Noritz NRC1111 pressure relief

[can200]

Just had this tankless water heater installed and have had two floods from pressure relief valve opening. The second flood occurred after we replaced the relief valve with a new one and also put an expansion tank on the cold water side about 7 feet away from unit. 7 feet because this is a location where I can hide it as I finish the room in the future as a basement bathroom. Plumber wants to move expansion tank closer to the tankless as the next move. This is a fine idea except I will not be able to hide the tank and something very unknown and mysterious is going on here and I am about to return the tankless and go back to tank. Any ideas on why my pressure relief valve continues to release water and the solution?

 

[Caduceus]

First, the T&P should always discharge to a safe place where flooding wouldn't occur or a scalding risk wouldn't be presented if it were to serve its purpose.
Second, the plumber should look into the obvious; Temperature, pressure or both. If the temperature is normal then test the pressure and expansion of the system using a gauge (this is advice for you and the plumber). The thermal expansion tank will also need to be charged to the buildings static water pressure to function appropriately.
If there is a recirculating loop on the piping system and a check valve separating the loop from the cold supply, the expansion tank will need to be within the loop.
That's where I would start.
If you could post some pics, maybe we could see something that is being missed.

 

[can200]

70 psi coming in, 70 in expansion tank, no recirculation, not sure how to check temp. Other plumbers have reported success solving these issues with an expansion tank even though they generally regarded as not needed for a tankless system.

 

[Caduceus]

As a reminder, the thermal expansion tank reading must be taken with the water shut off and pressure relieved from the piping system.
Is the T&P releasing while water is in use or when the tank is static?
I understand that most will agree that a thermal expansion tank is not needed and your pressure is at 70 psi, so you should then look into temperature. Is the tank over heating the water? Is it continuing to heat after flow has stopped?
A little more troubleshooting is going to be needed.

 

[KULTULZ]

So what is being said is that there is no or little water thermal expansion with a tank less water heater, and that negates the need for an expansion tank (on a learning curve here)?

The system pressure needs to be measured over a twenty four hour period to discover any pressure surges from heating events (considering it is a closed system).

The tattle-tale hand on the pressure gauge will show any surges over a time period.

 

[phishfood]

As a reminder, the thermal expansion tank reading must be taken with the water shut off and pressure relieved from the piping system.
Is the T&P releasing while water is in use or when the tank is static?
I understand that most will agree that a thermal expansion tank is not needed and your pressure is at 70 psi, so you should then look into temperature. Is the tank over heating the water? Is it continuing to heat after flow has stopped?
A little more troubleshooting is going to be needed.

What he said....

Something doesn't seem right here.