Thanks - it did seem high. Is it unusual for the water company pressure to be so high (and even more so for them to say that 85 PSI is "optimum")? I think I will go ahead and do both the PRV and the expansion tank.

It's a good idea. There is no guarantee that the pressure won't exceed 85, and it will shorten the life of the wh and other parts, and increases the risk of an unexpected flood.

If at times of high demand your supplied water pressure drops, your prv will not 'boost' it, it will only bring pressure down to spec. You won't notice any adverse effect otherwise.

An in-line pressure gauge permanently connected ( a decent stainless steel-bodied one) wouldn't be a bad idea, you will want some sort of pressure gauge on hand when you're setting up the expansion tank as you have to 'equalise' the pressure within the tank bladder, if you're doing it yourself.

In a municipal water supply system the static head pressure on the system is primarily generated by means of water towers.

Water exerts 0.434psi per/ft on a vertical column of liquid. (Vertical Static Head)

In order to insure that the system will maintain the required code minimum of 45psi the support tower is 105' tall above average terrain to the base of the storage tank. (105' x 0,434psi= 45.57psi)

The storage tank on the top of the tower may then be as much as 92ft higher. (0.434psi x 92' = 39.92psi)

From this we can easily see that when the storage tank is empty we still have a static head pressure of 45psi, however when the tank is full the static head pressure at the ground level would be 45psi + 40psi = 85psi.

The storage tank is then fitted with a pump controlled by a float system very similar to the fill valve in a toilet tank. Wherever practical the system is allowed to use the water from the tank during the day and the pumps are set up to refill the tank at night when the electrical supply for the pumps is on an off peak demand.

Generally a municipal water supply will provide the highest static head pressure to your house early in the morning when the tank is full, but that pressure may decrease throughout the day as the water level in the storage tank drops.

To this point we have discussed the pressure above average terrain however the actual pressure at your faucet is a net result of the physical differential between the elevation of your faucet and the top surface of the water in the storage tank.

If your house is substantially above the average terrain elevation at the storage tank your water pressure will be less than the average for the community, and conversely, if your house is below average terrain your water pressure will be higher. By example, I am familiar with one community in W.PA where the town is built on steep hills. While the municipal water system can provide water pressures within the prescribed code range of 45-80psi for the majority of the community, there are neighborhoods at the bottom of the hills where all structures must be fitted with PRV's. On the other hand, some of the structures on the top of the hills actually require boost pumps to maintain code minimum.

As you can see from this, during the course of a day the line pressure could vary by as much as 40psi, depending upon the actual level of the water in the municipal storage tank. For this reason we do not install pressure gauges on systems supplied by a municipal water supplier because the gauge would not provide any truly useful information, and it would only cause undue apprehension of the part of the untrained homeowner, which would then results in hundreds of unwarranted complaints to the water supplier.

Listen to lazypup on this topic....very helpful to me some months ago when my PRV failed and my house was subjected to 120-130psi street-pressure -- which is, in fact, intended nominal street-pressure for my neighborhood. My over-pressure problems/damages were over 6,000 before I woke up. Main evidence was my WH blowing out when I wasn't there. Painful.

lazy pup - Thank you VERY MUCH for citing your reference. I work for various private & public water systems and was told that there was no statute for how high the water pressure in municipal water system can be, which is apparently true. I had heard various opinions about what it "should be," and, of course pipe selection is a limiting factor. The information you provided gives me a better idea of what pressures to shoot for, and I will be able to advice my clients of the need for PRVs in the customer's homes if we exceed it. Again, thanks for citing code, and not just giving an anononymous opinion!!!

Lazypup was a knowledgeable and excellent plumber. Sadly, he passed away several years ago.