What can cause a tempering valve to fail?

Impurities and inconsistencies in the water itself can affect the valve. This includes hard water, unfiltered water, dirty water, and gunk in the water supply.

Tempering valves are covered under the parts warranty as they are supplied with the heat pump, for a period usually of 1-2 years (depending on manufacturer). However, be assured that any plumber can replace a tempering valve if it is outside of warranty.