Welcome! Here are the website rules, as well as some tips for using this forum.

# Pressurizing an Expansion Tank on Boiler

Member Posts: 6
Hi
I have an Extrol 30 expansion tank on my gas boiler, (hydronic system) and I wanted check pressurization and make sure it's working properly. I am able to isolate the boiler without draining the entire system to check, however do I need to drain the boiler at all? The tank is supposed to have a precharge of 12psi when empty and I thought if my boiler went down to 10psi when cold I could check the tank pressure then and not need to drain the boiler. If the tank is low I could pump it up to 12psi and make sure the boiler stayed below that mark since if the tank pressure is greater then the boiler pressure it should be able to fully fill with air to 12psi. Is my hypothesis off? What is it I am not thinking about or taking into consideration.
Thanks for your feedback.

• Member Posts: 11,062
Maybe. Your thinking about it right but if the tank is connected to the boiler you can't predict what slight pressure or vacuum from the boiler can affect the air pressure exerted on the tank bladder.

Why not do the job once. Drain the system and remove the tank. Install a ball valve between the tank and the system. Then install a tee between the ball valve and the tank. In the branch of the tee install a boiler drain valve (or you could use a pipe plug). When you want to check the pressure close the ball valve, open the boiler drain or plug
• Member Posts: 14
The OP is correct. The pressure difference between the boiler and the expansion tank bladder is zero whenever the bladder is free to move. [To argue otherwise is to assume that pressure doesn’t equalize across a fluid medium in a vessel. That logic would add gauges on the boiler in case it’s lower at the top near the north side because of the Coriolis effect. Don’t overthink that...]
It’s also true that if the bladder pressure is higher than the system pressure then something other than fluid pressure is restraining the bladder. The only things it’s got are a bladder and a shell. Since the bladder’s free to move, bet on the shell restraint the bladder because the bladder occupied 100% of the tank.
To summarize; yes to the OP, it’s only necessary to have the system pressure below the expansion tank bladder pressure to set the bladder pressure accurately. As a practical matter, no one uses the same gauge for both, so it’s a good idea to lower the system pressure enough to overcome any gauge error. You can find that error when the system pressure is high enough that they should be equal. Whatever difference is found is gauge offset. Gauges aren’t always perfectly consistent, so lower the system pressure by more than the measured offset.
You know the system pressure is high enough that they should be equal when the bladder pressure starts to rise without adding air to the tank. At that point it’s rising because the system is adding water to the tank pushing the diaphragm down and compressing the air.
• Member Posts: 1,819
Or , buy a new tank and charge it to 12# then change it out .... Or just wait for it to go ...
I have enough experience to know , that I dont know it all
• Member Posts: 9,219
Thank you, Impressive explanation.
However, IMO the simplest method of checking/charging the tank is to remove it from the system and set the PSI as desired....12-15 PSI. Reinstall as Ed suggested.
And then adjust the auto pressure fill valve to match.

You can use your newly added drain bib to get most of the air out of the boiler piping. Your auto air vent should take care of the rest.
• Member Posts: 14
Personally I too like to use valves to isolate components that can fail. In the long run, yes, it is easier to just isolate the tank so the boiler doesn’t have to be even partially drained. But... Since the OP doesn’t mention an isolation valve for the expansion tank, has been thinking through how to charge it and is actually at or very close to having pulled the system pressure down to the point where that’s possible, adding more work to drain the system below that tank to add a valve would simply be more work for no immediate benefit and it *shouldn’t* need recharging for several years at least. If it does it needs replacing.
• Member Posts: 6
WOW...thank you ALL for the great responses. I do have everything else isolated on the system, but when the tank was installed it was never done. I know for next time. This will be the 9th winter for the tank so it might need to be charged up....and with all your suggestions and feedback I feel much more confident about it. Famous last words I know:-)
• Member Posts: 2,521
edited November 2020
I have read literature in the past the indicates that the membrane inside the expansion tank may allow gasses to migrate thru the membrane over time (very similar to the way oxygen can migrate thru PEX tubing). This migration of gasses can account for loss of a few pounds of PSI over 9 years.

1.You are correct to drop the boiler pressure to 10 PSI (or lower) in order to get a 12 PSI reading from the tank service port.

2. You may find that you will need to add pressure (with a bicycle pump) to the tank.

3. If you have access to Dry Nitrogen (because you are in the HVAC trade and we use it for refrigeration work) you could add that gas in place of regular compressed air. (oxygen migrates faster than nitrogen)
Edward Young
Retired HVAC Contractor from So. Jersey Shore.
Cleaned & services first oil heating system at age 16
• Member Posts: 14
Re: nitrogen fill. FWIW, nitrogen diffuses through most types of rubber about 4x slower than oxygen although the gas molecules are very close to the same size. Diffusion rates have more to do with solubility, how well the polarity of the gas matches the polarity of the rubber, than anything else. Predicting the relative rate of diffusion for an expansion tank membrane with an oxygen barrier from the typical data for rubber isn’t going to be accurate for every make and model. I.e. “Your mileage may vary”. Air is 4/5ths nitrogen. A tank that’s down after years of service should already be oxygen poor and nitrogen rich, so the change in service intervals is more evident if the initial fill is nitrogen than the 3rd topping up.
• Member Posts: 9,219
But wait, there's more.
Should we overlook the fact that simple air expands with an increase in temp. Unlike N2 which has minimal temp/pressure factors. (why it is used to pressure test HVACR systems)
So as hot water enters the tank during the expansion process, the captive air on the other side of the bladder is heated, increasing it's pressure.
Now with a N2 charge we do not get this added pressure increase.
What to do now?

Sorry, couldn't resist stirring the pot (tank)!
• Member Posts: 1,414
I have thought about this for some time. This is how I would do it and it is dependent on whether one could isolate the boiler from the zones.

Isolate the zones by closing all the zone valves. This would eliminate static pressure from interfering with the tridicator reading.

Close the boiler water feed valve.

Access the expansion tank Schrader valve and pump air into the tank while watching the boiler pressure gauge. Stop putting air into the tank when the gauge reads 25 psi.

Open the boiler drain valve and drain out water until the gauge reads 12-15 psi. Repeat putting air into the tank as before.

Stop adding air when the gauge doesn't show any increase in pressure. That means that there isn't any more water in the expansion tank to expel. Set the tank pressure to 12-15 psi.

Open the boiler cold water feed valve. Open the zone valves.

The gauge should show 12-15 psi. Old gauges often don't register pressure correctly.

I think this would work.