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Here we explain hot water boiler (hydronic heat) pressure reducing valves and types of manual and automatic water feeder valves used on hot water heating boilers, including when and how to add makeup water to a boiler. A chart illustrates the adjustment to heating boiler operating pressure as a function of building height or the number of floors in a building being heated.
We include photographs of common pressure reducer valves and water feeders used on hot water heating boilers to aid in product identification, links to installation manuals, safety warnings, installation and pressure reducer / water feeder safety and maintenance tips.
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The photo above shows a modern automatic pressure-reducing water feeder valve on a hydronic (hot water) heating boiler - one of the safety controls which we discuss in this article. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
[Click to enlarge any image]
Hydronic (hot water boilers) in proper condition do not normally consume any water. Once the boiler and the baseboard, radiator, or convector piping are connected and filled with water, that same water remains in the system indefinitely. The water in the boiler is heated and circulated through the occupied space to provide heat to the building occupants.
Water may be lost from a hot water heating system, however, due to a small leak that may be un-noticed, or water may be removed from the system during servicing.
Some older heating systems may not have an automatic water feeder and may only provide a manually operated valve to add water to the boiler. To maintain the water level in these heating systems, water can be added from the building water supply piping manually by simply opening a make-up valve. A manual valve will simply be a shutoff valve that can be opened by hand to force water into the heating boiler.
On these older systems the "automatic water feeder" is often a bell-shaped device which opens and sends makeup water into the heating boiler and its piping whenever the heating system's internal water pressure falls below a normal level (perhaps 12 psi when the boiler is cold).
In our photo at left the gold colored bell-shaped device pointed to by the red arrow is an automatic pressure-reducing & water feed valve for a hot water boiler. An arrow cast into the valve base shows the direction of water flow (from building supply at right into the boiler at left in this photo).
The screw and lock nut on top of this valve permit adjustment of the automatic water feeder valve pressure (it's normally set to 12 psi).
The horizontal lever is a bypass that will send water through the valve on to the boiler at full building pressure (it's normally left "off" in the position shown here).
The separate heating system backflow preventer / check valve indicated by our green arrow in the photo above is discussed in more detail
The device to the right of the automatic pressure reducer (water feed valve) in our photo above is a back-flow preventer that is required in some municipalities. The back-flow preventer makes sure that water from inside the heating boiler cannot flow backwards into the building (and community) water supply. This prevents back-contamination of potable water piping from the boiler should the building water pressure fail.
See CHECK VALVES, HEATING SYSTEM for more information about backflow preventers and other heating system check valves.
Bell & Gossett's sketch of their pressure reducing valve is shown at left.
Well yes, and no. The valve can automatically feed water into a hot water heating boiler whose pressure falls below the set-pressure (12 psi at factory setting) if the water shutoff valve for the boiler has been left "on".
The B&G pressure reducing valve shown at left includes a bypass lever shown in the "up" or "open" position in the illustration. In this position water is fed into the boiler at street pressure. When the valve is "down" in the "closed" position, IF the shutoff valve before the pressure reducer is open, water is fed to the boiler if its pressure drops below the valve's pressure setting (adjusted by the lock-nut and screw on the valve top).
But the manufacturer warns that the purpose of the pressure-reducing (and automagic water feeding) valve is to fill heating boilers after installation or servicing. The company says
"It is not a safety device and is not intended to be used as as a water feed valve to control boiler water at a safe operating level".
OPINION: this is interesting since for closed-system hydronic heating boilers (not steam boilers) that's how lots of service techs and inspectors view this device.
In the company's service manual you'll see on the installation piping sketch (edited) for the pressure reducing valve (green, to the right of the blue shutoff valve at the left side of the sketch below) that they expect the installer to include a water shutoff valve (blue at the left end of the sketch below) before this pressure reducer.
B&G wants you to keep the boiler water supply valve shut except during service - as a precaution that allows easy detection of a boiler leak by noticing the reduced water pressure in that system. The company also points out that too-frequent feeding of water into a heating system can increase the effects of corrosion and system damage.
In my [DF] opinion, a typical building owner or occupant almost never thinks to check the water level in a hydronic heating system (hot water boiler) until there is some indication of a problem (like no heat or a damaged leaky boiler).
In our experience most hydronic heating boiler installers and service techs leave that shutoff valve "open" or "on" so that the boiler won't be at risk of being ruined or unsafe by operating at low or no-water level - their experience may be similar to mine. The manufacturer is telling you what's safe in some regards, but they may not have the same view of what people actually do in the field.
A safety improvement on hydronic boilers that gets around this argument is to add a low-water cutoff on residential hydronic heating boilers (hot water heating boilers).
That device, always present on steam boilers, is required by local codes in some jurisdictions for hydronic heating boilers too. See LOW WATER CUTOFF VALVES.
OPINION: on a hot water heating boiler that does not have a low-water cutoff valve installed, we would be reluctant to leave the building unattended for weeks or months during the heating season with the boiler water supply shutoff valve in the closed position.
We would think about leaving the water supply valve open - not what B&G recommends. That way if a small leak develops we're not at risk of destroying the boiler by firing it without adequate water in the system. This violates the B&G installation instructions for their pressure reducing valve - so be sure to review this concern with your trained, heating service company service manager.
In normal use, a hydronic or hot water heating system does not consume any water. Only if there is a leak (or during service) would the water level in a hot water heating system drop and need replenishment. That is an abnormal condition, and one that means the system needs to be repaired.
Keep in mind that devices like the B&G Pressure Reducer (notice they don't call it a "water feeder" even though it feeds water to the boiler) is intended for hot water or hydronic heating boilers.
But a hot water heating system might have a tiny, small, even hard-to-find water leak that goes un-noticed for some time, especially if the boiler water supply valve is kept open so that the pressure valve also feeds a little makeup water into the boiler when needed.
Conversely, on a steam heating system the boiler is expected to consume water at every operating cycle - the feed frequency is therefore much greater on a steam boiler and this B&G pressure reducer/water feeder would not be the proper device to use for water feeding.
For water feeders used on steam boilers,
Our photo at above left shows an Armstrong™ pressure reducing valve. The schematic at above right, courtesy of Carson Dunlop, explains how this pressure reducing valve works. [Click to enlarge any image]
Our photo (left) shows another model of pressure reducer valve on the water supply line to an Energy Kinetics oil fired heating boiler.
The lever on the top of this valve combines a full-open bypass (when the lever is vertical) and a set-pressure adjusting locknut (below the round lever body).
Safety note: If the pressure reducing valve is combined with a pressure relief valve, for safety these parts must be installed in the correct position.
As Carson Dunlop Associates shows in the sketch above, and as you'll also see in the B&G installation piping sketch, the pressure relief valve should always be installed closest to the heating boiler so that it can respond to an overpressure in the heating system.
What is the proper water pressure setting for a pressure-reducing valve or water feeder valve on a Hot Water Heating System?
What is the normal operating pressure of a hot water heating system boiler?
If you set the water feeder pressure too high it's likely that when the boiler heats-up to operating temperature, the water pressure will exceed the safety limit (30 psi) and the boiler relief valve will open to discharge excess pressure and water.
[click to enlarge this or any image]
Because a hydronic heating system's circulator pump has little "lift" capacity (it just pushes the water around the loop of boiler and heating piping and radiators), the heating system depends on its internal water pressure to lift hot water to radiators (baseboards, or convectors) on upper floors in the building.
A higher building may therefore need higher heating system water pressure in order to be able to circulate heat adequately to upper floors. The sketch, courtesy of Carson Dunlop, explains that a three-story building will need heating system pressure set up to at least 15 psi. when the heating boiler is cold.
But in most cases you should leave the water feeder at the psi set by the installer (typical factory setting is 12 psi) unless the building has unusual conditions (such as more than two stories) that require a higher in-boiler starting water pressure.
An automatic water feeder valve is included as part of some heating system expansion tanks on hydronic heating boilers - in a single unit such as those provided by Amtrol™, Extrol™, Fill-Trol™, or similar expansion tanks.
The water feeder is the brass assembly found attached to the expansion tank where piping from the boiler enters the tank on these units.
In our photo at left you can see the brass valve on top of the Fill-Trol™ expansion tank and underneath the cast iron air purge valve (red arrow).
The vertical copper pipe is feeding water from the building supply, at building water pressure into the brass valve and through it, upwards into the heating system distribution piping through the silver painted cast iron air purge valve.
This automatic water fill valve is not adjustable by the consumer and is factory-set to add water to the heating boiler whenever the system pressure falls below 12 psi.
The tank below the red arrow is, of course, an expansion tank. We discuss expansion tanks and their troubleshooting or maintenance at EXPANSION TANKS.
Details about automatic water feeders for steam heating boilers are at WATER FEEDER VALVE, STEAM. Excerpts are just below.
Keeping the proper amount of water in a steam boiler is a critical function for both safety and to avoid damaging the boiler itself. So on a steam boiler we consider the automatic water feeder to serve as a safety device too. Water feed valves on steam heat systems operate under very different requirements than those on hot water (hydronic) heating systems.
Automatic water feed valves on steam boilers (such as the McDonnell & Miller automatic water feeder shown at left) perform functions similar to the valve described earlier for hot water boilers.
Residential steam heating systems are mostly "low pressure" systems operating at less than 2 psi. In fact if you see the pressure set higher on a residential system it is often an indicator that the occupants/technician have been having trouble getting the steam heat to locations where it's needed. All steam heat systems all consume some water which escapes from radiator air vents during the time that the steam (heat) is first rising in the building. So unlike a hydronic water feeder, the steam boiler water feeder is going to be much busier, regularly adding makeup water to the steam boiler, but operating at low water pressures.
Steam heating systems without an automatic water feeder are less safe and risk serious boiler damage should boiler water be lost and should there be no low water cutoff installed on the system. That's why all modern steam heating boilers can be expected to include at least a low water cutoff valve.
Low water cutoff valves are discussed in detail at LOW WATER CUTOFF VALVES.
The following advice is adapted from Bell & Gossett's Instruction Manual V55999 Rev. M. Reducing Valves Installation & Service Instructions - a booklet that the installer is supposed to have been left for home owner use.
Select the Proper Pressure Reducing Valve
There are multiple B&G pressure reducing valve models, not to mention competing valves made by other manufacturers. Valves vary by their pipe diameter (1/2" or 3/4"), connection type (union or NPT/Sweat), and by factory setting and adjustable pressure range. The latter two key parameters are pretty simple as there are 2 ranges:
Pressure Reducing Valve Specifications
Pressure Reducer Valve Installation
Install the Pressure Reducer Valve Right side up:
Point in the right direction: The cast-arrow indicating direction of water flow must be in a horizontal position (and of course pointing towards the heating appliance).
Feed with cold water: We suspect that on occasion a plumber may supply the valve with hot water, thinking s/he is reducing the risk of damage that can happen by feeding cold water into a hot boiler. While the installation instructions don't explicitly say so, the illustration provided by the manufacturer shows that the pressure reducer / water feeder valve that is going to supply water to a heating boiler is itself fed by the cold water supply piping in the building. And the manufacturer's instructions consistently refer to supplying cold water to the equipment, for example:
Install a water shutoff valve on the inlet (water supply side) of the pressure reducing valve.
Install a bypass valve and piping arrangement for fast filling of an empty heating boiler if the pressure reducer valve model you're installing is not itself a "Fast Fill" model.
Watch out: if you install a bypass valve to allow fast filling of the heating boiler, make sure that this bypass is closed during normal boiler operation. Otherwise the boiler will be over-pressurized, the relief valve may spill, equipment or the building may be damaged, the equipment may be unsafe.
Install other safety controls: B&G recommends for safe boiler operation that a low water cutoff valve (LWCO), adequate burner safety controls (cad cell sensor etc), and a properly-installed float-operated water feed valve should all be installed. We are seeing more LWCO's installed on both hydronic and steam boilers, required by code in some jurisdictions. And it's rare to see a modern steam boiler that does not have the recommended automatic float operated water feeder. Without these controls, and without appropriate air controls (air purger and thermal expansion compensation), B&G warns that the boiler may not work properly nor safely.
Don't over-do the pipe joint compound: the manufacturer warns to use pipe joint compound sparingly (not "the bigger the blob the better the job" as Dan B. Martin used to say). The concern is that excessive pipe dope oozes into working control parts and can prevent safe, proper operation of the equipment.
Warning when using teflon tape on threaded pipe fittings: the same manufacturer warns that use of Teflon impregnated pipe compound and Teflon tape on pipe threads provides so much lubrication that it's easy to over tighten and damage the parts - risking unsafe or damaged equipment. Indeed we found that when tightening a part such that we are approaching the part's desired final position, the lubricant effect of teflon can cause you to turn the part too far - so you're tempted to turn it for another complete rotation to get it where you want - followed by breaking the part.
Disassemble some parts when "sweat fitting" (soldering) them in place: when installing a reducing valve using soldering, to avoid damaging the valve by overheating it, first sweat the union connection fitting onto the system piping, then connect it to the valve.
Water feed pressure adjustment - at initial installation
When the pressure reducing valve installation is complete and you are ready to fill the heating boiler, open the cold water fill valve to allow the system to fill until the preset pressure of the valve has been obtained. (Residential pressure typically starts at 12 psi cold).
If you are using a Fast Fill model pressure reducing valve, instead of filling the system through the bypass valve and piping you lift the fast fill lever to the vertical position, overriding the pressure regulating features of the valve and allowing water to enter the heating equipment and piping at building water supply pressure. So be sure to stop filling when pressure in the heating system is at the proper cold-water temperature pressure.
Watch out: after this initial boiler fill-up, close the fast fill lever - don't leave it up - open or the system will be unsafe and possibly damaged.
For pressure reducing valve feed pressure adjustment to meet varying building conditions such as tall structures that need higher starting pressure, see the detailed explanation found above in this article.
Inspect & Check the Pressure Reducer for Proper Operation, Clean the Strainer if Needed
B&G offers some excellent tips on how to check the pressure reducer for proper operation, and how to fix some common problems.
Using the Pressure Reducer Valve Repair Parts Kit
B&G provides a repair parts kit for this valve.
Water Pressure-Reducing Feed Valve Inspection Tips
Reader Question: What should we report if finding a water feed valve tied off as shown in this photo?
Tom Sherman - President, Absolute Home Inspection, Inc., 315-673-1755 provided this photo of a tied-off pressure-reducer water-feed valve and asked what we'd say about it, for an ASHI home inspector newsletter.
Home inspectors are expected to be competent to recognize conditions at a home that are improper, unsafe, not functional, etc. provided that the problem or component is readily accessible, visible, and under some other conditions.
In this photo we have a pressure-reducing water feed valve typically used on a hot water or hydronic heating system. The valve valve, this one is a B&G model, has the job of feeding makeup water to the heating boiler should the water pressure in the heating system fall below the set-pressure on the valve - typically 12 psi for most one or two story residential buildings, though the set pressure may need to be higher in taller structures.
The water feed valve's override lever in the photo has been secured to the water pipe by a black plastic tie.
The override lever is used to put the water feed valve into "bypass mode" by lifting the lever to a 90-degree position above the water piping and valve body - meaning that the valve is manually forced "open" to allow water to enter the heating boiler at whatever pressure is on the inlet side of the valve. (The heating boiler, not shown here, is always on the outlet side of the valve, and on most of these controls an arrow cast into the valve body indicates the intended direction of water flow.
In our photo we've drawn a blue arrow showing the direction of water flow for this valve. Click the image to see an enlarged version.
If the valve were in manual OPEN mode, the lever would be at right angles to the piping and valve body - its handle would be over the adjustment screw shown in the center of the valve top. So this valve is in the CLOSED position.
Why would someone put a plastic tie on the automatic water feed valve override lever to hold it in the CLOSED position?
Perhaps the heating service technician was annoyed by an owner's emergency service call complaining that the pressure / temperature relief valve was spilling and perhaps the tech found that the owner had been messing with the valve and over-pressurizing the boiler. If you set the boiler water pressure too high when cold then when the boiler heats up the pressure will exceed the (roughly 30 psi) pressure at which the relief valve will open.
But a more likely reason this valve was tied OFF or CLOSED was that it was leaking, feeding excess water pressure to the boiler on its own. One of the failure modes of these valves is that the valve begins to feed water when it should not. That can happen due to dirt and debris in the valve or for other reasons. Someone was trying to force the valve to behave itself.
Functional & Safety Worries with a Bad Behaving Water Feed Valve
Watch out: this is not only an improper heating system operating setup, it is potentially (though subtly) quite dangerous:
Watch out: The manufacturers of this type of automagic water feed valve recommend that additional manual control valves be included in the piping system, so in some installations you might also find that the automatic water feeder is not really so automatic.
Which is pretty interesting since so many people in the trades as well as among inspectors consider it a safety device.
This article series answers nearly all questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
Continue reading at CHECK VALVES, HEATING SYSTEM
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Question: why do I have to keep adding water to my boiler?
(Jan 16, 2012) Bob said:
why do i have to add water daily to my boiler
Question: expansion tank (or relief valve) keeps discharging water as service water pressure varies
I have a gravity feed system with water radiators in my home. When I leave the incoming water on the pressure in my system goes sky high the expansion tank discharges the water. When my system is full of water and I turn off the incoming water by the water feeder my system loses pressure and I have to add water again thru the water feeder. Is my water feeder the issue?? When the system loses pressure where is that water going?? The expansion tank is above my furnace in my basement. Any help would be appreciated!! Thanks, Fred
In regards to my gravity feed system "pressure" problem question -- Municipal water, system has been working fine for last 15 yrs. When system full of water home heats fine but when furnace off pressure drops. Thanks, Fred
If your boiler pressure is low and/or if you've had a leak and lost boiler water leading to low pressure AND if your water feeder valve is not working properly (or is not properly set) then you may have low boiler pressure.
Question: My house is too hot and the boiler circulator keeps going even if I turn down the thermostat
The circulator pump on my Weil Mclain boiler doesn't respond to the thermostat. I closed the manual flow valve to prevent the baseboards from "heating" during the summer. I recently had to open the valve to provide heat to the house now that it has gotten colder but when the house heated up too hot the flow valve would not close and the baseboards continued to receive hot water. How can i "fix" the valve to shut off the water supply to the baseboards? Will i need to purge the water from the zone to replace the valve? - House too hot 12/21/11
House too hot:
Fixing the problem by opening and closing the flow-control valve is a stopgap that as you have seen, is not a great solution.
Question: Top floor radiator is not getting water - should I up the system pressure?
I have a home with 3 levels above the basement, and radiators on every floor. The radiator at the very top floor does not seem to even contain water, and bleeding does not help. The radiators on the 2nd floor, just below the attic used to heat up fine, but now they do not seem to have water either. Obviously, there is a pressure issue here, but I had the water feeder valve for my hydronic system replaced just 4 years ago. The radiators in the basement and on the first floor work just fine. Can I turn the pressure screw on the water feeder to increase the pressure while the system has been operating for the past 2 months and the boiler is not cool? - Joey Butters 12/31/11
Joey I would NOT start by changing system pressure since there is a good chance that your top floor radiator is simply air bound. Hop over to our radiator troubleshooting articles at RADIATORS and you'll see how to check for and fix an air-bound heating radiator by simply opening an air bleeder.
i have a three zone ( individual circulator pumps) hydronic system that i recently changed the expansion tank and auto bleeders on and now it wont keep pressure for more than an an hour or so i have searched the system for leaks and cant see any. i have to keep adding water to get pressure back. what else could be wrong and can i leave the make up water valve (not the pressure reducing valve) but the one that feeds it open? is this dangerous?? it is a two story house - Martin Griffin 1/15/12
If the system is not maintaining pressure we need to look for a leak or a defective backflow preventer or check valve.
Question: The relief valve at my boiler keeps leaking, even after we replaced it
My boiler has been leaking at the pressure relief valve for 1 week, with the pressure at 30-35 when hot. I drained the steel expansion tank fully, and then re-started the system. The pressure initially was back down around 20, but within 1 day was back up in the 30s and leaking at the PRV. I had a technician come and install a new pressure reducing valve. We all thought that would fix everything.
We drained the expansion tank again before re-starting the system. After we re-started the system, the pressure continued to rise. one day later we're back at 33 PSI and draining from the PRV. I tried draining more water from the expansion tank and closing of the water valve from the city. The pressure in the system went down to 20. After running the heat, it's now hot and at 25 psi. Could I have just gotten a defective new pressure-reducing valve? Any thoughts? Help would be appreciated. - Josh 2/7/12
Great question Josh. There are actually several things that can cause the relief valve on a boiler to leak, among them is an automatic water feeder/pressure reducer that is not working properly, but there are quite a few other causes too. The fact that you drained your expansion tank and then found that you could drain it again makes me suspect that as the best starting place.
If it's an older bladderless type expansion tank (some call these compression tanks) it may be that it wasn't adequately drained. If the expansion tank is a newer type that has an internal bladder the bladder may be damaged or the air valve may be leaking out the air charge. Details are at EXPANSION TANKS.
At RELIEF VALVE LEAKS we provide a catalog of all causes of leaks at all types of pressure or pressure / temperature relief valves.
Question: the water level in my steam boiler keeps rising
my sight glass on manual feed boiler rising without adding water I have steam heat system with
And don't forget to double check that someone else has not manually added water to the boiler.
Question: where do I add water to my furnace
Need to know where I add water to my furnace. The low water indicator is on. Not sure where to look. And not sure if I have an automatic water feeder. Thank you LJV 109/23/11
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