Photograph of an attic expansion tank for a heating boilerAttic Expansion Tanks
Identify, troubleshoot & repair hot water heating expansion tanks & header tanks found in attics of older homes

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Attic expansion tanks or header tanks for hot water heating systems:

This article describes open or atmospheric-pressure expansion tanks sometimes found in the attics or on upper floors of older homes. Where an attic expansion tank is installed the boiler may depend on the expansion tank and its overflow for safe pressure release. Such systems are less safe than modern hot water heating systems that include a pressure and temperature relief valve right at the heating boiler.

We discuss how to identify, inspect, repair or replace an attic expansion tank. We also discuss the risks of leaks at an attic expansion tank and we include suggestions for improving the safety of the heating boiler and piping to which these tanks are connected.

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Expansion Tanks & other Tanks Found in Attics or Upper Floors of Buildings

Photograph of an attic expansion tank for a heating boilerAttic Expansion Tanks for Heating Boilers & Other Storage Tanks Found in Attics

Attic expansion tanks and pressure relief systems for boilers: Don't confuse an old heating system attic-mounted expansion tank like the one shown here for a water tank storage tank or a range boiler.

[Click to enlarge any image]

The heating system expansion tank will be connected to the heating system radiators or basement boiler and may have a simple overflow pipe to permit excessive water (or system pressure) to spill outside.

Overflow from attic expansion tanks that are open to atmospheric pressure typically run outside to a roof gutter and drain system (risking freezing and blockage in some climates) or the tank may be connected to a building drain.

Some attic expansion tanks were intended to be drained manually if needed and may not be piped to a drain.

Use of the Sight Gauge on Attic Expansion Tanks

Photograph of an attic expansion tank for a heating boilerThe expansion tank shown at left includes a water level sight gauge that could be used by the heating service technician in adjusting the hydronic heating system pressure and water level.

Watch out: Systems which rely on a remote attic-mounted expansion tank are less safe since than a boiler that has a pressure and temperature relief valve mounted right on or at the boiler.

A concern with any pressure/safety device that is remote from the vessel that it protects is that the safety device could become blocked, clogged, or otherwise unsafe without anyone in the building noticing the trouble. The result can be a very dangerous explosion.

It's an easy fix, usually, to just add a relief valve on or at the boiler.

In fact we often find that even though an attic expansion tank may still be connected to the hot water heating system, a technician has already added a temperature/pressure relief valve at the boiler.

We have on occasion found that the whole boiler has been replaced with modern equipment but nobody noticed that an attic expansion tank was still connected to the system.

Header Tanks: Small Open Cisterns Used as Attic Expansion Tank for Hot Water Heating Systems

At below left is a small wooden box lined with galvanized metal found in the attic of an historic home in Vermont, in New England.

IAttic cistern (C) Daniel Friedman

This header tank was quite small, certainly not large enough to be used as a water supply cistern. Its piping was incomplete, making clear that it had been abandoned.

It is probable that this was used as a hot water heating system overflow tank in an older home. Follow the piping to understand the design and function a component like this. I observed this attic receptacle while inspecting the Justin Morrill Smith Homestead in South Stratford, Vermont in the U.S.

This small open water tank handled both thermal expansion and boiler feed water for a hot water heating boiler, serving as its overflow system.

It was described to me [DF] by the building curator as a cistern but is much too small to function as a water storage system. It appears to be receiving water as an overflow system - notice the arched raised pipe feeding into the box and the lower drain line.

In modern installations header tanks are usually covered, possibly by plastic or in newer installations the whole header tank and its cover may be made of fiberglass or plastic rather than tin lined wood as in my photo. But all header tanks will be open to atmospheric pressure.

How is a header tank different from a pressurised expansion tank

Expansion tank details (C) D Friedman

Our photo above illustrates an older-style thermal expansion tank serving a heating boiler. This is NOT a header tank. Closed, un-vented expansion tanks will be usually located quite close to the heating boiler, above it, typically in the ceiling. And those tanks are a closed, pressurised container with the sole job of accomodating expansion in the hot water heating system during heating cycles. Such tanks are always connected to a system protected by a pressure/temperature relief valve.

A header tank is usually open to atmospheric pressure and is also used to add fill water to the heating boiler when needed.

[Click to enlarge any image]

Header tank for hot water heating systems, adapted from Newark Copper Cylinder Co., U.K. see

In most heating discussions and sources a header tank is in essence an expansion tank (to accomodate expansion when the boiler and heating system are operating and hot).

Unlike a closed-system, pressureised "expansion tank", a header tank is open to atmospheric pressure (thus it has to be located on the highest floor above the heating system and its radiators and piping, and unlike a closed expansion tank the header tank is also often used to fill the heating boiler when makeup water is required. .

A header tank may be filled automagically when more water is needed, using a float-type valve shown in the Newark Copper Co. header tank that we have illustrated above.

Or the tank may be filled only manually. that's what that manually-operated ball valve is for.

Opening the ball valve, in a conventional header tank plumbing set-up, will feed water through the header/expansion tank and into the boiler. You should not normally have to add water to a hot water heating boiler but might need to do so after servicing.

You will see these pipes connected to a header tank:

We might find a header tank in the loft of an older building where there will also be a cold water storage tank. Header tanks are more-common in the U.K. than in the U.S. or Canada, leading me to ask if your system is located in the U.K.

Watch out: un-vented cylinders, in not properly installed and protected, are dangerous. Un-vented cylinders must be fitted by a certified installer. An improper installation of an un-vented cylinder risks a boiling liquid vapor explosion - described at BLEVE EXPLOSIONS

True cisterns are larger and can be read about at CISTERNS

Header Tank Suppliers, Sources, Literature

Modern replacement header tanks are costructed of copper, stainless steel, or plastic and are sold in sizes from 18L to 55 litres. The size of header tank you need depends on the total voluem of hot water in the combined heating system boiler, piping, and radiators.

Harwood Associates in the UK has a brief web article on header tanks. Web article:

There that company notes two warnings:

1. Watch out for modifications to the original piping layout - someone who doesn't understand the operation may have changed pipes incorrectly

2. Harwood add that on some hot water heating boiler systems using a header tank the cold feed and vent pipes may be combined as one or even just have a cold feed pipe. This only acceptable if the boiler manufacturer says so in their literature.

Leaks in Hot Water Heating System Attic Expansion Tanks

Attic expansion tank leaks (C) InspectApedia DHWatch out: an attic expansion tank that leaks can damage ceilings and living area below, can cause costly mold contamination in a building, and might ultimately lead to loss of heat in the building's upper floors.

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Reader Question: attic expansion tank found leaking

I've been scouring the Internet trying to get a little knowledge before a handyman comes over here. I am a 65-year-old woman whose husband who could do everything had a stroke 9 years ago.

We live in a 90 year old house with radiators, and oil burner and an expansion tank in the attic. I noticed a wet spot on our ceiling upstairs and I've been in the attic (ugh!) a couple of times trying to see where it's coming from.

My husband kept trying to tell me the bottom of the tank, but I could see the wood around the pipe going out the roof was damp (and only there was damp) so I told him that's where it was and not at the bottom.

He wasn't even up there to see anything!

Attic expansion tank inspection for leaks (C) Hein Attic expansion tank inspection for leaks (C) Hein

Well, I put an old towel and a bucket under the damp roof section and figured it was ok until we could get a roofer here. But the wet spot on the upstairs ceiling got wetter rather than dryer. I just went up there and really looked around and it IS leaking at the bottom of the tank! What are the chances?

Photo at above left: top of the attic expansion tank. Photo at above right: leaks at the bottom of the attic expansion tank.

It's just a bead of water hanging off a rusty old nut or something that's at the bottom of like two tubes about a foot long running down the side of the tank.

I was looking at that nut and thinking something tells me this isn't going to be as easy as taking off that nut and putting on a new one. Is it? I love my old beautiful house my husband rebuilt room by room over 25 years before he had his stroke. I can't bear to have anything in it damaged. But we also don't have the money for a whole new heating system. And I LOVE the radiators.

Can you give me any direction at all? I have a roofer who knows "a little bit about everything but is no expert" coming tomorrow to do something about the roof and look in the attic for me. I didn't even read your whole article. Only enough to know you were talking about exactly what we have in our attic.

Please tell me -- if you know what I'm talking about, can something like that just be fixed or does this sound like a whole new expansion tank? And is that even possible? Is it expensive? Are we talking about a whole new heating system? If you wouldn't mind calling me, I would be so appreciative. My number is 845-561-8051. I couldn't believe it when I saw you are in Poughkeepsie! We live in New Windsor. What are the chances?  - D.H.., Poughkeepsie, New York 12/3/2014


Chances are you by no means have to replace the whole heating system just because the tanks leaking but there are some safety questions about pressure relief valves and the actual cause of the leaking that we will want to discuss.

I can see some helpful things about your boiler thanks to the photos you've sent along.

  1. The boiler already has a temperature/pressure relief valve installed right at the boiler itself. That means that the attic expansion tank - if it turns out to be what we think it is - can be abandoned completely if necessary, its piping capped off in the attic floor, and a modern Amtrol/Extrol type expansion tank can be installed at an appropriate location on the hot water heating piping at the boiler.
  2. The boiler bottom looks rusty - if this is a steel boiler it could be at or near end of life. IF it's cast iron and not currently leaking it may be OK.
  3. I see soot blow-out on the boiler covers around a plate used for a tankless coil (not installed on your boiler) - that suggests the boiler is leaking flue gases. Such leaks should be sealed as part of boiler servicing.

I'll say more when I've seen your photos of what's going on in the attic. An open question is why the expansion tank there is leaking:

Reader follow-up:

I'm just sending these to show where it leaked at the roof when I overflowed the system to fill it a couple weeks ago. So we have to fix that, too. But it's been raining all day and snow melted over the weekend and it didn't leak in the bucket I put up there at all.

I do know we need to get that fixed, too, though, because when I just went up there I could see a speck of daylight through the rafters there.

I also wanted you to see what the top of the tank looks like. Totally dry.

Thank you for your email about the boiler. We have a guy who services that every year who I absolutely LOVE. I totally trust his judgment about everything. But the last I talked to him I didn't realize it was leaking at the bottom, too, but I did ask if we needed it fixed or replaced if he did that.

He said, "I try not to." I have a list of questions for you and now even more after your email, but I'll let you look at these and then ask. Do you ever talk to people over the phone?

Leaky attic expansion tank (C) InspectApedia - HeinAs luck would have it, my hairdresser told me she has an awesome plumber, and she called him while I was there and asked him to help me if I called, and he said he would. Is this something any plumber can do? It's so old. Do you have to be familiar with old stuff like this?

I like the idea of just bypassing it and getting that other thing you talked about. About how much would a job like that cost -- time and material? Just roughly to the nearest hundred?


From your photos of the attic expansion tank it's clear that the tank has rust-perforated at its bottom - the main leak that is visible in the photo at above left.

The corrosion on the steel rods on either side of the sight glass suggest that the sight glass assembly has been leaking for quite some time as well, but at a slower rate.

Steps in Leaky Attic Expansion Tank Replacement

For safest heating system operation most likely your heating service technician will want to eliminate the attic expansion tank and install the appropriate expansion tank and safety equipment right at the heating boiler.

Watch out: Do not poke at the corrosion on this tank or it is likely to leak faster, causing more water damage to your home. And don't try to move the tank or disturb it before the heating system has been drained sufficiently to remove water from the expansion tank itself.

  1. Ask your heating service technician for his/her advice on how best to proceed in this repair. For best results you want the on-site person to accept responsibility for the repair work - so do not be over-directive even though we may include some suggestions here.
  2. I'd expect your heating service tech to temporarily shut down the heat and drain enough water to permit removal of this tank and to make necessary plumbing changes to permit closing off attic overflow piping to it.
  3. I'd expect your heating service tech to install a suitable expansion tank at the heating boiler
  4. The heating service tech may also inspect the temperature/pressure safety relief valve installed on your boiler or if necessary replace it to be sure that your system is properly protected.
  5. Ask the service technician to inspect the boiler for damage and leaks and to advise you of its condition. I noted signs that the boiler is rusted and possibly was exposed to flooding, and other signs of soot leaks at the boiler jacket or sides.
  6. Finally the service technician may see other heating boiler repairs that are needed for safe, reliable operation.

Let us know what you are told and let us know what other questions arise.

Reader follow-up:

I have no idea how that other address got in my address book. So a plumber was just here and I told him all your concerns about the boiler. He looked it all over real good and said it looked great. I asked about everything -- especially the soot. He said it looked like old soot stains but that everything looked really clean and it's a great system and he had one in another house he lived in and he would never change one like that if he had one again. He looked it all over real good inside and out. It's cast iron.

He's going to cap off the expansion tank upstairs at the pipes going to it in the basement and take the pipe out of the roof. Then we'll get that hole repaired for good. And he's going to just leave the old tank up there.

And he said there isn't enough damage up there that we would ever have to worry about mold or termites or carpenter ants. He said he can tell it hasn't been leaking that long and it really isn't that wet. It will dry up fast once the leak is taken care of.

And he's getting a new expansion tank and hooking it right to the boiler in the basement. I don't know what it will cost yet, but my friend said he doesn't rip people off. And we have to do it, so whatever it is, it is.

So he agreed with everything you said. :) And I felt very confident in him and was relieved that our furnace service guy has been doing a good job, because I like him so much. I gave him the opportunity to do the job, but told him somebody told me about a really good plumber yesterday if he doesn't want it. He wasn't offended at all and said, yes, call him, because he doesn't like doing those kinds of jobs if he doesn't have to. He confirmed everything you told me, too.

I can't thank you enough for taking the time to give me your expert advice. - D.H., Poughkeepsie NY 12/4/2014

Other Attic Tanks Mistaken for Heating System Expansion Tanks

Attic tank range boiler (C) D Friedman C Jones The image shown at left is also an insulated tank found in the attic of a California U.S. Home - this is not an expansion tank. Expansion tanks in attics are not usually insulated. This tank was. Read about it

In identifying old steel tanks found in building basements and attics, also see


Continue reading at EXPANSION TANKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see SOLAR WATER HEATER ANTIQUE - solar water heaters found in attics of some older homes

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ATTIC EXPANSION TANKS, HEATING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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