Good morning folks - From what I have been able to gather, the following seem to be a given when installing a thermal expansion tank with my new hot water heater. First, the pressure in the tank MUST match the PSI of the household water. Setting the tank at 20psi with water pressure at 80 psi renders the thermal expansion tank useless. The other given is that you cannot set the pressure (at least not on the 2 gal thermal expansion tank that I have purchased to go along with my 40 gallon electric water heater) in the expansion tank above 80. If both, especially the later is factual, then it looks as though I need some guidance on installing a valve to lower my water pressure from about 93-95 psi. I've read places where that valve does NOT have to be installed just after the water enters the house. Some prefer to install say after the first break for the outside hydrant (hose) so that we have good water pressure when washing our cars and trucks! That got me thinking - why not just reduce the pressure just ahead of the heater and expansion tank since I'll be plumbing that area anyway. Then I got to thinking a bit more, that would just put more pressure elsewhere in the house, correct, which is not a good thing (damage to hoses and washers and appliances...). I would be grateful for some assistance on this!

Thank you in advance.

Mike from Athens, OHIO

1) you do not set the pressure on a thermal expansion tank. install it as it comes from the factory. you are thinking of a well storage tank..
2) install the pressure reducer at a point where the water comes into the house. yeah, keep the hose faucet/s on the street side of the PRV if you want. code says the PRV must be installed when the house pressure is above 70 PSI.

Setting the expansion tank pressure, and you CAN adjust it, at the house pressure gives the maximum capacity, but almost any presssure below that will work, since it equalizes to the house pressure as soon as the water is turned on. A higher than house pressure just limits its effectiveness but it will still do its job when the pressure rises in the system. It makes no difference where you install a PRV as long as it is ahead of all the faucets in the house.

Re: Thermal Expansion Tank and Household Water Pressure
Author: hj (AZ)
Setting the expansion tank pressure, and you CAN adjust it, at the house pressure gives the maximum capacity, but almost any presssure below that will work, since it equalizes to the house pressure as soon as the water is turned on. A higher than house pressure just limits its effectiveness but it will still do its job when the pressure rises in the system. It makes no difference where you install a PRV as long as it is ahead of all the faucets in the house.

Okay - Let me see if I got this correct. First, the first post indicated that you SHOULD NOT adjust the pressure of the thermal expansion tank. I think you are saying that you can, but there's no need as long as the pressure in the tank is below the pressure in the "house." On one hand, I understand the concept behind the thermal expansion tank. As the water in the water heater expands, it needs a place to go. Enter the thermal expansion tank. Where the water gets muddy for me is the PSI in the thermal expansion tank vs. the line. Why are some saying they must match and others not so much?

as hj says (i think)... there is a certain air pressure behind the bladder. once you install the tank and turn the water on, the water pressure will push against the bladder and you will have equal pressure on both sides of the bladder. so less air pressure or more air pressure doesn't matter in the final scheme of things.

I guess where I'm getting lost is trying to understand why the bladder doesn’t completely fill with cold water and because the pressure is greater in the bladder than outside, it just doesn't compress the air. In other words, why doesn't the expansion tank just "do its thing" with cold water? Does that make any sense?

If the pressure is set low, then more water enters the tank, so the "air cushion"is smaller, but it will still absorb the expansion. If the pressure is higher, it ALSO will absorb the expansion, but not until the pressure reaches the level of the air pressure in the tank. For all practical purposes ANY pressure greater than zero and less than 150 will have some effectiveness.