Why must a mixing valve be installed

stevel87125

below the coil in a tankless coil boiler vs above the coil?

GGross

because the temperature held in the heat exchanger could be high enough to create extreme temperatures on the DHW side.

For example I have seen first hand a mod/con combi boiler that was set up to heat baseboard high temp, when it would switch to DHW the boiler fluid was hot enough that we measured over 140 at the tap for a period of time

pedmec

to prevent scalding of the people using the hot water

stevel87125

I should say, why below vs above the coil

bburd

stevel87125 said:

I should say, why below vs above the coil

Because below the coil, the mixing valve and connecting piping will stay cooler than above, since heat rises.

stevel87125

Could the heat build up in an above the coil installation, that it becomes too hot for DHW, even for an instant?

EBEBRATT-Ed

I don't know the exact reason. But that is what the installation instructions say. I was told installing it below the coil makes the thermostatic element last longer.

mikeymo

Should you put a one way valve below the DHW and before the mixing valve?