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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 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
This website answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. The photo at page top shows a modern Extrol(R) bladder-type heating system expansion tank.
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.
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 degF) 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.
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.
Periodic draining of non-bladder boiler expansion tanks:
So Now, Finally, Let's Drain that Heating Boiler Expansion Tank
Many expansion tanks use a special drain valve that permits air to flow into the tank as water is drained out.
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.
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.
Really Old "Expansion Tanks" are Attic Cisterns Open to the & Air Won't Need Draining
Frequently Asked Questions (FAQs) About Heating Boiler Expansion Tanks: types, troubleshooting, draining, maintenance, repairs or replacements
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.
Questions & answers or comments about heating system expansion tanks: their function, size, location, maintenance, and need for draining (on some models)
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