shown in this photo (that large brown tank over our client's head) need periodic service: because air in the expansion tank can become absorbed into the heating water over time, eventually the expansion tank can become waterlogged.
As we detail below in another photo, the green arrow indicates the expansion tank drain valve and the blue arrow marks the isolating valve that gets shut off first.
So if no one has ever drained the expansion tank, or not for years, chances are it's ready and waiting for that attention. Or as we've explained above, it's past due for draining.
How to determine when the expansion tank needs drainage:
If the heating boiler expansion tank is heavy (try pushing it up or tapping on it) or if the relief valve is leaking, we probably need to drain the tank and let air return to it.
Watch out: don't go pushing the expansion tank all over the place. Just a tiny test push is sufficient. If you wiggle the tank around too much you're asking for a pipe leak. Also some expansion tanks are strapped tightly against the ceiling so you can't move them anyway.
Watch out: modern internal-bladder type expansion tanks such as AMTROL's EXTROLl®, the FILL-TROL® [separate article] Diaphragm-Type Expansion Tank do not need periodic draining - those tanks use an internal bladder. If your Amtrol expansion tank is waterlogged there is a problem to correct and the tank probably needs replacement.
So Now, Finally, Let's Drain that Heating Boiler Expansion Tank
Watch out: this hose idea is convenient and is recommended by some folks who lack experience with expansion tanks. But the hose only works if a special air-admittance valve is installed on your expansion tank. If your tank just has a regular boiler drain valve installed, the hose thing will be very difficult to work - in that case plan some trips with a bucket held under the opened tank drain valve instead.
Both of the expansion tanks we show with our arrows in the photos above had a simple boiler drain. We open the drain and let it spill into a bucket. That lets air bubble back up into the expansion tank - something that would take forever if we had a hose hooked up here.
Watch out, the expansion tank water could be hot, and is usually a bit smelly and dirty.
This article section has been moved to FIND the EXPANSION TANK
Indirect fired water heaters: In some other buildings domestic hot water is produced by cycling hot water from a hydronic or steam boiler through a loop inside of a steel tank. The water in the tank is heated by the water in the coil.
using this approach use the term indirect-fired water heater and such systems are sold by various companies
such as the SuperStorTM unit shown here as the white tank to the right of the heating boiler.
The range boiler is an old concept in use for about 100 years. Indirect fired water heaters are a modern system and are in current sales and use.
See HOT WATER TANKS, INDIRECT FIRED for details about this type of water heater.
See WATER HEATERS for details about residential hot water systems.
This article section has been moved: see AMTROL's EXTROL®, the FILL-TROL®
Amtrol Extrol Models #15 through #90 come with a 5 year warranty. All other EXTROL models come with a 1 year warranty. *
Reader Question: I drained the expansion tank but the system pressure is still too high; high radiators in the house are cold & the relief valve spills
I have an bladderless expansion tank and was wondering if draining it incorrectly will not fix the problem. Here is the background of the situation.
It was noticed that a radiator one floor above the boiler (1940s oil boiler converted to natural gas) contained a lot of air in it, which was drained. Shortly after that, it was noticed that the relief valve was dripping.
I conducted a heat test to see what pressure the boiler got to, being suspicious that the expansion tank was waterlogged. I turned off the boiler and let it cool down to ~140°F and closed the feed line and expansion tank isolation valve.
I depressurized the system at the relief valve and replaced it since it was leaking. I then drained the expansion tank and took out about 2 gal. After that, I opened the isolation valve and then slowly opened the feed line valve, which re pressurized the system to around 15 psi.
When the system started the next morning (night time set-back), the pressure looked good and there was no dripping at the relief valve after an hour of operating, however, at two hours the pressure was at 30 psi and the relief valve was dripping.
I'm very stumped by this, because the procedure I've used seems very logical, but hope there is an issue with what I've done. We've been in this house for two previous heating season and have had no issues. There may have been leaks in the past since the relief valve outlet was slightly corroded. Any help would be greatly appreciated. - H.H. 11/20/2013
Let's SUPPOSE for a moment that the expansion tank is fine - that is, there are no leaks anywhere. Let's pretend that you drained enough water out of the tank (which can be tricky). This would mean that the expansion tank is essentially empty.
Then the next day you see the tank is water logged again.
You saw "... at two hours the pressure was at 30 psi and the relief valve was dripping. "
The fact that the pressure is up to 30 makes me suspect that your system could have
- a bad automagic water feeder - feeding in too much pressure and water
- a tankless coil that leaks house-side water pressure into the boiler
If your system has a tankless coil you can check for a coil leak by TEMPORARILY shutting off water ahead of the valve. IF pressure still rises then the coil is leaking into the boiler.
If you don't have a tankless coil try this test anyway. If boiler pressure doesn't soar up to 30 when it heats up then the water feeder is probably leaking into the system.
Don't leave water shut off to a boiler or it could be ruined, dangerous, burn down, explode up or what have you if it runs out of water.
The system doesn't have a tankless coil and I don't think the automatic water feeder is leaking.
I took some more observations of the system last night. At the beginning the system was cooling down from running previously running and the boiler temperature was 110°F and pressure was 10 psi (why I think the automatic water feeder is not leaking).
I then turned raised the thermostat to fire up the boiler. It took about 50 minutes to reach 180°F (the temperature the aquastat is set to), during which time the pressure increased. At 160°F, it was 20 psi. At 180°F, the pressure reached 30 psi and the relief valve starting leaking again.
I then turned the system off and let it cool down, which dropped the pressure too. When the system cooled to 140°F (20 psi), I isolated the expansion tank at let things cool further. When it was 120°F, the pressure dropped to 12 psi and then I opened the expansion tank isolation valve. I could hear water flowing through the valve and the system pressure increased back to 17 psi.
Based on these results, it seems like the expansion tank is working properly but there isn't enough air in the system to compensate for the thermal expansion (which agrees with the earlier observations that this problem began shortly after I bled a lot of air from one of the radiators). I'm thinking I didn't drain the expansion tank properly and will try to do it again.
Do you have any suggestions or comments to what the problem might be? I can conduct additional diagnostic tests if you'd like. If you think the expansion tank needs to be drained again, could you provide a procedure to do it (I'm pretty sure I followed the correct one, I might have just done it when the system was too warm, but I don't see how that would have been a problem).
I've attached some pictures of the system. I now know that the lack of drain for the relief valve is very dangerous and this will be rectified once this current problem is fixed.
The fact that you see 30 psi at just 180 °F tells us that the starting pressure in the boiler is too high.
At cold boiler you should see about 12 psi. Slightly more for a home more than two stories high. Try to be sure that you are getting ALL water out if the expansion tank. Water will reenter it but starting with a full tank of sir is usually right and sufficient.
1. Empty the bladderless expansion tank completely.
2. With the system cold, check that the starting pressure is about 12 psi. (Keep in mind that pressure gauges on heating boilers are not lab-grade precision instruments.) If the home is taller than 2 stories see our article about setting boiler pressure to account for that difference.
3. Check that with low pressure the relief valve is not dripping.
4. Turn on the boiler and watch it go through a heating cycle. The relief valve should not spill, the temperature should be below 200F, and the pressure should be below about 28 psi.
Let's take it from there.
Does it matter what temperature the system is at when I drain the expansion tank? Also, I wonder if the proper way to drain my expansion tank is to use the drain valve (other end of tank in the picture I sent you) and open the union between the isolation valve and the expansion tank (it is the only fitting in the system that has Teflon tape and not pipe dope, which would imply it has been opened at a different time from the rest of the system).
As for the 10 psi pressure when cool, I was thinking the gauge could be a little off or maybe the automatic water feeder pressure was reduced because the expansion tank doesn't have any extra capacity.
The expansion tank was still waterlogged. When attempting to drain it the first time, I must not have opened the valve enough to overcome the vapour-lock in the tank. During the second attempt, the "glugging" sound was evident and it eventually drained. Now the pressure is fairly constant during operation and the peak pressure is about 25 psi. The cold pressure still is around 10 psi though, and was wondering if I should adjust the automatic water feeder to increase the pressure or is that pressure adequate for a one-story house?
Reply: no need to increase boiler cold pressure as long as highest radiator/baseboard is getting hot
Here's the problem - I'm just guessing because I have not see the whole expansion tank and all its fittings: You've got a container full of water with probably only one outlet - on the container bottom.
And I saw no convenient valve to drain the tank - there is a special drain valve designed for this situation, one that admits air into the tank while it is letting water out. What I saw on your system was just a union (a dis-assemblable connection) on the horizontal line near the tank - probably you've got some other lower drain point.
It's hell getting air up into the tank by that route unless you completely open the pipe and wait patiently for the tank to sloooowly drain. Or you can rig up something that pushes air into the tank intermittently to help get ALL of the water out. T
his design, unless we retrofit a better tank drain system or an air inlet higher on the tank, is so annoying that that's why we want to be sure to drain the tank fully - to get as long a life from the service job as possible. Over time the air in the tank is absorbed into water and the tank's compressible air volume shrinks - which is why ultimately the TP valve leaks.
With that chest beating done, back to your last question. As long as the highest radiator or baseboard in the home is getting hot we know the pressure is adequate. So I would not mess with the automagic pressure-reducer water feeder.
Except, to argue with myself, I'd like the cold start pressure to be slightly higher than the absolute minimum needed to allow the circulator pump to push hot water all around the heating system. That little margin, say 1-2 psi, might prevent a cold radiator under varying load or weather conditions. Usually the factory-set pressure on a water feeder/pressure reducer valve is 12 psi cold.
If you haven't seen it,
And if you're feeling curious about the physics involved in the pressure increase as we heat up water,
Reader Question: water coming out of the top brass venting/valve atop the expansion tank
I found information on the expansion tank noted in the subject line on your web page. This is the kind I have on my gas furnace.
There is water coming out of the top brass venting thing above the tank. My water pressure cold is almost down to zero.
The unit runs and heats the house but wondering what is going on here something is off. Any suggestions on what to do would be greatly appreciated.
If the expansion tank you are using is an internal-bladder type and water is coming out of the air charge valve as you describe, I suspect the bladder is burst - it's time for a replacement tank bladder or if that's not an option on your tank, a new expansion tank is needed.
Watch out: before even considering expansion tank replacement, if the water leak you are describing is not at an air charge vent on the expansion tank itself (a part that is indeed not present on most bladder type expansion tanks for heating systems) then you are probably talking not about a problem with the expansion tank but rather a problem with an automatic air bleeder vent - a separate part intended to bleed air from a hot water heating system.
If the part that is leaking on your boiler looks somewhat like the device shown at left, that's an Automatic Air Eliminator - Auto Vent No. 67 from Maid O Mist in our example. (Your heater may use a different brand or product). [Click to enlarge any image]
If that's the leak source the part replacement is usually a minor repair.
Temporarily you can usually stop the leak at an automatic air vent by screwing closed the valve cap atop the valve. But that will also prevent the valve from venting air and thus can lead to an air-bound heating system and thus to loss of heat in one or more areas of your home.
Therefore while you want to stop any immediate leak to avoid making a mess or dripping onto electrical components, the proper repair is to replace the automatic air vent promptly.
Reader Question: how do I know the right PSI for my expansion tank?
Oct 5, 2014) Luis R. said:
How I do know the correct PSI for my new expansion tank? Do I need a Fill Trol-valve in my new diaphragm expansion tank system?
I did not put a Fill troll valve, do I need one?, and do not know if I have to put more air into the new expansion tank that already came pre-charge with 12 PSI, and I do not know how much more will be need it . My hot water heating boiler system is Gas; water 30PSI, min. relief valve cap. 144, water BTU/HR 125.200 input 180.000, output 144.000.
The tank you installed comes with a proper air charge in it. Don't change it unless the installation instructions give a procedure for some special case.
SOME but no all model of expansion tank include an automatic fill-trol valve while others do not - check the model that you installed.
See PRESSURE & TEMPERATURE SETTINGS, CONTROLS for advice on setting boiler pressure
Reader question: 11/3/2014 Mike said:
Great site I have a question on a building of a larger size, the expansion tank is 180 gallons and is of a bladderless type, now the boiler was operating with under 5 psi.. after it cooled I went around looking where the feed for cold water goes to the system which led me to the Watts SB1156F..
it has the fast fill lever on it, so I opened it and noticed the pressure gauge on the boiler started to rise.. I also noticed the water level in the expansion tank rose but when I returned the lever back to it's normal position I noticed water was still flowing but at a much lesser amount on the water meter.
I have two questions about the incoming water, why did the boiler not get any pressure until I bypassed the regulator setting, and why was the flow still trickling in after I reset it back to it's normal position?
Now the expansion tank it's full of water and I'm trying to figure out how to drain it, the terms they used back in the 60's are a little different but they at least tagged each valve with a brass letter and number, one of them in question is called expansion tank guard..
it comes from the circulation pumps and it leads to the bottom of the tank, another valve is the supply to the boiler just after the watts regulator. should I turn off the one that's called expansion tank guard and the supply for the boiler while I drain?
Mike I've been thinking about your question and am not sure I can diagnose the issue accurately from just your e-text.
If water pressure in the boiler drops, typically there's a leak to be found and fixed.
The pressure regulator may also be improperly set or in failure if it's not keeping the boiler pressure at normal cold levels.
11/5/14 Mike said:
Well Dan I fully drained the tank and I came back the next day and after getting the system back up everything was ok, it's a bit hard to describe over text I still don't know why the boiler before was not adding water to it and it was operating at 0 PSI.. Odd that the boiler did show pressure after I opened the fast fill (after boiler cooled) now it's fine.
I had no clue when the tank was last drained but the last date (and only date) someone wrote was it was drained back in 1988, and I don't think a tank can go 26 years without another draining?
I had no other option but to use the hose on the tank, I did have to blow into the hose and the tank would drain for about 5 min, then I would crack open the hose and air would rush in, as the tank was getting lighter the rushing air into the tank was enough to have it moving a bit.
11/6/14 Matt said:
I just had a certified tech replace the relief valve for my boiler water. I have the extrol (modern) tank with a bladder. I had no problems with the relief valve that I could see, but it was 10 years old and the tech suggested replacement. After he replaced it, I had significant water and it was because the bladder was now ruptured.
The company of the tech stated that it is "common" for the extrol bladder to rupture after replacing the relief valve - is that true? I don't know if this was an unlucky series of events or if the tech made a mistake. They didn't charge me labor, but did charge about $170 for a new bladder.
Your plumbing tech may certainly know stuff we don't, but I don't see the connection between installing a new TP valve and a ruptured pressure tank or expansion tank internal bladder. I've looked and not seen that warning among Amtrol's installation guides.
I'm also confused about why touching (to replace it) a relief valve would come anywhere near the expansion tank or its bladder. I don't know what a tech might do that would damage an expansion tank unless during system re-pressurization someone inadvertently submitted the equipment to unusually high pressures or jammed a screwdriver somewhere into an expansion tank.
Why not ask the company's service manager to help you out with an explanation of what happened and why.
Thanks Daniel, and I agree. I probably should have added more info (I was in a bit of a hurry the first time). In addition to the boiler water relief valve, I had the combo backflow preventer and air vent also replaced, so a little more work that did get near the Amtrol tank. I don't know anything about how to service a boiler, but I'm guessing that he had to vaccum some water and potentially perform other things that, if he over-pressurized the Amtrol® tank in starting back up, could have caused it to rupture, but I'm not sure.
Before I called back the service company, I was interested in your thoughts. I didn't think that answer made any sense either, so I'm glad to hear that. Now that you have the full story, any additional info would be helpful. Thanks again.
Sometimes when a hydronic system is drained for repairs it is re-filled under pressure, even using a pony pump, as part of the procedure to purge air from the lines. Still I'd be surprised if pressures during those tasks would burst the tank bladder.
Vacuuming? really? Indeed if the water pressure in the system were dropped rapidly and significantly, that might cause an internal pressure tank bladder to burst, tear, rupture.
I await with interest hearing what the service manager has to offer. I'd expect that if you come across as interested and not threatening s/he should be willing to offer a more cogent explanation.
Reader Follow-Up: case closed - service tech uses vacuum to speed the job - blows the expansion tank bladder
11/7/2104 Matt said:
He wouldn't admit that, simply kept repeating that these fail all the time and it was old (10 years) and should have been replaced anyway. My guess is that they probably pay about $20-$40 for these tanks and charge $165 to replace them (excluding labor). Nice racket for them. I did get about 50% of my money back, but needless to say, I'm switching service companies effective today.
Reply: if something "fails all the time" we might want to ask "Why?"
You have to love it, right. "They fail all the time" may indeed be absolutely true, because all the time the service procedure the company is using is one that destroys a component that was not intended for the negative pressure they apply.
I once inspected a brand-new home in which new gutters had been installed and it was raining. Roof runoff was pouring behind the gutter and on my head as I rang the doorbell. The builder himself answered the door.
I tried to be tactful. "Do you think this looks right for all this water to be running behind your gutters?" I asked.
"Yeah" he replied "they all do that".
Well indeed all of HIS gutters did that because his gutter installer always installed the roof drip edge improperly (behind the gutter rather than extending out over it).
Continue reading at AMTROL's EXTROL®, the FILL-TROL® or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: is it safe to remove and replace an old steel expansion tank with the newer type? Which is better?
is it safe to remove the horizontal type tank and replace it ? Is the old type better? is it ok to let the valve relieve the pressure and not have a tank at all? - Anon 9/11/11
Anon, you can usually replace an old bladderless expansion tank with a much smaller internal diaphragm tank, as we describe above. The new tank will be smaller yet can handle the same job. Newer internal-bladder heating system expansion tanks such as the Fill-Trol® Diaphragm-Type Expansion Tank have the advantage that you don't need to periodically drain the tank to re-charge its air.
In choosing the size of a heating system expansion tank, the service technician will consider the size of the building being heated, or more accurately, the volume of water in the boiler and heat distribution piping & radiators.
Question: New steam heater installed for first floor; old boiler had two expansion tanks - was it ok to remove one?
I am a home owner, i had a plumber switched and installed a steam heater in the first floor. when he saw my oil boiler that had two Expansion Tanks, he said it would work more efficient if he removed one on the two. He removed a green horizontal one at no charge, but took the Expansion Tank with him. Now that same steam heater he installed does not warm up at all. Is this because he removed one Expansion Tank.
Laura: steam boilers won't use an expansion tank, not normally, as the boiler is making steam and open to the atmosphere up through the radiators and steam vents.
There are some steam heat systems that use steam to distribute heat to upper building areas and use hot water from the same boiler to circulate through hot water radiators or baseboards in a lower building area. Maybe that's what you've got ?
Question: I have to empty our boiler's expansion tank every month - the relief valve spills; how far away can I put the new replacement tank I'm planning?
I have oil burner and I have to empty the expansion tank about once a month. I know it's full when the relief starts passing water. It's an old tank that lay's on top of the heater. My question is if I install the new type what is the max distance can it be from the heater? Is 8' to far? - Tony 6/5/2012
Question: what do I need to do after my contractor removed an attic expansion tank?
Would you be able to help me determine what to do when my contractor removed my tank. I'd like to send you a before and after photos if possible? - Contractor removed attic tank 7/8/12
We will try. Use the CONTACT US link at page top or bottom to send photos for comment, best accompanied by a specific question or concern. Keep in mind that if your building is quite old there may have been an old, unused, atmospheric pressure expansion tank in the attic.
Question: why would my heating system's expansion tank lose its air after just 3 or 4 days - the sight glass fitting shows no air
what makes expansion tank lose air in 3-4 days. I checked for air leaks with soap bubbles around tank and sight glass fittings, found none. Serviceman says domestic hot water coil may be leaking. How does that make tank lose air? Replacing coil will be expensive because of position of furnace. - J Warden 10/1/2012
J Warden, I have to agree that I'm confused by this question. I've not found a hot water heating system whose expansion tank included a sight glass (a feature I find on steam boilers) - can you give me some details: boiler brand, type, model, and some photos to allow further comment?
In any case, if your expansion tank (if that's what we've got here) is losing its proper air charge every few days, there is either a leak out of the tank piping and fittings.
Question: confusion about how to drain an expansion tank and what boiler pressures should be found
I drained exp.tank and shut valve and bleeder valve. I noticed the entire system was low. I brought system up to proper pressure and bled all radiators. Then I opened the valve to the expansion tank. When water entered tank the pressure for the heating system dropped. Do I now add more water to system or will pressure come back when water is heated .? Thank you - Lee Davidson - Filling entire system 10/8/12
Sirs, drained the exp. tank with tank supply valve closed. Closed drain valve and bleeder valve. Entire system pressure was low so brought it up and bled whole house system. Bleeding drops pressure so brought up then i opened tank supply valve this makes pressure drop but i think it will come back when water is heated. Right or wrong? - Lee Davidson 10/8/12
Lee, I agree with your procedure except the last step. On opening the expansion tank connection between the tank and the boiler, I would expect the boiler pressure drop briefly as water feeds into the expansion tank to bring it (and its air) up to the standard heating system cold pressure, eg. 12 psi presuming that the boiler is cold.
But unless your system lacks an automatic water feed valve, the automatic water feeder would be expected to add the necessary makeup water. Or doesn't your system gave one?
Lee, about your second question:
Watch out: some readers have been confused about the bladderless expansion tank drain procedure: one has to close the water feed valve into the tank (from the boiler) before opening the tank drain. Otherwise we are not draining the water out of the tank to let air in, we're just running water through the heating system: from the building water supply, through the water feed valve, through the boiler, and up and out through the expansion tank drain - accomplishing nothing.
Question: once or twice a year the expansion tank needs to be emptied - else heating system pressure goes above 30 psi. How do I stop this problem?
Hi. I seem to be having an issue once or twice a year that the expansion tank needs emptied, i.e. pressure goes above 30. Is there something I can do to stop this? Thanks. Sandy. email@example.com - Sandy 11/12/12
Question: Comment - thanks for discussing old bladderless expansion tank systems
Thank you for this informative information. Many articles just mention the expansion tank with bladders but I have an old bladderless system and I was not sure if I should let the water out The over pressure valve was periodically going off. Thanks again, I believe you solved my problem. - John OHare 11/16/2011
Thanks for the nice comment, John. We work hard to make InspectAPedia information accurate, complete, and unbiased and are thrilled when a reader reports that our data has been helpful. Indeed, there are still lots of buildings around whose heating systems use an older type of steel bladderless expansion tank - a component that needs periodic attention to keep the heating system working safely.
An bladderless expansion tank that loses its air charge not only causes the relief valve to spill, it is a more subtle safety hazard - as a spilling relief valve may eventually clog up from mineral deposits and crud, then stop opening, risking a BLEVE or boiler explosion. Sp if you see water on the floor under a relief valve it is something that should never be ignored.
Just how often we need to drain the water out of a bladderless heating boiler expansion tank varies from one installation to another; some need attention twice a year while I've worked on other systems that ran OK for three years or more before the expansion tank became waterlogged. If your bladderless tank doesn't need attention too often, say not more than once a year, there is no reason it can't continue in service.
Question: banging pipes vs new Amtrol Extrol® expansion tank
(Jan 11, 2013) Rick said:
I have a fairly new Amtrol expansion tank (~2 years old). Lately we are getting banging in the pipes when the heat is circulated. I know I probably have to bleed the system, but the valves that are on the unit are the handle-less type that have to be turned with a wrench to open, and they take a while to seal again once closed.
Will loosening the valve at the top of the expansion tank a little more allow the trapped air to escape eventually - or will I have to bleed the lines manually with a hose and bucket? I have loosened it a bit and I can hear air escaping occasionally and I am hoping this will eventually bleed the heating system.
Also, I cannot find any other leaks in the system to allow air in. All help is appreciated.
Reply: Rick, it should not be necessary ever to bleed a captive-air type expansion tank such as the Extrol® unit and I'm not sure how a waterlogged expansion tank would cause banging pipes in a hot water heating system. Instead please take a look at our banging pipes diagnostic article:
Reader Question: Water-logged Amtrol Extrol Expansion Tank No. 30 - vs tank mounting position?
(Jan 29, 2013) claude st-pierre said:
Hi I find your site very helpful.
I have an Amtrol Extrol Expansion Tank No. 30 (with disphram inside). The pressure inside the system was under 12 psi before I pumped it back up to over 15 PSI (18 PSI). The tank has a tendency to loose its air over time, especially after many days of cold weather (minus 15 Fareiheit).
The tank was installed in 2007 and is mounted with the water inflow into the tank at the bottom and the air valve on the top. The maintenance guy mentioned that this type of tank could be mounted upside down and that does not matter but on the Amtrol web site, they always show the Extrol tank with the water intake at the top and the air valve at the bottom.
Could mounting an Extrol tank upside down reduce the life of the tank or reduce the capability of the tank to keep the pressure under 30 psi when the heating system works at its maximum temperature?
Reply: Claude, Amtrol does not specify tank mounting position in the installation instructions (found at the References section of this article) for the Amtrol Extrol Expansion Tank No. 30 but the company does provide these notes:
You state "The tank was installed in 2007 and is mounted with the water inflow into the tank at the bottom and the air valve on the top." which leaves me a little uncertain. There is only one water connection on the Extrol expansion tank and one air valve fitting. The air valve is used to adjust the initial tank pressure to match the boiler cold or fill pressure at the time of installation.
As water entering the tank enters an internal bladder, I'm doubtful that the tank orientation affects its position. I think that the reason you see the tank mounted with it water entry port at the top is that it is being connected to the bottom of a typical air purging device whose tapping for that purpose is on the under-side of the air purger.
If your expansion tank is water logged and it's an Extrol unit it needs to be replaced as most likely the bladder has burst.
Reader Question: water-logged expansion tank prevents boiler operation?
2/27/2014 Trish said:
Can a waterlogged expansion tank cause an oil boiler to not kick on?
In short, no, normally a waterlogged expansion tank doesn't keep a boiler from turning on, but there could be these exceptions:
The principal effect of a waterlogged expansion tank would be spilling at the temperature-pressure relief valve.
Residential hydronic heating boilers (hot water heat) usually are rated for a maximum of 30 psi - the TP valve will open at that pressure and spill water.
I suppose if a boiler lost water and didn't include an automatic water feeder, a low-water cutoff safety switch on such a boiler in that circumstance would shut down the boiler.
Some heating systems might include a pressure control sensor switch that would shut down the boiler if its pressure became abnormally high - a condition that follows a waterlogged expansion tank, but that's not a control I've seen on a residential heating boiler.
Question: separate tank for electric water heater?
(May 31, 2014) Matt said:
I have a HWB heating system with an expansion tank. I am now installing an electric water heater. Does this need a separate expansion tank? Thank you.
You might, Matt, depending on local regulations and the operating properties of your system, including water pressure.
Take a look at our article on the topic over at
and let me know if questions remain.
Question: have to add air to expansion tank
(Oct 16, 2014) Tom B. said:
My Amtrol-Fitrol Model 110 needs to be topped-up w/air to every now and then to bring it up to 12 PSI. I figure it needs replacing. There is a shut-off valve in line immediately before the tank. Can I just shut off that (I guess water inlet valve) and swap out the old tank for a new one of the same model and manufacturer? Thanks.
Look for a leaky air valve or a burst internal bladder in the tank - replace it.
Question: troubleshooting expansion tank
(Oct 18, 2014) Swen said:
Hi there and thanks for running this site. Great resource since hydronic heating is not commonplace in the US yet (at least not in CA).
I am trouble shooting a broken brass fitting on my hydronic system and here is what I found:
- The valve to the Extrol expansion tank was shut-off
Here are my questions:
- Is the return side installation of the Extrol tank a serious issue?
BTW, the brass fitting burst during the summer month without any system load (it is a weak fitting - or rather connector type element - most likely not suited to the application).
Thanks for your answers in advance,
The Extrol tank comes pre-charged at the factory recommended pressure and will work on either side of the boiler water circulating loop but I would indeed prefer to follow the manufacturer's instructions - as described in the article above.
Probably 12 psi cold is the start pressure you need, but first read the details of how to decide on and set heating boiler pressure found in our article at
Question: The Amtrol Extrol Model 30 is leaking from the pressure gauge
(Oct 25, 2014) Jason Little said:
Came home to a flooded basement. The Amtrol Extrol Model 30 is leaking from the pressure gauge. Is this a easy repair?
I'm not sure what's going on with the pressure gauge: water intrusion vs. a damaged gauge vs. some other trouble. If the gauge was immersed by flooding or is damaged/leaking, I'd start by replacing the pressure gauge.
Question: do I really need an expansion tank on my boiler?
(Oct 28, 2014) Kevin Parry said:
The water in mt heating expansion tank is gurgling and the pressure gauge continually fluctuates, how I can I stop this?
(Nov 10, 2014) Anonymous said:
can a boiler operate without a expansion tank
The boiler will "run" but not operate properly without SOME means to handle thermal and air expansion in the water in the system - else it will dump the relief valve, that itself leading to an unsafe system.
Older boilers sometimes used a pressure overflow system that would simply dump boiler water at a safe location from an upper building floor. The pressure in the system was set by the piping height. No modern heating boiler uses such a system nor would it be considered safe by modern standards.
Question: Our water does not stay one temperature.
(Nov 11, 2014) July said:
Our water does not stay one temperature. For example, if we turn the shower on, it will go from extremely hot to very cold, the whole entire shower. Any suggestions as to why this might be happening?
You may be simply running out of hot water, or there may be a mixing valve control problem. If your water heater is a 2-element electric model it may have a burner out lower element.
Take a look at water heater troubleshooting articles found beginning at WATER HEATERS
Question: chilled water expansion tanks
(Nov 13, 2014) Steve said:
Do chilled water expansion tanks need to be inspected annually per any sort of legal requirement for commercial buildings?? Thanks.
Steve, I don't know. Such a requirement would originate in the equipment manufacturer's maintenance instructions, in a building code requiring following those instructions, or perhaps in your local, municipal, or state building department.
11/19/2014 Robert said:
My residential gas boiler pressure is operating at 30 psi. The pressure relief valve drips water on cycles. Temperature does not exceed 200 F. I have followed the proceedures for draining a bladderless hot water boiler expansion tank. water drained from valvle on tank into several buckets and when water was just about emptied air was slurrping back up into the tank.
I let the tank stand for a period of time. Closed the drain and slowly opened the incoming valve. Water entered the tank and pressure dropped to 24psi. Ran system for a couple days and pressure is back up to 30 psi and the tank was filled up again. Folowed same proceedure again but this time the pressure did not drop and stayed at 30 psi.
Or the pressure reducer / water feeder may be at fault
Reader follow-up: 11/24/14 Robert said
There is no tankless coil. Replacing the pressure regulator valve for incoming water supply would be the solution?
Robert the proper solution depends on a proper diagnosis of the problem. We don't know what symptoms were observed at the heating system you are describing. If the TP valve was leaking (for example) that might be caused by any of several reasons: over-pressure in the system, a tankless coil leak into the boiler, or a failed expansion tank. I'm reluctant to agree that a specific part replacement is the right repair when I don't know how the problem was diagnosed.
I have a an old non-blatter type expansion tank that is waterlogged..I am going to drain it this weekend following your description...I have been giving the entry and drain valves high penetration WD-40 for the last 2 days to avoid valve stem problems..now the question.
I wouldn't bother to try pre-charging the tank just to help it drain, but I would open a tapping or fitting that lets air into the tank and system to speed the drain-down.
Question: proper repairs to an expansion tank, air vent, pressure reducer?
11/20/2014 Anonymous said:
I am purchasing a house with an oil-fueled hot water boiler (NewMac NBR-2002). During an inspection of the system, it was determined that the expansion tank and the automatic air vent needed to be replaced, and my technician also indicated that the pressure reducing valve would likely need to be replaced. Instead of crediting me the cost of those repairs, the owner instead had another heating company replace the expansion tank and the air vent, and that technician declared that the pressure reducing valve was working just fine.
My technician told me that one has to run the system for as long as a week to know whether that valve is actually working properly. There is clearly a difference of opinion here. Comments? By the way, we live in an area with very hard water.
If the water feeder-pressure reducer valve has failed you would probably see the boiler over pressurized and leaks at the relief valve. Maybe your tech was being safe (spending other people's money to reduce his risk) or maybe he or she saw something not reported here. But the seller's repair was a reasonable step. For the valve to also be a problem would at this point be a double fault - less likely. If the repair already done was not effective you should see that as excessive boiler pressure - typically in 1-5 days. Look again.
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