fourth year
This customer asked about a Watts 100XL. Unless your are the cheapest S.O.B. on the planet, you would not use that relief valve on an ASME rated water heater. And I doubt that you would install an ASME tank that is only rated at 100,000 btu. 100L t&p valves are almost exclusively used on domestic water heaters and SMALL commercial ones. Those heaters are ALWAYS rated at 150# unless you know of a renegade out there that I have never come across. 40 years ago the typical T&P valve that was installed was 125#, but the heater was still rated at 150 psi working pressure.
