Expansion tank pressure?

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Jan 20, 2016
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Hello fellow TFers,
I am installing a 5 gal pressurized expansion tank on my fresh water line and was wondering what is the correct pressure to set for it? Manufacturer set it to 20psi and mention to set pressure to line pressure. My pump pressurizes to 40psi so I inflate the tank to 40psi, is that correct?

Pressurize to 20 which is close to your "kick in" pressure on the pump. If you pressurize to 40 psi it will defeat the purpose of having the tank at all, which is to contain an amount of water under pressure to stop the pump from cycling as often.
You want to leave it to what the factory has it set at. If you pressurize it to the same as your water pump you are defeating its purpose and it will be useless.

Why a 5 gallon size? For most boats our size, a 2 gallon (1 gallon capacity) seems more than adequate. The last one we installed, I bought at one of the box stores for ~$50.

Edit: I have to type faster. What bayview and Gary said. :thumb:
I suspect it is for a dual pressurized system. Water and wine...

Thank you all I will deflate it to the original 20psi.
Why a 5gal ? well no special reason except that I got a good deal on one, around 45$CAD for a domestic Watts one. Capacity is 4.5gal, 2.8gal acceptance to be exact. I have enough spa to fit it under my head sink.
Mr RTF, I have a separated special hidden secret tank for the holy nectar, I do not want to risk to mix it with this water that is just good enough to clean the glasses :)

Should be a pound or two below the cut-in pressure of the pump. Usually that's in the low 20's.
Yep, you need to know the "cut in" pressure of the pump, that is the lowest pressure system sees when pump comes on. Then tank, with water side vented and drained, should have bladder pressurized to a few to maybe five psi less. That way as pump cycles on and off there is always some water under the bladder.

I don't think you can go wrong to just start with 20psi. It will get you in the ballpark.

To find cut in pressure, let pump fill tank with water. Run til it stops (max pressure). Open a spigot and let water drain til pump kicks on. Turn pump off at breaker straight away. Measure air pressure at bladder. Bladder pressure will be the same as cut in pressure.
The bigger the tank, the longer the time between pump cycles. Less wear and tear on the pump and contacts.
"The bigger the tank, the longer the time between pump cycles. Less wear and tear on the pump and contacts."

YES! 20 gal would be fine too!
General rule for compression tank in domestic pumped system is 2 psi below the cut-in pressure of the pump as others posted. That insures that there will be residual water pressure in the tank when the system approaches the cut in pressure of the pump. If the pressure's too high, water flow can stop before the pump has a chance to start up and deliver flow.
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