Fine, learn me expansion tanks

After a little over two years and countless theories and experiments, I've finally determined where my water pressure spikes come from. 

Hot water and thermal expansion.

We have a "closed" system here at the house, with a backflow preventer on the main. This gives expanding water nowhere to go, except the overflow on the hot water tank or any faucets.

Well the overflow on the tank is brand new as of when we moved in, as is the house pressure regulator/back flow preventer, and the meter itself. 

The water pressure spikes throughout the house after using a bunch of hot water, like a shower or the dishwasher. 

I've been trying to size tanks but the calculators I've used ask for measurements I don't know how to make, and the tanks I've seen for sale cap at 155psi. This is a problem because my pressure peaks at 175psi. 

Our nominal pressure is set at 70psi. We have a 40 gallon hot water tank. 

I'm sick of blowing seals in my bidet, toilets that sound like bombs going off, dripping shower heads and puddles on the floor by the water tank. 

So let me recap in a list real quick.

40 gallon tank

70 psi nominally

Surges to 175 after hot water usage. 

Main line is 3/4", drops to 1/2" before the hot water tank and splitting to cold for the rest of the house.

Almost all tanks I've seen are rated for a max of 150psi.

How big of a tank do I need and where should I put it with regards to the plumbing around the HWT?

If you don't get a more direct answer, I'll see if I can turn anything up among the engineers I work with.

Mine is mounted right above the water heater in the garage. It's about 2 gallons. 

175psi is pretty impressive but your expansion tank shouldn't need to be rated that high because it is there to take care of that problem. 

If your system pressure is 70 psi you probably need to charge the bladder in the tank to 70 psi or a little higher but don't quote me on that. 

Toyman! said:

Mine is mounted right above the water heater in the garage. It's about 2 gallons. 

175psi is pretty impressive but your expansion tank shouldn't need to be rated that high because it is there to take care of that problem. 

If your system pressure is 70 psi you probably need to charge the bladder in the tank to 70 psi or a little higher but don't quote me on that. 

 

"175psi is pretty impressive but your expansion tank shouldn't need to be rated that high because it is there to take care of that problem.".

This is correct. You are installing the expansion tank to solve the high pressure issue by allowing expansion into the tank instead of excessive pressure build up. The tanks you are finding rated for 150 psi will cover your needs when properly sized. A 2 gallon expansion tank should cover your needs (40 gallon heater) but there is no harm in being over-sized aside from cost/space.

The bladder should be pressurized to match your normal system pressure (70 psi in your case). 

In reply to fusion66 :

Ok, that makes some sense. So if the tanks come in "charged" at 40, just pump it up to cover the difference between peak and nominal. 

Stupid homeowner/plumbing company blogs making things more confusing than they need to be.