Water Heater Pressure/Temp Relief Valve Discharge

[bligh]

Pv=nrT
Pressure x volume =(some fixed number) Temperature

In other words when you heat up water it expands. Hence the pressure rise. But you can’t really ‘stop’ the expansion as the physical force of the pressure exerted by the temperature rise is really high. So instead of your water heater and all your hot water pipes springing leaks, a little bit of water weeps out the pressure relief valve to stave off that massive pressure build up. The pressure relief valve is doing it’s job. It weeps then it stops until the next cold/hot cycle. Most boats have an accumulator tank that could absorb that expansion. However in marine plumbing I often see a one way check valve on the cold water supply to the water heater which prevents the accumulator tank from absorbing that half a teaspoon of water that needs to be bled off from the temperature rise. So if your not into removing that check valve or you don’t have an accumulator tank (AKA expansion tank), you next option is to install an expansion tank between the cold supply and the water heater. You’ll need to set the pressure in the tank on the air side with a regular bicycle pump to whatever pressure your domestic pump puts out less 10 to 25 percent. (If your pump puts out 50 psi, set your expansion tank pressure to 35 to 40 psi.) That will fix your weeping pressure relief valve.

Regardless, I didn’t like my relief valve weeping at first either, but mine is directed overboard so I just got used to it. Besides the mechanical space in my boat is crowded enough and expansion tanks fail too. YMMV

 

[OldDan1943]

As the water in the tank heats up it expands and builds up pressure. Instead of triggering the relief valve, the water flows into the expansion tank. Your pressure relief valve will last longer and your hot water pressure will be smooth, rather than exploding from the faucet as it releases the built up pressure.

I had a new hot water heater install at my condo. The new code requires a expansion tank installed on the hot water outlet. I guess you could try that. The expansion tank had no facilities to change the pressure so I guess it is just at the HW pressure.

 

[FF]

The T&P discharge valve is for either pressure and and/or temperature.

Could an engine loop to the unit be heating the water above the discharge setting?

If so plumbing supply houses have T&P valves for commercial or farm service with 180F ratings.

You will need an anti scald (not tempering) valve in the system.

 

[boathealer]

As the water in the tank heats up it expands and builds up pressure. Instead of triggering the relief valve, the water flows into the expansion tank. Your pressure relief valve will last longer and your hot water pressure will be smooth, rather than exploding from the faucet as it releases the built up pressure.

Yep. In typical boat systems the flexible water hoses can absorb the expansion.

In my new-to-me boat some time ago, mine was leaking because the boat is plumbed with PEX and it does not expand very well. I put an accumulator tank in and all is well.

 

[sunchaser]

Pv=nrT
Pressure x volume =(some fixed number) Temperature

In other words when you heat up water it expands. Hence the pressure rise. But you can’t really ‘stop’ the expansion as the physical force of the pressure exerted by the temperature rise is really high. So instead of your water heater and all your hot water pipes springing leaks, a little bit of water weeps out the pressure relief valve to stave off that massive pressure build up. The pressure relief valve is doing it’s job. It weeps then it stops until the next cold/hot cycle. Most boats have an accumulator tank that could absorb that expansion. However in marine plumbing I often see a one way check valve on the cold water supply to the water heater which prevents the accumulator tank from absorbing that half a teaspoon of water that needs to be bled off from the temperature rise. So if your not into removing that check valve or you don’t have an accumulator tank (AKA expansion tank), you next option is to install an expansion tank between the cold supply and the water heater. You’ll need to set the pressure in the tank on the air side with a regular bicycle pump to whatever pressure your domestic pump puts out less 10 to 25 percent. (If your pump puts out 50 psi, set your expansion tank pressure to 35 to 40 psi.) That will fix your weeping pressure relief valve.

Regardless, I didn’t like my relief valve weeping at first either, but mine is directed overboard so I just got used to it. Besides the mechanical space in my boat is crowded enough and expansion tanks fail too. YMMV

The ideal gas law does not apply to solids or liquids. That professorial note aside, many electrode heated tanks have boiling around the electrode releasing gas bubbles that indeed can cause problems with the relief valve popping off. As you and others have noted let the expansion tank work by removing the tank mounted check valve as one cure. Another is to turn down the temperature on the tank thermostat to avoid boiling at the electrode.

 

[catalinajack]

My water heater temp cannot be adjusted. It is factory set at 140 degrees. Don't know for certain but it may be that many marine water heaters cannot be adjusted. Also, I wonder whether the water surrounding the electrode would still boil even at a lower set temperature as the electrode is either on or off.

The ideal gas law does not apply to solids or liquids. That professorial note aside, many electrode heated tanks have boiling around the electrode releasing gas bubbles that indeed can cause problems with the relief valve popping off. As you and others have noted let the expansion tank work by removing the tank mounted check valve as one cure. Another is to turn down the temperature on the tank thermostat to avoid boiling at the electrode.

 

[catalinajack]

My valve is plumbed to the shower sump. I have long since stopped paying attention to it.

Pv=nrT
Pressure x volume =(some fixed number) Temperature

In other words when you heat up water it expands. Hence the pressure rise. But you can’t really ‘stop’ the expansion as the physical force of the pressure exerted by the temperature rise is really high. So instead of your water heater and all your hot water pipes springing leaks, a little bit of water weeps out the pressure relief valve to stave off that massive pressure build up. The pressure relief valve is doing it’s job. It weeps then it stops until the next cold/hot cycle. Most boats have an accumulator tank that could absorb that expansion. However in marine plumbing I often see a one way check valve on the cold water supply to the water heater which prevents the accumulator tank from absorbing that half a teaspoon of water that needs to be bled off from the temperature rise. So if your not into removing that check valve or you don’t have an accumulator tank (AKA expansion tank), you next option is to install an expansion tank between the cold supply and the water heater. You’ll need to set the pressure in the tank on the air side with a regular bicycle pump to whatever pressure your domestic pump puts out less 10 to 25 percent. (If your pump puts out 50 psi, set your expansion tank pressure to 35 to 40 psi.) That will fix your weeping pressure relief valve.

Regardless, I didn’t like my relief valve weeping at first either, but mine is directed overboard so I just got used to it. Besides the mechanical space in my boat is crowded enough and expansion tanks fail too. YMMV

 

[sunchaser]

My water heater temp cannot be adjusted. It is factory set at 140 degrees. Don't know for certain but it may be that many marine water heaters cannot be adjusted. Also, I wonder whether the water surrounding the electrode would still boil even at a lower set temperature as the electrode is either on or off.

After 9 years our Seaward heater exploded due corrosion of the Al tank. It was replaced with a glass lined Torrid unit that has a temperature adjustment. Adjusting the temperature downward helped reduce the pop off. The final solution was to remove the tank mounted check valve and let the pressure go to the expansion tank.

As already mentioned, home water heaters are now plumbed with a small expansion tank. My home plumbing guy says these expansion tanks are now being pushed by code and insurers.

 

[catalinajack]

Well, there you go, a water heater with an adjustment feature. By the way, my 6-month-old heater is a Seaward. In any case, even if I had an adjustable heater I would keep it at 140 degrees and live with the very minor bleed-off. It gives me plenty of hot water for a shower such that I use much less to get a nice, long, hot shower. I have no interest in a tempering valve for the same reason. I also have an engine loop so that when we arrive at a destination the water is at 180 degrees. We are just very careful when in the shower and since we use a powerful 110VAC fresh water pump we have no line surging and thus cannot get scalded once the temp is adjusted. Guests, extremely infrequent, are warned. In fact we demonstrate how to use the shower for them. Same with sink water, warning ahead of time.

After 9 years our Seaward heater exploded due corrosion of the Al tank. It was replaced with a glass lined Torrid unit that has a temperature adjustment. Adjusting the temperature downward helped reduce the pop off. The final solution was to remove the tank mounted check valve and let the pressure go to the expansion tank.

As already mentioned, home water heaters are now plumbed with a small expansion tank. My home plumbing guy says these expansion tanks are now being pushed by code and insurers.