LARGER VIEW of a heating boiler expansion tankBoiler Drain Procedure Damages Expansion Tank Bladder
How use of a vacuum to speed boiler drain-down damages the expansion tank (compression tank) bladder

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Hot water heating system expansion tank damaged by boiler drainage procedure:

This article reports damage to the internal bladder of a heating boiler expansion or compression tank. Damage was caused by use of a pony pump to speed the draining of a hot water heating boiler during service. The article includes steps in determining what went wrong and why.

The article concludes with a warning that using pumps on heating systems to speed the service drainage process may be risky.

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 compnent.

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Boiler Drain-Down Service Procedure Using a Pump Damages Expansion Tank Internal Bladders

Reader question: 11/3/2014 Mike said:

Great site I have a question on a building of a larger size, the expansion tank is 180 gallons and is of a bladderless type, now the boiler was operating with under 5 psi.. after it cooled I went around looking where the feed for cold water goes to the system which led me to the Watts SB1156F..

it has the fast fill lever on it, so I opened it and noticed the pressure gauge on the boiler started to rise.. I also noticed the water level in the expansion tank rose but when I returned the lever back to it's normal position I noticed water was still flowing but at a much lesser amount on the water meter.

I have two questions about the incoming water, why did the boiler not get any pressure until I bypassed the regulator setting, and why was the flow still trickling in after I reset it back to it's normal position?

Now the expansion tank it's full of water and I'm trying to figure out how to drain it, the terms they used back in the 60's are a little different but they at least tagged each valve with a brass letter and number, one of them in question is called expansion tank guard..

it comes from the circulation pumps and it leads to the bottom of the tank, another valve is the supply to the boiler just after the watts regulator. should I turn off the one that's called expansion tank guard and the supply for the boiler while I drain?


Mike I've been thinking about your question and am not sure I can diagnose the issue accurately from just your e-text.

If water pressure in the boiler drops, typically there's a leak to be found and fixed.

The pressure regulator may also be improperly set or in failure if it's not keeping the boiler pressure at normal cold levels.

Reader follow-up:

11/5/14 Mike said:

Well Dan I fully drained the tank and I came back the next day and after getting the system back up everything was ok, it's a bit hard to describe over text I still don't know why the boiler before was not adding water to it and it was operating at 0 PSI.. Odd that the boiler did show pressure after I opened the fast fill (after boiler cooled) now it's fine.

I had no clue when the tank was last drained but the last date (and only date) someone wrote was it was drained back in 1988, and I don't think a tank can go 26 years without another draining?

I had no other option but to use the hose on the tank, I did have to blow into the hose and the tank would drain for about 5 min, then I would crack open the hose and air would rush in, as the tank was getting lighter the rushing air into the tank was enough to have it moving a bit.

11/6/14 Matt said:

I just had a certified tech replace the relief valve for my boiler water. I have the extrol (modern) tank with a bladder. I had no problems with the relief valve that I could see, but it was 10 years old and the tech suggested replacement. After he replaced it, I had significant water and it was because the bladder was now ruptured.

The company of the tech stated that it is "common" for the extrol bladder to rupture after replacing the relief valve - is that true? I don't know if this was an unlucky series of events or if the tech made a mistake. They didn't charge me labor, but did charge about $170 for a new bladder.



Your plumbing tech may certainly know stuff we don't, but I don't see the connection between installing a new TP valve and a ruptured pressure tank or expansion tank internal bladder. I've looked and not seen that warning among Amtrol's installation guides.

I'm also confused about why touching (to replace it) a relief valve would come anywhere near the expansion tank or its bladder. I don't know what a tech might do that would damage an expansion tank unless during system re-pressurization someone inadvertently submitted the equipment to unusually high pressures or jammed a screwdriver somewhere into an expansion tank.

Why not ask the company's service manager to help you out with an explanation of what happened and why.

Reader Follow-Up:

Matt said:

Thanks Daniel, and I agree. I probably should have added more info (I was in a bit of a hurry the first time). In addition to the boiler water relief valve, I had the combo backflow preventer and air vent also replaced, so a little more work that did get near the Amtrol tank. I don't know anything about how to service a boiler, but I'm guessing that he had to vaccum some water and potentially perform other things that, if he over-pressurized the Amtrol® tank in starting back up, could have caused it to rupture, but I'm not sure.

Before I called back the service company, I was interested in your thoughts. I didn't think that answer made any sense either, so I'm glad to hear that. Now that you have the full story, any additional info would be helpful. Thanks again.



Sometimes when a hydronic system is drained for repairs it is re-filled under pressure, even using a pony pump, as part of the procedure to purge air from the lines. Still I'd be surprised if pressures during those tasks would burst the tank bladder.

Vacuuming? really? Indeed if the water pressure in the system were dropped rapidly and significantly, that might cause an internal pressure tank bladder to burst, tear, rupture.

I await with interest hearing what the service manager has to offer. I'd expect that if you come across as interested and not threatening s/he should be willing to offer a more cogent explanation.

Reader Follow-Up: case closed - service tech uses vacuum to speed the boiler drain-down job - blows the expansion tank bladder

11/7/2104 Matt said:

Spoke to service manager. He said that these "fail all the time" during valve replacements and that they routinely put negative pressure on relief valve to remove excess water. I explained that it was working prior and failed after his tech replaced the valves, likely due to over pressure / rapid change in pressure.

He wouldn't admit that, simply kept repeating that these fail all the time and it was old (10 years) and should have been replaced anyway. My guess is that they probably pay about $20-$40 for these tanks and charge $165 to replace them (excluding labor). Nice racket for them. I did get about 50% of my money back, but needless to say, I'm switching service companies effective today.

Reply: if something "fails all the time" we might want to ask "Why?"


You have to love it, right. "They fail all the time" may indeed be absolutely true, because all the time the service procedure the company is using is one that destroys a component that was not intended for the negative pressure they apply.

If we consider that an expansion tank bladder and its contents may be sitting at 12 psi or higher and that someone applies a powerful vacuum to the system such that the bladder is exposed to a strong negative pressure it's no surprise that an internal bladder might burst, particularly if the exposure were rapid.

I once inspected a brand-new home in which new gutters had been installed and it was raining. Roof runoff was pouring behind the gutter and on my head as I rang the doorbell. The builder himself answered the door.

I tried to be tactful. "Do you think this looks right for all this water to be running behind your gutters?" I asked.

"Yeah" he replied "they all do that".

Well indeed all of HIS gutters did that because his gutter installer always installed the roof drip edge improperly (behind the gutter rather than extending out over it).

I doubt the service company is being deliberately dishonest. More likely they've got an idea to speed their service procedure and don't appreciate the effects of being in a rush.

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