To keep your heater from exploding.
Tankless water heater t and p valve.
Gas fired tankless water heaters will usually require a pressure relief valve while electric on demand water heaters may not.
Here s an excerpt from that manual.
If all goes well a tpr valve never gets used except during testing.
In my application code requires a pressure relief but not a temperature relief t p.
This is just a pressure relief valve i used with a tankless water heater.
You shouldn t need an expansion tank on a tankless water heater.
This is one of those things that should never be needed unless something goes very very wrong with the tankless water heater.
My personal opinion of tankless heaters is that they are way too expensive to install and maintain never mind that few plumbers know how to service them.
I d check the pressure on the house before you go to the extent of changing the t p valve.
The takagi gas fired tankless water heater manual specifies a pressure relief valve on the hot water outlet.
They are prone to failure particularly if there is any water condition hard water etc at all.
I m betting the pressure is a little high and the act of water hammer is pushing the valve open a little.
This valve may sit quietly unnoticed on your water heater but it has a very important purpose.
On the side or top of your water heater tank is a valve connected to a metal or plastic discharge tube pointing downward.
Tankless heaters are required to have a pressure relief valve.