Water heater T&P valve problems

[Jim26m]

@jviss, you are a details kind of guy. Excellent! No slipping anything past you.

Agree that the T&P is a safety. However, T&P valves are available in a wide range of pressures/temps. While he chose to put one in at 150psi, he could have chosen 75psi or less just as easily. Who knows what others are running? I was only addressing @Wanderwind 's thought that; it seems strange that everybody isn't freaking out when their relief valve lets 12 oz of water loose into the bilge on a warm up cycle. I'm sure it's happening on a lot of boats. I've seen stuff that people have ignored that blew my mind. Leaking 12oz into a bilge that's out of direct sight, and a little spurt from the faucet is something that over 90% of the general population wouldn't even notice, much less investigate. Maybe more sailboat owners that do their own maintenance would look into it. Pretty sure you would be on it like white on rice.

You are correct. If you're cycling the system up to and over 150psi, using the relief as an operating control, it's not good, and I wouldn't want to do it. I'm not sure if that shower hose is rated for plumbing pressure, but it wouldn't normally be shut off and holding line pressure I assume. I recommended he hook the gauge to the drain valve on the heater. He chose to connect the gauge to the shower hose so he could get a quick comparison between hot and cold readings. Good idea, but, I have been very uncomfortable every time he notes that the pressure has gone high in that hose.

140 degree water is NOT SUPPOSED to be available at faucets/fixtures typically for the very reason you mention. A tempering valve is a great idea for safety, especially if you have young kids aboard, or anyone who can't sense heat and react quickly.

 

[Mike Oldak]

I stepping in a little late with less knowledge, but 140 psi is WAY too much pressure and so is 140 degrees at the tap. I think your water heater is stuck in the on position and heating the water way above normal and creating the incredible pressure.

I had 105 psi in my house when they were fixing the water mains and blew out 3 toilets and one hose. I think 40-60 psi in a residence is normal and probably more than for marine applications. Also 120 may be a better temp. but Google says domestic water heaters are set to 140 degrees.

I'd be very concerned and call the heater mfr. in the morning.

 

[Jim26m]

Mike, he's been working through his system to get it ship shape. The pressure is related to expansion and will not be his typical operating pressure. You're correct. It's higher than is desirable.

His thermostat is factory set at 140 deg. It's holding right at that, so he's good as far as equipment operation for electric heating mode. He hasn't reported on engine heating mode, but doesn't expect trouble based on past usage.

Storing at a higher temperature is safer from a health standpoint, and it essentially gives you more usable hot water (since you're blending cold with it to get it down to usable temperature). So it's typical to store it hotter than you use it. A tempering or mixing valve will automatically (and safely) blend the water to the correct temp as @jviss indicates, before distributing to the points of use.

If he calls the manufacturer, they'll tell him the thermostat is working perfectly and that he needs to take care of his expansion problem. He's just finished working this out for himself.

 

[Ron20324]

For grins, I looked up what I think is my tempering valve on the boat. 125 max pressure, max input hot water, 200, max output 80 to 120 degrees.

IIRC, household water should be 120, but 140 if you have a dishwashing machine?

 

[Jim26m]

For grins, I looked up what I think is my tempering valve on the boat. 125 max pressure, max input hot water, 200, max output 80 to 120 degrees.

Good info.
If I recall correctly, 120 degrees will not burn an adult male unless exposed over 5 minutes (didn't look it up so - grain of salt...). It's a balancing act between hot enough storage to kill critters like legionella, and cool enough point of use so it doesn't scald the kids or impaired. Tempering valve such as you have is a good way to go. Wouldn't want to wash my hands in 200 deg, 125 psi water