Hot water heating system expansion tank / compression tank drain procedure to re-charge lost air:

This article describes the steps in draining of a steel compression or expansion tank used on hot water system heating boilers - hydronic heat. Draining water out of a waterlogged expansion tank is a common heating boiler service procedure used to replace a lost air charge.

If this step is not taken when needed the waterlogged compression / expansion tank will cause boiler leaks and improper, even unsafe boiler operation.

In this article series we provide a heating system expansion tank / compression tank Troubleshooting & Repair Guide that will address just about any problem traced to this heating system component.

How to Drain & Re-Charge the Air Needed in a Heating Boiler Expansion Tank, step by step

Periodic draining of non-bladder boiler expansion tanks:

Older heating system expansion tanks such as the one shown in this photo need periodic service: because air in the expansion tank can become absorbed into the heating water over time, eventually the expansion tank can become waterlogged.

Watch out: your expansion tank may not need this service procedure: Newer internal-bladder type expansion tanks don't need draining but might need an additional air charge, as we explain further

at EXPANSION TANK DIAGNOSIS

As we detail below in another photo, the green arrow indicates the expansion tank drain valve and the blue arrow marks the isolating valve that gets shut off first.

So if no one has ever drained the expansion tank, or not for years, chances are it's ready and waiting for that attention. Or as we've explained above, it's past due for draining.

How to determine when the expansion tank needs drainage

A traditional rule of thumb practiced by old heating service technicians (including me - DF) was simply to always drain all of the water out of the bladderless-expansion tank at every annual service.

If a call-back customer complaint led to a need to drain the expansion / compression tank more often than once a year we figured something else was wrong and needed to be diagnosed and fixed. At that point we'd check to see if the expansion tank were waterlogged.

If not we'd look for a different problem such

as LEAKS into the BOILER CAUSE RELIEF VALVE LEAKS

If the heating boiler expansion tank is heavy (try pushing it up or tapping on it) or if the relief valve is leaking, we probably need to drain the tank and let air return to it.

Watch out: don't go pushing the expansion tank all over the place. Just a tiny test push is sufficient. If you wiggle the tank around too much you're asking for a pipe leak.

Also some expansion tanks are strapped tightly against the ceiling so you can't move them anyway.

Watch out: modern internal-bladder type expansion tanks such

as AMTROL's EXTROLl®, the FILL-TROL® Diaphragm-Type Expansion Tank do not need periodic draining - those tanks use an internal bladder. If your Amtrol expansion tank is waterlogged there is a problem to correct.

It might be a slow loss of the tank's air charge over several years, fixed simply by adding air.

IF the tank has a pinhole leak or a if your bladder-type expansion tank has suffered ruptured bladder, the tank probably needs replacement.

See EXPANSION TANK DIAGNOSIS if you missed that step
or

see EXPANSION TANK PRESSURE ADJUSTMENT if you need to adjust the pressure in a bladder-type expansion tank.

How to Drain that Heating Boiler Expansion Tank

Many expansion tanks use a special drain valve that permits air to flow into the tank as water is drained out. But some don't.

Here we give a step by step procedure for draining a traditional (bladderless) boiler expansion tank in order to then get the proper air charge into the tank.

Photo: we used a pony pump to force-drain an expansion tank that wouldn't drain on its own. You should not need to use such heroic procedures if your expansion tank has the proper drain fitting.

Really? What if no water comes out of the drain but the tank is full? If the heating boiler's expansion tank won't drain when you open the valve

see the temporary and permanent solutions at

EXPANSION TANK WON'T DRAIN

Watch out: Without the proper air charge in the expansion tank the boiler relief valve will leak and the heating system may not operate properly nor safely.

• Turn off the heating boiler 
  
  at its electrical service switch.
• Identify the expansion tank feed pipe.
  
  There will usually be just one pipe leading to the expansion tank.
  
  That pipe will be found coming from the boiler or perhaps a nearby heating water pipe to the tank to permit the tank to accept the expansion of water in the heating system (or expanding air entrained in the heating system's water) as the boiler gets hot. In our image at left, the red arrow points to the feed pipe.
• Turn off the isolating valve that should be located on that hot water feed pipe 
  
  between the expansion tank and the boiler or heating line.
  
  This temporarily isolates the expansion tank from the rest of the hot water heating system. If you don't do this and you start draining the expansion tank you may end up draining water from your heating piping too - not what you want.
  
  We only want to drain the water that is inside the expansion tank. In our photo at left the blue arrow points to the isolating valve.
  
  Technical note: an isolating valve often is omitted if the expansion tank is an internal bladder Extrol type tank since those don't need to be drained.

• Connect a garden hose to the expansion tank drain valve 
  
  and position the hose so that it drains into a floor drain or convenient slop sink. In our photo the green arrow points to the expansion tank drain valve. [Click any image to see an enlarged, detailed version.]
• Open the expansion tank drain valve 
  
  to allow
  • water from the expansion tank to drain out
  • air from the room to enter the expansion tank

Watch out: this hose idea is convenient and is recommended by some folks who lack experience with expansion tanks.

But the hose only works if a special air-admittance valve is installed on your expansion tank.

If your expansion tank just has a regular boiler drain valve installed, the hose thing will be very difficult to work - in that case plan some trips with a bucket held under the opened tank drain valve instead.

Watch out: using a pony pump to speed drain of a heating boiler itself can damage a bladder-type expansion tank.

Those details are

at BOILER DRAIN DAMAGES EXPANSION TANK

The expansion tanks we show with our arrows in the photos above had a simple boiler drain. We open the drain and let it spill into a bucket. That lets air bubble back up into the expansion tank - something that would take forever if we had a hose hooked up here. This can be a tedious process.

Some expansion tank drain valves include an extra feature, an air inlet that allows air to enter the expansion tank through an air inlet on the drain valve at the same time that water is draining out of the expansion tank.

Watch out, the expansion tank water could be hot, and is usually a bit smelly and dirty.

• Let all of the water out of the expansion tank.
  
  You could get away with a bit less than all, but all is good. This lets us start with an expansion tank full of air. Don't worry, water from the heating system will be back in there in a jiffy.
• Close the expansion tank drain valve.
• Open the isolating valve between the expansion tank and the heating boiler 
  
  or heating boiler piping (photo below left). You will doubtless hear some water entering the expansion tank.
  
  That's because the water in the heating system is sitting there somewhere between 12 and 28 psi and your expansion tank was full of air at "0" psi. When water stops running into the expansion tank, don't worry, it won't be waterlogged again.
  
  Rather it will contain some water, and some air, both at whatever pressure is that of your heating system at the moment.

Our photo above shows an expansion tank isolating valve. Below it and to its left you can see the top of a B&G water feed valve that sends water into the heating boiler.

• Check for leaks.
  
  Make sure that the isolating valve (that no one had touched for a few years until you got to it) and the expansion tank drain valve (that no one has touched for a few years before now) are not dripping.
  
  If you see a tiny drip around the valve stem of either valve, you can usually stop that by gently tightening that large packing nut that surrounds the valve stem. GENTLY. Don't over-tighten nor break anything.
• Turn the boiler back on.
• Check for leaks one last time 
  
  when the boiler has heated up and the system is at full operating pressure. You should be ok.

Below: The boiler drain valve on an oil fired heating boiler is located low on the boiler at the bottom of the boiler's water jacket.

 

Can't Find the Expansion Tank?

This article section has been moved

to FIND the EXPANSION TANK

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-03-29 by InspectApedia Editor

@Eagle Eyes,

Thank you so much for the very nice note. We've worked hard on this material for many years so we are especially grateful when readers find it useful and trusted. We also welcome any questions or content suggestions that you may have.

On 2023-03-29 2 by Eagle Eyes

Thank you for this article. The solution might have been very simple, but without this knowledge things can go horribly wrong. I've used my boiler for twenty years without bleeding the expansion tank, not knowing the type of tank and that it should be bled.

This past week I noticed the pressure above 30psi with pressure valve dripping and temperature was literally off the charts. By following your advice everything is back to normal and has remained normal. Thanks again for your article.

On 2022-12-25 by InspectApedia (Editor) - could I empty the expansion tank when the furnace is operating

@anthony,

No, it's best to turn off the heater, let it cool down (to avoid risking a scalding burn), then follow the directions on the page above.

Watch out: some boilers, such as some older cast-iron models, can be seriously damaged, even cracked, if you send a significant volume of quite cold water into a quite hot boiler.

That's another reason to shut off the system and let it cool down - warm is OK, quite hot is not.

If you're worried that the boiler may still be too hot, then, after draining the expansion tank, as the water supply is opened to allow water back into the boiler (some water will enter as the air in the now-empty expansion tank is now being brought up to system operating pressure), open the supply valve just a crack to feed water slowly.

On 2022-12-25 by anthony

not sure what I read could I empty the expansion tank when the furnace is operating

On 2022-12-19 by InspectApedia (Editor) - header tank is overflowing

@Anonymous, Nigel:

Thanks for the photo. It's one of the marvels of internet that we find systems that are foreign to us. What you show is not something familiar to me except on VERY old hot water heating systems that used an attic expansion tank that overflowed and drained to the building exterior.

I'd need to know more about your heating system: type, components, age, brand, model.

I'm left guessing that this is an open-type hydronic (hot water) expansion tank that is located in the top of a building and that is an alternative to the newer closed tank or bladder type expansion tanks used on newer heating systems.

If I'm on the right track, and if your header tank is overflowing, and if we're really talking about a heating system expansion tank and not something else, then it's possible that your automatic feeder or manual water feeder to the boiler is over-feeding the system.

On 2022-12-19 by Nigel

@InspectApedia (Editor), i mean if the header tank is full surely water should be gushing out the overflow pipe and then outside the side of the house rather than filling up in my loft. Bare in mind the ball valve apparently needed replacing which it now as, i just wanted to make sure if it filled up again the overflow pipe drained it before it filled up in my loft

On 2022-12-19 by InspectApedia (Editor)

@Nigel,

I'm unsure what you mean by your "overflow" but

if you are referring to the pressure/temperature relief valve on hour heating boiler, as long as the boiler pressure stayed below 30 psi the TP Valve wouldn't be pushed open. So leaks at the expansion tank might have simply been enough to keep the pressure below the relief valve open point.

On 2022-12-19 by Nigel

My header tank was full to the brim and leaking in my loft can anyone tell me why my overflow didnt kick in ?

On 2022-12-03 by InspectApedia (Editor) - pressure reducing valve is not working

@maurice j schranz,

If your heating system expansion tank is a type that uses an internal bladder you should not need to adjust its air charge. I'm puzzled by your statement that you need 40 psi in that tank.

The water pressure in a residential heating boiler typically is around 12 psi cold and up to somewhere under 30 psi hot. So 35 or 40 psi sounds way too high.

Post a photo of your expansion tank and we can comment further.

On 2022-12-03 by maurice j schranz

My pressure reducing valve is not working. I'm going to replace it as my water psi has been jumping from 40 to 120 psi. My T/P is not leaking. My question is that I have an expansion tank with a bladder ahead of the water heater.

I drained the water heater and tried to put air into my tank with a hand pump because it was 35 psi and I need it to be 40 psi. It hangs down from the water line but I can not pump any air into it. Air does come out out it when I check the pressure. Should I remove it and try pumping it on the floor.

I see no drain for the tank if it was water logged.

On 2022-08-23 by InspectApedia-911 (mod)

@Scott,

Thank you so much for your very nice note. It means a lot that you took the time and trouble to comment as you did, and of course I'm very happy that our information proved helpful to you.

Indeed we have worked hard for over twenty years to make information at InspectAPedia.com accurate, in-depth, and without bias, so I am of course very grateful when a reader reports that our website has been useful.

To that end, we appreciate hearing any comments, critique, suggestions, or further questions that you may have about any of our diagnosis/repair articles.

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On 2022-08-23 by Scott

This explanation/instructions are A+. I have an older boiler that works well, but has the older style expansion tank. I scoured the internet looking for solutions and came up with nothing… until now. I emptied my tank and the boiler and expansion tank seem to be working perfectly again.

I’ll be emptying the tank annually now just to get ready for heating season. Thank again. Great content!

On 2021-09-16 by inspectapedia.com.moderator - will the tank be pressurized after the tank fills with water

@Jack,

Thanks for a helpful expansion tank question.

Provided that the pressure/reducer water-feeder valve is on and working properly, after draining the expansion tank and allowing it to fill with air,

when you open the water supply to the boiler the water feeder will re-fill the system to the proper starting pressure (e.g. 12 psi cold) and the tank will thus also be pressurised to 12 psi.

The volume of water in the tank may be less than half the volume of the expansion tank; but that's not a worry.

Note, too, that these instructions are for a boiler expansion tank that does NOT use an internal bladder. Modern expansion tanks such as the Extrol use an internal bladder and do not need to be emptied. If you find that a bladder-type expansion tank is waterlogged then its bladder is leaking and the bladder, or the entire tank, needs to be replaced.

Thank you also for your generous comment and offer to help support InspectApedia.

We've worked hard on this data for decades so we are really grateful when a reader finds it useful and trusted.

The website is supported by the pittance of income from on-page Google-placed advertisements which, though sometimes an annoyance, also are sometimes pertinent to the topic.

The most helpful support we can receive from our readers are questions, content suggestions, editing suggestions, criticism, all of which help improve the accuracy and clarity of our information.

On 2021-09-15 by Jack

After draining an boiler expansion / compression tank, and when I open the expansion tank water feeder valve, will the tank fill automatically? If so, do I need to watch the boiler pressure as the tank fill? (I do not have an automatic main water feeder; I have to manually raise the pressure as needed).

Also, will the tank be pressurized after the tank fill with water (or when the water stop filling? If so, will the water automatically stop when the tank is half full?

Thanks for your ‘InspectAPedia’! This information is so great; I feel I am taking a boiler course. Let me know how I can support ‘InspectAPedia’ ‘Encyclopedia of Building & Environmental Construction, Diagnosis, Maintenance & Repair.
I would appreciate a response. Thank You!!

On 2021-02-13 by danjoefriedman (mod)

Thank you for the very nice comment John. We've worked hard on this material for many years so I'm always really really grateful in a reader finds it useful. Should you have questions, critique, or content suggestions at any time those are also welcome and helpful.

On 2021-02-13 by John

You sir are amazing! Thank you so much for writing detailed instructions on here on how to empty the expansion tank; I have been having a hard time with my pressure and i finally read your blog and got it figured out; I appreciate you taking the time and write this. Cheers. ~John

On 2018-10-20 by (mod) - internal bladder tanks don't need to have air added

Richard

If your expansion tank uses an internal bladder you do not normally need to drain it.

Perhaps you can attach a photo of the installation and I can comment further.

On 2018-10-20 by Anonymous

@Richard VERNON jr,

I do not have an isolation valve between the bladder and boiler. ? I need to drain excess water out of expansion tank.

On 2017-12-08 by Gene - Expansion tank does not drain completely.

Since no one commented I figured it out myself. After I removed the air inlet screw and water drained as much as possible, but there was still water in the expansion tank

. I took a piece of 12 ga copper wire and ran it up thru the air inlet valve, up the air inlet tube and up into the expansion tank. It was a straight shot up. I did this a few times.

This must have freed up the blockage in the air inlet tube allowing air into the tank and the remainder of the water in the expansion tank drained out freely.

We have been in this house about 20 years and this is the first time the expansion tank drained completely.

Expansion tank does not drain completely. Isolation valve shut off. Hose attached, air inlet screw removed. Water drains partially.

Tank still sloshes and water gurgles at valve and only small amounts of water continue to drain. Sounds like bleed air is not getting into the tank once the initial tank air pressure is relieved during drainage. How do I get this tank to drain completely? I have blown mouth air through a short drain hose and some water comes gushing out.

On 2017-10-29 by (mod) - Should you wait for the system to cool before draining the tank?

Interesting question. Thanks for asking, Matt.

My first thought was that letting the system cool down avoids a scalding burn risk.

Aside from taking care not to get burned, the most-recent time that I watched a hot water heating system service tech drain an old-style expansion tank to refresh its air charge was in Two Harbors MN. The technician had shut down the boiler for cleaning and inspection but it certainly had not reached room temperature. He wasn't concerned about excess air in the system as a result of the expansion tank drain procedure.

What happens, even with a rather hot boiler, is this:

The water valve that permits the heating system or boiler water to enter the expansion tank is closed before opening the tank drain.

Then the expansion tank drain is opened; most of these old tanks use a special expansion tank drain valve that lets air into the tank as water drains out.

We end up with an expansion tank empty of water, and filled with air at atmospheric pressure which technically is 14 psi (1 ATM) but in a practical sense is "zero" psi with respect to the boiler.

As soon as the tech closed the expansion tank drain and next opens the valve that permits boiler water to enter the expansion tank, water will flow INTO the expansion tank, rather than air flowing out of the tank.

That's because our tank was at "zero" psi while the boiler will be, at the very least, at 12 psi - above the expansion tank pressure.

So we won't have air flowing back out from the expansion tank into the boiler as the boiler cools back down. Rather we'll have SOME of the water in the expansion tank flowing back into the boiler.

Just how much water might that be? HOT WATER PRESSURE EXPANSION RATE at https://inspectapedia.com/plumbing/Hot_Water_Expansion.php

gives us the science and tells us that the volumetric expansion of water as we heat it is small: 1 deg C rise in the water tank = 1.0002 Unit Volume Rise

If you invert that you'd have the volume of water that pushes back into the boiler from the expansion tank: not much.

Great question!

On 2017-10-29 by Matt from Minnesota

Should you wait for the system to cool before draining the tank? If the system is hot and you drain the tank, could the air in the tank get sucked back into the system as it cools?

On 2016-07-28 by (mod) - no water comes out of the expansion tank when I open drain

Darn I hate when that happens; I suspect that the tank is waterlogged but the drain is also clogged or the wrong type of drain valve is installed.

See the solution

at EXPANSION TANK WON'T DRAIN

On 2016-07-28 by Danny jenks

What if tank is full of water but no water comes out of drain?

On 2016-03-12 by (mod) - What causes the overflow?

No, KCK. Heating boiler expansion tanks are closed vessels, though bladderless expansion tanks will have a drain valve and (rarely) some may have a pressure relief valve.

On 2016-03-11 y KC Kang

There is an overflow from the outlet of the expansion tank.
What causes the overflow?

On 2015-10-20 by mod - How much pressure should be kept in the expansion tank with a bladder?

Bill: see EXPANSION TANK PRESSURE ADJUSTMENT

On 2015-10-20 by Bill tremmel

How much pressure should be kept in the expansion tank with a bladder

...

Continue reading at BOILER DRAIN DAMAGES EXPANSION TANK or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• EXPANSION TANKS, BOILERS - home
  • AMTROL's EXTROL®, the FILL-TROL®
  • ATTIC & ANTIQUE EXPANSION TANKS, HEATING
  • BOILER DRAIN DAMAGES EXPANSION TANK
  • EXPANSION TANK AIR VALVE LEAKS
  • EXPANSION TANK DIAGNOSIS
  • EXPANSION TANK DRAIN & AIR RE-CHARGE
  • EXPANSION TANK IO MANUALS & SIZING GUIDES
  • EXPANSION TANK LEAK REPAIRS
  • EXPANSION TANK PRESSURE ADJUSTMENT
  • EXPANSION TANK SIZING GUIDE
  • EXPANSION TANK WATERLOGGING CAUSES
  • EXPANSION TANK WON'T DRAIN
  • FIND the EXPANSION TANK
  • HEADER TANKS
  • RELIEF VALVE LEAK DUE TO EXPANSION TANK
  • TANKLESS COIL INTERNAL LEAKS INTO the BOILER
  • THERMAL EXPANSION of HOT WATER
  • THERMAL EXPANSION TPR VALVE LEAKS
  • WATER PRESSURE TOO HIGH: DANGERS

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EXPANSION TANK DRAIN & AIR RE-CHARGE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING BOILERS

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