Bladderless expansion tank on gas fired heating  boiler (C) HHHeating Boiler Expansion Tank FAQs
Diagnostic questions & answers assist in leaky or water-logged boiler expansion tank service & repair

  • EXPANSION TANK DIAGNOSTIC FAQs - CONTENTS: questions & answers help troubleshoot problems with hot water heating system expansion tanks - also called compression tanks. Why is the TPR (relief valve) dripping? How do I know if the problem is the expansion tank? How do I diagnose or fix expansion tank problems.
  • POST a QUESTION or READ FAQs about heating system expansion tanks: their function, size, location, maintenance, and need for draining (on some models)

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Hot water heating system expansion tank / compression tank installation, troubleshooting, repair diagnostic questions & answers:

Reader questions about diagnosing leaky TPR valves, leaky expansion tanks, hot water system pressure problems & explanations of how expansion or compression tanks actually work on hydronic heating systems help diagnose problems with these components.

This article series provides a heating system expansion tank (compression tank) troubleshooting & repair guide.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Diagnostic Questions on Heating Boiler Expansion Tanks - Troubleshoot, Understand, Fix

LARGER VIEW of a heating boiler expansion tankThe photo at page top shows a traditional steel bladderless compression tank on a hot water heating systems. Most techs and owners call these "boiler expansion tanks".

At left is a modern Extrol(R) bladder-type heating system expansion tank.

[Click to enlarge any image]

These questions & answers about the expansion tank or compression tank used on hot water heating boilers were posted originally at EXPANSION TANK LEAK REPAIRS.

On 2017-03-29 by (mod) - if a tankless coil is leaky which way does the leaking water move: into the boiler or into the building water piping?


You should not have to drain the expansion tank more than once. It's normal for the boiler water to be smelly - the same water cycles around in the boiler and piping system for years.

I'm unclear what you mean by "burner pressure at 15 lbs" - the burner doesn't have a water pressure. You probably meant boiler pressure - and yeah, 15 psi would be in normal range. And 42 psi is normal range for a building water supply system pressure.

I don't consider the smell an issue unless you thought you were getting the same odor from your building water supply (as opposed to drainage from the boiler).

Water treatment on heating boilers is important on commercial systems (for corrosion protection) and of course on steam boilers that keep taking in new water. It's much less common on residential boilers.

Antifreeze is used in some residential boilers - so if there were a tankless coil leak that happened to find that unlikely scenario I described before (boiler leaks into building water supply) that would be dangerous unless food-grade antifreeze were used. As I said, if a coil leaks the more likely scenario is that boiler pressure keeps creeping up and the TPR valve leaks.

On 2017-03-29 by Bill

Thanks for the info...very helpful. Drained the expansion tank several more times. Again, same smell from that water as in the potable water...only much stronger. Also, getting brown sediment in the tank water.

Maybe iron bacteria growth since we have been getting small pieces of metal like grit in the potable water along with the brown sediment in the tank water (staining the slop sink). Maybe should pull the submersible pump up some.

Checked burner pressure at 15 lbs. when firing. Well water tank pressure at 42lbs. Does not suggest a coil leak but asked to check water for antifreeze VOC as well as iron bacteria. Will write back when we figure this out. Thank again

On 2017-03-28 by (mod) A heating boiler expansion tank water may smell from these sources:

A heating boiler expansion tank water may smell from boiler treatments, antifreeze, or just old heating water that may contain bacteria, rust, etc.

Boiler water could, in theory, leak into the building's potable water supply when a tankless coil is installed in the boiler, and IF the tankless coil has a perforation or leak. But ONLY when the boiler pressure is higher than the building's water supply pressure.

That is not normally the case. Normally a residential heating boiler ranges from about 12 psi (cold) to under 30 psi (hot) while house water pressure typically is above those levels.

(But not all the time, since on a private well system some pressure switches "cut in" at 20 psi).

But here's a critical clue to the presence of tankless coil leaks:
As usually, for at least part of the time, the house pressure is higher than the boiler internal pressure, if there is a coil leak water will leak INTO the boiler, raising its pressure above normal, showing up as relief valve leaks at the boiler or as abnormally high pressure readings on the boiler pressure gauge.

Search using the search box just above to find our article on TANKLESS COIL LEAKS for complete information.

Thanks for asking, Bill.

On 2017-03-28 by Bill - I drained my expansion tank and the water had a very strong chemical smell.

I drained my expansion tank and the water had a very strong chemical smell. I have that same smell in my potable well water supply (being tested). Is it possible that the antifreeze in my heating system is bleeding into the potable hot water supply within the tankless coil in the burner (there is no hot water heater)?

On 2017-03-13 by (mod)

See where this question and reply are already posted

On 2017-03-13 by Anonymous

The valve on the top of the expansion tank is dripping water.

On 2017-03-02 by (mod) - should water be in the expansion tank?

It's normal for there to be water in the bladder in the lower portion of the tank. That ought to give a different sound when tapping there.

On 2017-03-02 by cliff have a 4gal honeywell expansion tank w/butyl membrane above my boiler/furnace. was told to tap on it to give me an indication as to whether or not it needs to be replaced. when i tap on top half i get a ping, tapping on bottom half gives me a solid thud. is this normal?

On 2017-02-15 by (mod) - should there be air in the expansion tank?

Yes, Anon. If there were no air in the expansion tank there would be nothing that could be compressed in the tank (water is not very compressable).

Without that "spring" of compressed air to absorb an initial pressure increase in the system when the water is heated, pressure would cause unsafe leaks such as at the pressure relief valve.

On 2017-02-15 by Anonymous

Is there supposed to be air in the expansion tank or holding tank on a boiler system

On 2017-01-07 by (mod) - a dripping relief valve is unsafe and needs immediate attention

A dripping relief valve is unsafe. The problem could be a waterlogged expansion tank, over pressure in the system, over temperature in the system, a defective TP valve, or something else.

If your tankless coil is leaking into the boiler that'd cause overpressure. Check the pressure gauge to start.

Watch out: failing to fix a leaky relief valve risks a BLEVE EXPLOSION - search to read details.

On 2017-01-07 by cliff

just left question on water valve drip forgot to include it is oil fired boiler furnce

I have a boiler furnace that heats water both for heating and domestic use. I just took notice that the water relief valve with pipe to floor is slowly dripping. good size wet spot on concrete floor. The drip is from inside if the pipe running to floor, nothing wet where valve is at top side of furnace.

I has a Honeywell Thermal Expansion Tank with Butyl membrane. Never saw it leak before. The furnace and everything around it was replaced in the early 2000's. Anything wrong or is this normal?

On 2016-12-22 by (mod) - If your expansion tank does not include and air adjustment valve, then the pre-charged pressure is probably not adjustable.


If your expansion tank does not include and air adjustment valve, then the pre-charged pressure is probably not adjustable.

As long as the tank is the correct size for your system, and your boiler pressure when cold is at 12 PSI, and your boiler pressure when the boiler is hot is below 30 PSI, then the system should work correctly.

On 2016-12-22 by John

So, novice here - hydronic expansion tank died, rusted through (old). I have a new one - got the right size, etc, no worries there. The furnace reducing valve is 12psi; the furnace psi shows 20 when it's cranking out hot water. How much pressure should the expansion tank have, and what can I do if it's already installed but overpressurized?


On 2016-11-20 by (mod)

Steve your question on boiler expansion tank sizing is online in a detailed article I'll post at EXPANSION TANK SIZING GUIDE

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.
please help - Laura 10/30/11


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.

Usually on a modern hot water (not steam) heating system you wouldn't need two expansion tanks, certainly not if one of them is a new internal bladder type. So, recognizing that I can only guess from afar, it sounds like two different problems.

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



Amtrol makes both thermal expansion tanks and hydronic expansion tanks used on heating and plumbing systems. Amtrol's expansion tank installation instructions specify "Install the EXTROL on the supply side of the boiler, on the suction-side of the heating circulator(s) (see Figure 1)." That is, the location is more critical than the distance.

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?

Or finally, if at a low pressure heat makes it to the highest radiator or baseboard you are provably OK.

Lee, about your second question:


It is a mistake to set the water pressure in a hot water heating system to an abnormally high level, and further, you shouldn't need to.

Presuming that your equipment has a water feed/pressure reducing valve in place, that valve will automatically feed water to the system up to the usual COLD water starting pressure. Typically that's around 12 psi, though it may be higher for some taller buildings.

Expecting the problem to be fixed when the water is hot is also an error. It's the cold starting pressure that we attend.

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. - Sandy 11/12/12



Sure. You can eliminate the need for periodic draining the heating boiler expansion tank by replacing it with an internal bladder type tank like the Extrol shown at the top of thus article.

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:
BANGING HEATING PIPES RADIATORS(more often a concern on steam heating systems) or if your heating system is only a hydronic unit (hot water heat only)


Bubbling or rumbling heating system noises in hot water heating piping can be caused by air in the heating lines. If the amount of air becomes excessive the heating system may be unable to circulate hot water and extra steps to bleed unwanted air will be required.

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?


Claude, Amtrol does specify tank mounting position in the installation instructions for the Amtrol Extrol Expansion Tank No. 30 (found at the References section of this article) in text, from which I quote:

Mount tank vertically in downward position only. Ensure the piping can support the entire weight of the tank when full of water.

and in illustration shows it suspended below the air scoop/air vent on the supply side of the boiler. The company provides these notes:

1. Although the EXTROL does not have to be installed at the base of an air purger, this installation is recommended to reduce air in the system and provide a compact installation. Some method of air elimination is required to ensure an oxygen-free system.

2. The EXTROL must not be placed overhead in the floor joist space. When replacing a plain-steel tank, the EXTROL should be relocated per Figure 1. [see our schematic drawings in the article above]

3. Do not place the EXTROL on a dead-end pipe. Air can collect in the EXTROL, causing internal corrosion and possible leakage.

4. Radiant systems with non-barrier tubing or high oxygen content should use a Radiant EXTROL ® expansion tank with a plastic liner. See the EXTROL brochure, or call (401) 535-1216 for sizing assistance.

* Models #15 through #90 come with a 5 year warranty. All other EXTROL models come with a 1 year warranty. See reverse side for warranty information.

* Do not hang models SX-30V through SX-160V. They are floor standing models only

- Extrol® (2014)

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.

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. Follow Amtrol's instructions.


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.

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
- The expansion tank (and the air trap) are on the return side and not the supply side

Here are my questions:

- Is the return side installation of the Extrol tank a serious issue?
- What is the right pressure to preload the entire system (My heating system is in a basement in a two story above ground house)?
- Is there ever a need to check pre-load charge of the Extrol tank in accordance of the set system pressure (see above)?

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: 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.

Question: old atmospheric (non-bladder type) expansion tank is waterlogged. Should I pre-charge it after draining?

Atmospheric expansion tank on hydronic boiler (C) Daniel Friedman Two Harbors MN

Carsten said:

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.

Is it worthwhile to charge the expansion tank with approx 20 psig air pressure from a small compressor before closing the drain valve and opening the connection valve from the boiler..good idea or bad idea ?

Reply: No.


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. The atmospheric expansion tank shown just above your question includes a special drain valve that is designed to also make it easier for air to enter the tank while its accumulated water is being drained.

See EXPANSION TANK DRAIN & AIR RE-CHARGE for the procedure to drain and re-charge an atmospheric expansion tank on a heating boiler.

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.

Question: system pressure does not fall when I empty the expansion tank

(Nov 25, 2014) Robert said:
While the boiler was at 130F I turned off the incoming water supply and emptied the bladderless expansion tank again. Pressure did not drop. Closed the expansion tank drain. With water supply still off, ran the boiler through one cycle. Temp at 200F and pressure still at 30 psi and minimal water dripped from PR valve. Opened expansion tank drain valve and emptied tank again. Pressure still at 30 psi. Drained all three zones and boiler. No water coming in and no water in the system. Pressure reads 28 psi. Could it be as simple as a faulty pressure gauge at this point? If so, would the new gauge have to be installed with the system empty? Any other suggestions?

Added information to last post 4 hours ago...Boiler is turned off with water supply off. Left boiler drain valve, zone drain valves and expansion tank drain valve open since last post. Pressure gauge has dropped to 20 psi. Is it possible that there is an air lock in the system that is slowly releasing? Or something else?

Update post from last post 12 hours ago... With the boiler still off, all valves open and system empty the pressure gauge indicates 12 psi now. ?


If the boiler is cool and pressure is stable at 12 psi that's normal. If pressure drops there's a leak out; if pressure increases there's a leak into the boiler or a bad water feeder or pressure reducer. Some more subtle relationships between system pressure and the compression tank (expansion tank) are discussed at EXPANSION TANK WATERLOGGING EXPLAINED, most of which are more likely to occur with a compression tank that uses an internal bladder - which yours does not.

Question: Utica Starfire boiler pressure readings very high

(Jan 24, 2015) Les B said:
I am a little concerned.I have decided to check out my gauges on my Utica Starfire Boiler, I was shocked to find the pressure reading was pinned. Could this be correct? The relief valve isn't leaking nor do I see any other leaks. I have fairly high water pressure in the house, is it bypassing and going to the boiler? The expansion tank sounds full on the top side and hollow on the bottom, as I have read is normal. Any thoughts?



If the boiler pressure were "pinned" at a number over say 30 psi, and if that were actually the pressure in the boiler, you'd figure that the temperature/pressure relief valve would be spilling.

But indeed you don't want a water logged expansion tank.

Watch out: if the pressure is confirmed to really be over 30 psi and your TPR valve is NOT spilling then the system is very dangerous and should be turned off immediately - from a safe remote emergency-off switch.

Question: how do I get air out of my baseboards?

(Jan 30, 2015) Carl said:
How do I get the air out of the hot water baseboard lines in my house. It sounds like a raging river going through the lines. Tried bleeding the top floor bleeders and made it worse. What do I look for? What do I do?



Procedures for bleeding out air from an air-bound heating system begin at


There are several approaches including using a pump at the boiler if bleeders don't work.

Expansion Tank / Compression Tank Location Recommendations

LARGER VIEW of a heating boiler expansion tank

Question: want to install an Amtrol EX-30 expansion tank on cooler return side of the boiler

(May 8, 2016) Bruce Benner said:

I need to install an Amtrol EX-30 expansion tank on a boiler that has the circulating pump on the return side of the boiler. I am confused as to where it should be installed?

I would think it should go on the supply side with the hotter water and thus more thermal expansion. Any advice would be appreciated.



In a water piping system filled with water, excluding surges when valves open or close, water pressure is pretty much the same throughout the system. But a look at Amtrol's Extrol hydronic expansion tank shows this instruction from the manufacturer:

"Install the EXTROL on the supply side of the boiler, on the suction-side of the heating circulator(s) (Figure 2)."

That means the Amtrol Extrol tank is typically installed on the hot side of the boiler, close to the boiler, on the hot water supply pipe ahead of the circulator pump(s).

In the live in this article series (or by searching for this captialized phrase) see the article titled AMTROL's EXTROL®, the FILL-TROL® where you can find the manufacturer's installation instructions as PDF files in the article's REFERENCES section. There and in our article itself you'll see that typically the expansion tank is on the supply side.

Here is a direct link to Amtrol's installation instructions:

So before moving the expansion tank to the return side I'd contact the company for advice. Keep in mind that the company offers a variety of expansion tank designs including a floor-standing model might let you locate the expansion tank where you have room, but piped to the supply side of the hot water piping loop.

Repairs to Leaks in Expansion Tanks on Hydronic Heating Systems

Please see EXPANSION TANK LEAK REPAIRS where we discuss diagnosing and repairing leaks in boiler expansion tanks / compression tanks as well as relief valve leaks traced to problems with the expansion tank.

Relief Valve Leaks Traced to Waterlogged Expansion Tanks


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: can't find the expansion tank for my heating boiler

(Jan 5, 2016) Jim said:
Just moved into an old house and the only pressure tank I can find is the one next to the well pump. Is it normal to not have an expansion tank or (obvious) relief valve anywhere?


Thanks for asking, Jim.

It may depend on where you live and local custom, but most of the private homes I've inspected over the last 40 years did NOT have a hot water system expansion tank. More likely it's a component people add when they're having trouble with the hot water system.

If your home heating system is by hot water, there will be an expansion tank protecting the boiler.

See FIND the EXPANSION TANK for help finding the expansion tank when you don't see one at the boiler itself

(Jan 6, 2016) Jim said:
The hot water heater is oil-fired and not used to heat the house. Maybe that's why there isn't one. The hot water takes a minute to reach the fixtures (especially upstairs), but the water is hot and pressure is not an issue.

(mod) said:
Thanks for the added detail; indeed, as I opined, most residential systems for heating hot water don't have an expansion tank. If you are not having a problem with leaks at the TP valve on the water heater you're probably OK.

Search for ATTIC EXPANSION TANKS to see how older heating system hot water expansion was handled.

Question: ruptured bladder in Amtrol AX-240V expansion tank - temporary repairs

(Sept 24, 2016) Dan said:
existing heating system has an Amtrol AX-240V expansion tank. Discovered it has ruptured bladder. New tank is three weeks away. What if anything can I do to operate boilers until I can replace tank


You may need to add an expansion tank elsewhere on the system. It should be possible to pipe a conventional or alternative expansion tank right on the existing piping as long as it's sized correctly.

Watch out: without an expansion tank the increase in system pressure from just normal operation can cause the relief valve to spill. If this happens the siguation can become dangerous, as mineral deposits and crud at the relief valve, brought by the spillage or discharge, can ultimately cause the valve to stop opening at high pressure - and that in turn can lead to a dangerous BLEVE explosion. Search for BLEVE EXPLOSION to read more.

You might also try lowering the HI limit on the boiler aquastat.


(Sept 25, 2016) Dan said:

We have two boilers (50 HP each) that supply the facilities heating including hot water through indirect tanks. I have decreased one of the boilers temperature to minimum. We have just began having call for in floor heat so demand on boilers is low. typically at night in-floor heat pumps will operate. Daytime temperatures in Nova Scotia still reaching 20 degrees Celsius 68 F.When you say add an expansion tank can you elaborate as the one specified is not readily available? One question in regard to proper placement of Back flow preventer. The literature I have read is that it should be on supply side of Pressure regulator. Seeing ours was installed down stream from Pressure regulator would this be an issue in maintaining boiler pressure


I would absolutely trust the manufacture's literature over urban legend. It's always safest to give the manufacturer's technical support line a call to ask directly.

Question: boiler pressure keeps going too high and spilling the relief valve

Josh said:
My boiler pressure keeps going too high and leaking through the pressure relief valve, i changed pressure relief valve ,water feeder and bled the heater lines still nothing can it be the expansion tank


Check for

a defective water pressure reducer/feeder or wrong water feeder pressure settings

abnormal water pressure at the building side

OR - here's a cool one: your boiler could have a tankless coil that's leaking house hot water INTO the boiler.

Let me know.

Question: how to choose the right size of expansion tank

Steve said:
Expansion tank sizing: In a very large old home, there is a huge boiler(size of a small car) and an old style expansion tank in the rafters. The tank recently started leaking, so I'd like to replace it with the newer bladder type. I understand for sizing calculations, you need to take the system volume, temperature differential, etc. into account. But with a house so large with dozens of radiators & seemingly miles of piping, I can't begin to imagine how you could calculate the total volume. Isn't there a way to make this determination using the size of the old expansion tank? Its 12" x 60", so about 30 gallons. Thanks for the help.


Steve, please see complete details at EXPANSION TANK SIZING GUIDE


Continue reading at EXPANSION TANK LEAK REPAIRS where we discuss diagnosing and repairing leaks in boiler expansion tanks / compression tanks as well as relief valve leaks traced to problems with the expansion tank, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see this

Article Series Contents

Suggested citation for this web page

EXPANSION TANK DIAGNOSTIC FAQs at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman