Bladderless expansion tank on gas fired heating  boiler (C) HHHeating Boiler Expansion Tank Leaks
Diagnose & fix air or water leaks at hydronic heating system expansion tanks

  • EXPANSION TANK LEAK REPAIRS- CONTENTS: questions & answers help troubleshoot air leaks or water leak 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)
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Hot water heating system expansion tank / compression tank leak diagnosis & repairs:

Questions & answers about diagnosing leaky expansion tanks on hot water (hydronic) heating systems, leaks at relief valves caused by expansion tank problems, air leaks at expansion tanks, and leaks in the internal bladder of bladder-type expansin tanks.

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]

Article Contents

Diagnosis & Repair for Leaks in Expansion Tanks on Hydronic Heating Systems

Question: expansion tank leak repair with epoxy?

(Mar 29, 2015) Sam said:
Hi there! I've learned a lot about expansion tanks from this site, so thank you very much for that. I'm encountering a problem with mine, though, and I can't seem to find anyone else that's experiencing the same thing, so I want to ask here.

Our expansion tank has a leak and is dripping very small amounts of water onto the floor. It's not a lot, and it only drips when the heat is running (or rather, it only drips a noticeable amount when the heat is on), and is little enough that I haven't put much thought or effort into what's happening until now.

The problem I'm encountering though, is that the leak is not at any connection point--it's not where the air goes in near the bottom of the tank, and it's not at the top where the tank threads into the pipe. It's just on the side of the tank. It's easy to spot because there's a line of rust going down the side of the tank from the spot.

I had read that a hole could be patched with the proper epoxy (I bought some JB waterweld), and I cleared away the rust and found, what appears to be, a perfect circle of paint missing (about the size of a pencil eraser), that will very slowly fill with water, and once there's enough water, it drips down the side. I'm now hesitant to patch this up, since it looks like it was a design of the tank to have it leak in this spot.

So I'm curious--are any expansion tanks designed with a spot to let out a small amount of water? If not, is this a spot I could just cover with epoxy and be good to go? If so, does that leak mean it needs to be replaced? I'd much prefer to just epoxy it :). I'd attach a picture if I could, but I just want to emphasize that it's just a random spot on the side of the tank (the upper part, where the water is), and not at any connection point. Everything else about my system seems fine--no valve leaks or issues, pressure and heat are normal, and the tank itself still has air and when tapped the uppper part sounds different than the bottom part.

Thanks for any info!



There are special drain valves installed on some bladderless expansion tanks that incorporate an air inlet to make draining the tank easier. But there are not systems intended to only drain "a small amount of water" - given the trouble and cost of the usual way that chore is performed, by a heating service tech, the tech drains the entire tank down to empty.

YOur tank sounds as if it's ready for replacement- leak signs of water leaking out mean end of lilfe.

Question: Amtrol Extrol #30 expansion tank has pinhole leak; should I adjust the tank pre-charge pressure?

LARGER VIEW of a heating boiler expansion tank2016/06/06 Charlie said:

Hello, I found a pin hole leak in my Model #30 Amtrol Extrol Expansion tank. I have shut the system down and have ordered a replacement tank. I have read and seen all over the web that you must determine the system pressure and adjust the tank pressure to match the system pressure. If the pressure is 30#, then the tank pressure should be increased to 30#. The system is a forced hot water system. I have ordered a pressure gauge to check the inlet pressure, but am I wasting my time and money? Lots of confusing info out there. And lastly, do I need to purge the system of air once the new tank is installed?

Thanks so much. -Charlie

Reply: No.

Charlie I think the confusion is caused by readers and writers who mix up expansion tanks on hot water (hydronic) home heating systems (boilers) with the pressure tank used to regulate water pressure on a pump and well system.

On a hot water heating system the (much smaller) expansion tank that uses an internal bladder, such as the Amtrol Extrol in-line (smaller) expansion tanks in series EX-15 through EX 90 comes with a 12-psi factory pre-charge that will be fine for just about any residential home heating system except where the boiler is heating a taller building.

At initial installation of the pressure tank the heating tech, following Amtrol's instructions, sets the Extrol pre-charge psi to the same as the "fill pressure" of the boiler.

The factory pre-set pressure (12 psi) assumes the tank is being used on a hot water heating system whose starting or fill pressure (controlled by the pressure-reducer water-feed valve) is about 12 psi cold. Typically a residential hydronic heating boiler operates between 12 psi cold and under 30 psi hot, with a TP relief valve that will open at 30 psi.

The "fill pressure" at the expansion tank needs to be amended only if the setting of the automatic pressure-reducer/water feed valve on a typical hydronic heating boiler is also different. For example, your heating installer *might* need to adjust the pre-charge pressure if your heating system operates at higher pressure ranges, such as in taller buildings.

See PRESSURE & TEMPERATURE SETTINGS, CONTROLS - for details about that situation. Amtrol also sells much larger floor-standing expansion tanks SX-30V through SX-150V for larger heating system applications.

In sum you DO NOT NORMALLY NEED TO RE-ADJUST the PRE CHARGE on your closed-system Amtrol-Extrol heating boiler expansion tank, and trying to do so in response to a leak in that tank won't work and may be unsafe. .

Watch out: Here is a direct quote from Amtrol's Extrol literature:

RUPTURE OR EXPLOSION HAZARD. Do not expose product to freezing temperatures or temperatures in excess of 240°F. Do not adjust the pre-charge or re-charge this Product except during installation or regular inspection.

Replace the Product and do not adjust the pre-charge if corroded, damaged or with diminished integrity. Adjustments to pre-charge must be done at ambient temperature only. Failure to properly size the Product or follow these instructions may result in excessive strain on the system and may lead to Product failure, serious or fatal personal injury, leakage, and/or property damage.

Watch out: As your heating boiler pressure tank is leaking it needs to be replaced, not adjusted. Else it's likely to fail to operate safely. The risk may seem subtle but it's serious: a heating boiler pressure tank that's waterlogged is likely to cause the relief valve to drip or discharge; that in turn, un-noticed and un-repaired can lead to dangerous BLEVE EXPLOSIONS.

Also see RELIEF VALVE LEAKS to understand the causes of TPR valve leaks and why a leaky TPR valve may stop leaking because it's clogged, leading to a nasty situation.


Completely different: on a water supply pump and well system using a building water pressure tank, the pressure tank is pre-charged to 2 psi below the water pump control switch CUT-IN pressure. E.g. on a 30/50 pump control switch the cut-in is at 30 so you set the **WATER TANK ** precharge to 28 psi. Building water supply system pressure tank adjustment (THIS IS NOT YOUR CASE) are discussed at WATER TANK AIR HOW MUCH TO ADD.


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: 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.

Boiler Relief Valve Leaks Caused by Waterlogged Expansion Tanks

Question: relief valve is leaking - and the expansion tank is full of water

(Oct 9, 2015) Samantha said:

I have a boiler that was leaking out of the pressure relief valve from the time I moved in and seemed to only get worse after I had it serviced and the guy doing the work replaced the valve. Since then I've learned more about the system and well the problem I've noticed is the expansion tank is full of water. Now that I've done some reading, I know this is the problem.

I also know that the tank was likely not serviced in a very long time and my question is whether the tank might be in working condition after just service where it's emptied and the air replaced, like I've seen instructions online, or if the tank should be replaced right away to fix the problem. Thank you for your help


Samantha at EXPANSION TANK WATERLOGGING CAUSES we describe what goes wrong with a conventional, bladderless exansion tank that leads to waterlogging.

This article EXPANSION TANK DRAIN & AIR RE-CHARGE explains how to drain the heating boiler expansion tank and get an air charge back into it.

Watch out: the advice about re-charging an expansion tank applies only to atmospheric or older-style heating system expansion tanks that do not use an internal bladder. If your heating system expansion tank uses an internal-bladder type design such as Amtrol's Extrol hydronic heating boiler expansion tanks, and that tank is leaking, it should be replaced.

Watch out: also keep in mind that there can be other causes of relief valve leaks including an aquastat setting that is overheating the boiler or a pressure reducer water feeder valve that is not working properly. And relief valve leaks are unsafe, risking burns, loss of heat, or even a BLEVE explosion. The various causes of relief valve or TPR valve leaks are explained at RELIEF VALVE LEAKS

Question: TPR valve leaks - is it the expansion tank

(Feb 16, 2015) ROBERT said:
Water dripping from the extension pipe. Is there a problem with the Expansion Tank?


A waterlogged expansion tank is just one of several conditions that can cause this unsafe condition.



Watch out: this is an unsafe condition. A leaky TPR valve, if it stops leaking, may no longer be protecting the system - ask for help from your heating service technician.

Question: PR valve leaked, drained expansion tank, system went right back up to 30 psi

(Dec 1, 2014) Samuel said:
I just had someone come take a look at my crown water boiler because it's leaking from the PR valve, he said my expansion tank was waterlogged so he let some water out of the tank , which reduced the pressure, then turned the heater on for about ten minutes, the pressure went up to 30 psi and then water came out of the PR valve again,he said I need to replace the expansion tank, is that true? If so is there a certain brand I have to use or can I use any tank with the correct psi I need for my home?x



The TP valve is indeed supposed to open at about 30 psi - so your system is at excessive pressure. It is possible that the expansion tank is waterlogged and can only be fixed by replacement IF

- the tank is one that uses an internal bladder and the bladder has burst
- the tank does not use an internal bladder but is damaged, say perforated - which you'd probably see as a leak on the tank itself.

There are other possible causes for excessive heating system pressure including a damaged or mis-set water feeder pressure reducer. If the boiler water pressure is excessive (say over 12 psi) when the system is COLD then there may be a feed problem. Just how much pressure is too much in a cold boiler? It depends on building height. Your home may need a bit more than 12 psi cold.

(Dec 1, 2014) Samuel said:
Is there any way to check if the bladder has burst or not, there is no perforations /no leaks on tank, and what does a water feeder pressure reducer look like and how can I check to see if its damaged or mis-set, my house is an upstairs,downstairs,and basement. THANK YOU FOR GETTING BBACK TO ME SO FAST!


What kind of tank is it, Samuel? Brand, model, size. Use our CONTACT US link to see an email for sending photos and I will comment further.

In the More Reading links above see WATER FEEDER VALVE HYDRONIC BOILER

Question: relief valve drips, temperature is below 200, I heard air entering the expansion tank

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.


If the system includes a tankless coil it may be leaking into the boiler

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.

Air Leaks at Expansion Tanks

Case 1: air leak in an atmospheric non-bladder type expansion tank

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

Older atmospheric pressure tanks used at heating boilers may corrode through and leak or may develop leaks at piping connections. A leak in the bottom of the tank or its piping typically shows up as a water drip or as build-up of mineral salts at the point of leakage. A leak in the upper portion of the tank, while rare, can occur, causing slow loss of air out of the expansion tank and thus a water-logged pressure tank. Since atmospheric pressure tanks (boiler compression tanks) also become waterlogged by air absorption into the heating water over the course of the heating season, it can be difficult to determine if the expansion tank is waterlogged due to a hidden air leak or due to normal waterlogging.

To diagnose this condition, with the expansion tank isolated from the heating boiler (by closing the valve that allows boiler water to enter the tank, pressurize the tank to 20 psi, then if there is no other fitting to which a pressure gauge is already attached or can be attached, attach an water pressure gauge to the tank's drain valve. Monitor the tank for a drop in pressure. A pressure drop over 24 hours suggests that the tank is leaking. If you don't see water leaking at the tank it's probably an air leak.

Measuring static and dynamic water pressure (C) Daniel Friedman

See WATER PRESSURE MEASUREMENT for advice on how to buy or make a portable water pressure gauge that will attach to the tank drain.

It may be possible to make a temporary repair of a leak in an atmospheric expansion tank, but the tank needs to be replaced.

Case 2: air leak in an internal bladder type expansion tank

LARGER VIEW of a heating boiler expansion tank

An air leak in the internal bladder of a bladder-type hydronic (heating boiler) expansion tank may never show up as water outside the tank itself. Rather the tank gains water into what was supposed to be its air space, ultimately causing dripping or leaks at the boiler relief valve during each heat-on cycle.

To diagnos this condition, if the expansion tank has an air adjustment port, with the boiler off and cold and at its starting (typically 12 psi) pressure, remove its cap and press briefly on the center pin of the schrader valve or air valve.

If the air valve is on the upper end of the expansion tank, and if water squirts out the expansion tank then the bladder isleaking and the tank needs replacement. However if the tank is only partly waterlogged this test may be ambiguous as just air may exit (until the tank is fully waterlogged). You could try allowing all of the air out of the expansion tank while looking for water discharge at the end of that step, but even then, if the leaked-in water is residing only at the bottom of the tank (in the space betweeen bladder and tank walls) nothing but air may come out of the valve. If the tank feels abnormally heavy it may be waterlogged.

If the air valve is on the bottom of the expansion tank and if water squirts out of the tank's air valve, then the tank bladder is leaking and the expansion tank needs to be replaced.

Case 3: air leak through the air valve at the expansion / compression tank

On occasion a defective Schrader valve or debris on the valve seat (because you were fooling with the valve, right?) can cause the air valve on an expansion tank to leak. The result is loss of the tank's air pre-charge and a waterlogged expansion tank.

To diagnose this condition and repair it, you'll re-pressurize the tank to its proper level and monitor the air pressure in the tank.


Case 4: air leak through the bladder of an internal bladder expansion tank

As NJT documents below, it may be possible that over a long period of time the air charge in the bladder type expansion tank may fall slowly as air permeates the bladder wall itself - without a pinhole or tear type leak. The result will be a slow drop in the expansion tank's air charge.

Note that I have not seen this effect documented in the manufacturers' literature. If you suspect this condition, with the boiler off and cold and at its starting (typically 12 psi) pressure, return the expansion tank's pre-charge to that same pressure.

Question: reader points out air leaks through membrane of internal-bladder-type exansion tanks

18 March 2015 NJT said:
You said the following on this page and I do not agree:

"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."

The newer bladder or diaphragm tanks DO need regular service, no more than bi-annually. This is because this type of tank will lose 1-2 PSI of air per year from air molecules migrating through the rubber, much as a childs toy balloon will slowly deflate.

Manufacturers will even admit this, but you'll have to search for this information from them.

This type of tank will last a LONG time if properly installed and maintained.

NEGLECTING to maintain the charge on a bladder or diaphragm type tank will cause premature failure of the diaphragm or bladder by stretching it all out of shape inside the tank.


You are quite correct that an internal bladder type expansion tank should not need frequent service - unless the tank has ruptured. I'm interested in the air migration through rubber citation and would be very grateful if you could find a citation for us. I've found a few weak references, none citing air loss through the rubber of the diaphraghm.

riteboiler dot com ". Gauge glass seals and valve packings on compression tanks may also be a slow escape path for air and eventually cause the tank to lose its air cushion. System connection to a compression tank should always be with a B&G Airtrol Fitting which acts like a trap to help keep air in the tank."

Taco doesn't mention it.

19 March 2015 NJT said:

My personal experience and testing. I have observed over the years the loss of a few PSI per year in general. Some more, some less, but always a loss. I have NEVER observed a tank that was still holding the same amount of air it started with years before. Same observation on well tanks.

I have seen manufacturers references to same over the years and I'll try to find again.

By the way Dan, I really want to thank you for an EXCELLENT resource! You have put a LOT of work into this, and it's VERY rare for me to feel the need to contradict anything I've read here. Bravo!

p.s. I also believe that some of the air loss may be through the Schrader valve. I've found this a number of times with the 'spit test'.

Also, in addition to toy balloons, motor vehicle tires probably lose air to the same process of migration through the rubber (and probably Schrader valves also)

This fellow calls it 'oxygen diffusion', I presume he means what I was calling 'migration'.



We have had confirmed reports of pinhole leaks in the internal bladder of pressure tanks of various uses, in heating system expansion tanks, hot water system expansion tanks and water supply system expansion tanks. A pinhole leak is very difficult to spot itself but is detected by symptoms: a slow loss of air charge *and* the accumulation of water in the air space of the tank where no water belongs. This phenomenon may be mistaken for gas migration through the rubber of an expansion tank.

I am researching further for gas permeability of rubber membranes used in expansion tanks - which would be mimicing an RO system - and which I have not yet been able to confirm with solid research. The author of the article you cite is a smart contractor but not a permeability expert. We consider his views with care and respect but not as a final authority. More work is needed and will be reported here.

Thank you for bringing up this interesting possibility.

We have added additional research citations
at REFERENCES and have clarified the text in our article



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