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# pressure and size of expansion tank for 3rd floor apartment (boiler 50k btu in the basement)

Member Posts: 21
edited August 2019
everywhere are different suggestions for size and pressure of expansion tank for apartment located on 3rd floor,baseboard finned heaters,50k btu gas boiler located in basement.Currently installed # 30 extrol tank with fractured diaphragm.It's very,very tight place to work in,and few people told me the #15 tank would be sufficient for my needs and much easy to install.

## Comments

• Member Posts: 18,177
if a # 30 will fit, use that Only about 5 bucks difference in price
You need to know or calculate the volume of water in the system to get an exact answer.
The tank can mount anywhere near the boiler
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 21
as i said,the place is very ,very tight.Looks like wall was built after plumbing installed. How can I calculate the tank size and pressure ?
• Member Posts: 20,404
Pressure is easy -- you need enough pressure at the tank to ensure positive pressure throughout the system. 3 stories? Tank in the basement? OK -- call it 24 feet. You need at least 16 psi, and 20 would be better.

But... that has nothing to do with the size of the tank. That is determined, as @hot_rod said, by the total volume of water in the system.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 5,966
Tank size-volume of water in system.
Pressure-distance in feet from boiler to top of radiation x .433. I usually add 4 psi to that.
Set your static pressure to that. Make sure tank is charged to that amount before you mount it. Using the Webstone Expansion Tank Valve allows you to easily check the charge properly, and easily replace a tank without having to drain the system.

That’s pretty much the only way to determine. Don’t know what else you read online.
steve
• Member Posts: 18,177
Without some data it is somewhat of a guess.

Here is the Amtrol sizer, residential 50K, fin tube shows a #15

The commercial sizer allows you to add more info.
Fill pressure, max pressure and capacity. The tank is a bit larger.

The #15 may be adequate, if you do pre-charge to 20 psi from the factory 12 psi, you do reduce the expansion capability.

I'd still suggest a 30

One of the reasons tanks don't last is under sizing, where the diaphragm is stretched to it's extreme at high temperature.

Undersized tanks may cause the relief valve to seep, is there a drain close by if that does in fact occur?

Seems if a #30 is in the space now???

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 995
99 % of the time, it is not a diaphragm failure. The tank looses about 4% of air per year through the diaphragm. Remove the tank, pressurise to required pressure, wait 10 minutes check pressure.
• Member Posts: 18,177
Also push in tire stem "schrader" valve on bottom. if water comes out, the tank is done.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 21
yes,now is #30 in space,drain is not close by.