The takagi gas fired tankless water heater manual specifies a pressure relief valve on the hot water outlet.
Tankless water heater t and p valve.
So consult the manufacturer s installation manual.
The t p relief valve short for temperature pressure relief valve is a critical safety device on your 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.
Here s an excerpt from that manual.
This valve may sit quietly unnoticed on your water heater but it has a very important purpose.
Gas fired tankless water heaters will usually require a pressure relief valve while electric on demand water heaters may not.
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.
The valve is called a t p valve or tpr valve for temperature and pressure relief.
They are prone to failure particularly if there is any water condition hard water etc at all.
You shouldn t need an expansion tank on a tankless water heater.
To keep your heater from exploding.
I m betting the pressure is a little high and the act of water hammer is pushing the valve open a little.
On the side or top of your water heater tank is a valve connected to a metal or plastic discharge tube pointing downward.
This is just a pressure relief valve i used with a tankless water heater.