As I understand the plumbing code 80 psi is the upper limit of static water pressure.

If you open a cold water faucet, close it, and get 145 psi you need a PRV (pressure reducing valve). If you have one it has failed and needs to be replaced.

The gizmo on your water heater is not a PRV it is a T&P (temp & pressure) safety valve.

With a PRV installed you will have a closed plumbing system and code requires a thermal expansion device. The easiest to install is a thermal expansion tank (for potable water). Usually installed on the cold supply to the water heater.

More info on thermal expansion if you click here

110 PSI is too high. You need a new PRV(pressure reducing valve) on your incoming supply line or your's needs to be adjusted. If you really meant the TPV(temperature pressure relief valve) on the heater then the problem is the heater building pressure as the water heats up.
Normally the buildup goes back out the water line to the street. With a PRV the pressure stays in the house. You needs to install or replace an expansion tank so the water has somewhere to expand into.

Thanks,
My termnology was off, I was thinking PRV as Pressure Relief Valve.

So Yes I have replaced the TRV on the water tank since it was not reseating.

The pressure in the house after that was 145 PSI which dropped after turning on a cold water tap.

I do have a Pressure Regulating Valve on the main line which I adjusted to see if this corrected the problem. It did not. So yes I understand I have a closed system.

At this point, I can probably assume that I need an expansion tank and the problem is due to thermal expansion.
I will do that.

MY one question remaining is that when testing for thermal expansion, I tested by turning several different cold water taps on ( only any one tap was turned on at a time). The pressure dropped from 145PSI to 110 psi on one test, 90 psi on another, yet all the way down to 20psi on the third.

Why such a huge difference in pressure drop and is this indicative of some other problem?

My understanding is that when dong this test pressure should drop down to pressure that the pressure rgulator has been set to provide to the household. so should not the values be about the same?

Thank you very much for your help.

David

If you have more than 80psi static water pressure after opening and closing ANY valve on the cold side then your existing PRV is set wrong or has failed. That will need to be remedied.

Since you have a PRV you have a closed system and require a thermal expansion tank. no ifs, ands, or buts.

Different pressure drops noted at different valves can simply be the flow rate of the valve opened and how the house is plumbed.

The acid test for thermal expansion is a pressure gauge with a tattletale needle. On a closed plumbing system, after the water heater fires and without opening any valve the pressure will go up to what you noted. Every valve and appliance in the house will be subjected to pressure in excess of their rating.

QUOTE," The pressure dropped from 145PSI to 110 psi on one test, 90 psi on another, yet all the way down to 20psi on the third."

Fresh Water reaches maximum density of 62.4lbs & 231cu.in/gal at 39degF. When heated from 39degF to the boiling point water expands at a linear constant until it achieves approximately 10% expansion. At the boiling point, when water flashes to steam, it expands another 1628 times it volume. (At standard atmospheric pressure the boiling point is 212degF however the actual boiling point increases proportionate to the pressure.)

Your water is entering the structure through a defective PRV(pressure reducing valve) which is allowing the cold water static pressure to rise to equilibrium with the prime mover (municipal water main). Under normal circumstances when the water is heated the increased volume caused by expansion would backflow to the municipal main, however there is a backflow preventer in the PRV which prevents the water from backflowing, therefore code requires that we must install a thermal expansion tank on the cold water inlet line to the water heater whenever a PRV or backflow preventer is installed on the main water supply line. (NOTE-The thermal expansion tank must be correctly sized to your tank. At a minimum the expansion tank should be equal to or greater than 7% of the volume of your water heater).

Water is not compressible and in your case there is no expansion tank, therefore some of the volume will be absorbed by the water hammer arrestors. The T&P valve on your water heater is preset to blow off it the internal temperature exceeds 210degF or if the pressure exceeds 150psi. Keep in mind that the 150psi is a factory preset and their may be a slight tolerance which would account for the 145psi you noted. In addition, unless you are using a laboratory grade pressure gage there could be some tolerance in your gage.

The moment you open any faucet the excess will immediately be relieved and the pressure will do down to or below the pressure from the prime mover (municipal main) depending upon the volume of flow, which in your case is the 110psi you noted.

Up to this point we have discussed "Static Pressure", which is the pressure that is apparent on the system when there is no flow.

In all cases when flow begins the pressure in the system will immediately drop from static pressure to "Dynamic Pressure' (working pressure).

Dynamic Pressure is Static pressure less "Vertical Head Loss" and "Friction Head Loss".

Vertical Head Loss is a physical constant equal to .434psi per vertical foot of rise. Exxample_let us assume your water main enters the basement at floor level and you have a shower on the first floor. Allowing 7' vertical from the water main to the 1st floor and another 6' vertical rise from the 1st floor up to the shower head we have a vertical rise of 14'. The Vertical Head loss to the shower head would then be 14 x .434= 6psi. This means the pressure at the shower head will be at least 6psi less than the pressure at the water main.

In addition to the vertical head loss, we must consider the "Friction Head Loss" which is a pressure loss caused by the friction of the water rubbing the pipe walls as it flows or eddy currents caused by fittings in the pipe run. When considering Friction Head loss the pipe size is a constant, therefore if the demand volume increases the velocity of flow must increase proportionally, however as the velocity of flow increases proportionally the friction increases exponentially. By Example: in a 3/4" copper pipe with a flow rate of 1gpm the friction head loss is 0.001psi/ft, and if we increase the flow to 3gpm the Friction loss is 0.009psi/ft however if we increase the flow rate to 10gpm the friction head loss would then be 0.084psi/ft.

We must also keep in mind that the friction head loss includes all piping from the prime mover, which in this case is the municipal main to the point of use.

For estimating Friction head losses we must know the actual length of all pipe from the prime mover to the point of deman, plus the fitting allowances (Fitting allownace's are expressed in equivalent lengths of straight pipe) for the run, then consult published Friction Head Loss tables for the type and size of pipe being considered.

The bottom line, You must immediately either reset or replace your pressure reducing valve to limit the incoming static head pressure to not more than 80psi, and you must install an approved "thermal expansion tank" on the water heater cold water inlet line or risk a catastrophic failure of a fitting, valve or line in your house.

Thankyou Justalurker and lazypup.

I am getting an education here. Everyone on this forum is extremely helpful.

I have bought a new PRV Watts AUB25-Z3 same as old one.
I understand when installed correctly, this valve will allow some backward flow so it is not a real closed system after all. It will be installed in the morning. I will then recheck everything and see where we go from there.
Lazypup, that was an amazing lesson, I will print it out and put it in my plumbing book!

thanks to all

David

Hi all,

Replaced PRV today and everything seems OK.
muncipality pressure comng in is 110.
PRV set at 55 PSI
Once hot water tank got fully heated house hold pressure stabalized at 65 PSI.

Might wait a couple of days and if everything seems ok might lower the pressure a bit more.
Probably will keep my eye open for one of then tattle tale gauges, sounds like a useful device.
thanks for all your help.

David

David,

The static pressure is about right but it's still the spike you need to consider.

Here's the gauge click here

I believe they sell them at Home Depot for around $12

I'd still T in a thermal expansion tank in the cold service to the water heater.