Plumbing: Domestic: Pressure reducers and hot water heaters.?

If the pressure is increasing when the water heater is off, this means your pressure reducing valve is not functioning properly. It is allowing the pressure to continue to slowly increase.

You will need a new pressure reducing valve.

You mentioned that you were getting 180 degree water. When you say the water heater is off, do you mean the electricity or gas are turned off to the water heater?

Also, when you say it is slowly increasing, is this over a matter of a few minutes or more than an hour or two? If it is increasing in a few minutes, the pressure reducing valve is likely the culprit. If it is taking a couple of hours, you will need to turn the gas/power off to the water heater as a test.

A residential water heater has settings that do not normally allow the water heater to be set above 150-160 degrees. If you have a residential water heater, and it is reaching 180 degrees, there is something wrong with the water heater.

I would suggest you turn the gas or power off to the water heater as a test first. If the pressure continues to rise, the water heater is not the cause of the pressure increase.

If the pressure does not increase when the gas or power are turned off to the water heater, the water heater is the problem.

If you have an electric water heater, and the pressure does not increase when the power is off, you probably have a grounded heating element. A grounded element can continue to heat the water even when the thermostats have reached the set temperature. This increased temperature also would explain the increased pressure. When you heat water, the air in the water expands. (all water has air in it unless it is treated to remove the air) The higher the temperature, the more the air will expand, and as a result, the higher the pressure.

If the heater is a gas water heater, and the pressure does not increase when your turn the gas off to the water heater, you could have sediment buildup that has caused the thermostat to malfunction.

First of all 120 psi is higher than most domestic pressures it shouldn't be much higher than 80 psi. The most likely reason is the temperature of your water heater 180F is way too high it shouldn't be higher than 150F and even that can cause scalding especially in children. Most water heaters come factory set at 140F and many places are making it code that a mixing valve must be used to reduce the temp at the taps to 120F. I suspect that if you turn your temp down where it should be it will resolve your pressure problem.

If it happens when the heater is off then the increase in pressure appears to be too much to be associated with the expansion of the cold water entering. Also vapour pressure of water at 180F is only about 7psi so this does not account for it either.

Hopefully your drum is rated for you full supply pressure, normally the prv is only for run away overheating cases where you boil the water to much higher temps. I would check the regulator if I was you, they very commonly start to leak at as they get older. You can also run the test by running your boiler 100% cold. or close the inlet valve if closing the inlet valve stops the rise then you know it is the regulator.

Good designs should always have the drum rated above supply pressure and not rely on a pressure regulator. You can check your drum for its rated pressure it should be stamped on a name plate. prv is normally only for run away overheating cases, which can cause the drum to explode due to the expansion of the water with temperature.

It sounds like your PRV has failed, IF, you are in fact getting such high pressure with the heater OFF. You should replace it if this is the case. You also should have a thermal expansion tank because you have a PRV. The PRV will not allow the expansion of your hot water to be pushed back through your main.

180 degrees is extremely dangerous.

the jump to 60 psi, seems to be a water hammer pressure rise (notice that you have a one-way valve at the inlet line to the heat exchanger). mentioned the situation of your expansion tank, most of water heaters do have an built in expansion tank.

the slow rise to 180psi could be caused by a leak in the heat exchanger.