Rinnai on demand T&P weeping

[Daniel Collick]

Model: RU180IN
Serial: 21PF.BA-134479
Temp set at 120 degrees
House water supply pressure has been verified using different gauges at a couple different locations.
Factory T&P failed as well as 2 others including Cash Acme and Watts.
Each T&P starts to drip.
It’s stupid but Rinnai isn’t being very helpful.
Any ideas because it comes up when I Google search the issue.

 

[WorthFlorida]

Is this a new problem? You should have an expansion tank installed when the Rannai was installed. A good suspect Is the expansion bladder failed and the tank is water logged.

Turn off the system and check the pressure of the expansion tank. The installation manual should recommend the required pressure. If water spits out if the Schrader valve the bladder failed and a new tank is needed.

T&P opens at around 150 psi. A pressure gauge with a garden hose fitting can be placed anywhere on an outside spigot or an opened drain valve.

 

[Sponsor]

 

[Jeff H Young]

Could you share the pressure reading? is the house water system open or closed? Expansion tank? Ptrssure reducing valve on the main? a dripping t and p valve can mean it actually works not nessesarily a failed valve.

 

[Daniel Collick]

This is not my install. Home owner installed it and wants me to make it my problem since we go to the same church.
It’s an open residential system, water heater is for domestic hot water supply only.
No circulation pump.
I checked pressure at the unit and at a hose bibb using two different gauges and consistently measured 60psi.
No PRV on the house.
No expansion tank. I always put ET’s on tank type, and even tankless when they have a circulation loop. It’s been a little while but things change, so if the manufacturers are now recommending expansion tanks on tankless w/no circ pump, that’s no biggie, it’s just news to me.

 

[WorthFlorida]

Place the pressure gauge on a drain valve for the Rinnai. Your problem is on the hot side but it was a good fine that your cold water side is constant.

Did you use a pressure gauge with a tattle tail needed?
Do you have city water or well water?
Have you ever checked the cold water temperature?
Do you know if the T&P weeping while hot water is being called for or after the hot water use has ended?

While the hot water is in use a pressure increase to open the T&P should never happen. I'm suspecting after the hot water demand is zero, the Rinnai is still heating the water for a period of time or the residual heat, with its heat exchanger being very small, cannot accept any kind pressure increase do to expansion. There is a chart somewhere on line the volume of water expansion VS temperature increase.