Sizing expansion tank

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Jlinde37
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Post by Jlinde37 » Wed. Feb. 12, 2020 6:43 pm

Hey all. I currently have 2 small expansion tanks Installed on my system. ( I know, not ideal but I had the other one laying around so I decided to install it to see if it would fix the issue) Both are set at 13psi air with no water pressure. Both plumbed on the suction side of the pump on the primary primary loop. The problem I have is when boiler is cold, no fire, I set water pressure to 12 psi and as my DS3200 heats up to 180 I get too close for comfort at around 28psi. I know the boiler holds about 40 gallons and I have 1000' of heated slab and about 200 feet of 3/4" copper for hwbb. I figured by adding the second expansion tank would solve this issue, but it didnt seem to make a difference. So now I let water out of heated boiler to run about 20psi hot.
When I shut down and cool down for cleaning my pressure has dropped to 0psi.
Do I need a monster expansion tank? Thermal expansion calculators from google are giving me mixed results.

 
Jlinde37
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Post by Jlinde37 » Wed. Feb. 12, 2020 6:46 pm

The gray tank is on my boiler bypass

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Rob R.
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Post by Rob R. » Wed. Feb. 12, 2020 8:15 pm

sounds like you need a bigger expansion tank.

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Lightning
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Post by Lightning » Wed. Feb. 12, 2020 8:33 pm

I'm not sure if its proper or not, but I filled my expansion tank to 14 psi with air (given no boiler pressure). Then once all the water in the system is around the low set point of the aquastat, I let water out to bring the boiler pressure back down to 15 psi. My thinking is that this way the tank has the most room for expansion between the low and hi set points of the aquastat.

 
Jlinde37
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Post by Jlinde37 » Wed. Feb. 12, 2020 8:43 pm

Lee, I pretty much do the same thing. But then when boiler goes cold for cleaning I'm left with no pressure. Maybe even vaccum after oxygen is burned out. Maybe I'm worrying about something that I shouldn't. And keep doing as I'm doing. All is fine up and running. But my mind dont work like that and If i feel something is not absolutely correct it bothers the crap out of me. I'm thinking a a cold boiler set at 15psi should stay at 15psi at 180° when given proper expansion

 
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Lightning
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Post by Lightning » Wed. Feb. 12, 2020 8:47 pm

Oh... for that concern the automatic fill valve will let water in if the boiler pressure falls to 12 psi.

Do you have an auto fill valve?

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Jlinde37
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Post by Jlinde37 » Wed. Feb. 12, 2020 9:04 pm

Negative.. I manually fill with hard piped ball valve.

 
lzaharis
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Post by lzaharis » Sat. Feb. 29, 2020 1:03 pm

If you go to the AMTROL or B+G web site home pages they provide you
with the information you need to properly size and purchase the correct expansion tank.

Having two small bladder tanks may be causing some of the issues with the "Point Of No Pressure Change" in your system. Each gallon in your systems capacity will expand a certain amount in a hot water heating system which would explain the higher pressures you see.

1. Did you install that air purger on that bladder tank??

2. where is your circulator????

 
Jlinde37
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Post by Jlinde37 » Sat. Feb. 29, 2020 1:33 pm

Yes I have the one tank on the bottom of the spirovent. The other tank is t'd into my boiler bypass. Both on the supply side of boiler. The pump is aft tanks on the boiler return. Thus 'pumping away' from expansion tanks.

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