Tankless Heater Piping

CLamb

This diagram is from a Weil-McLain SGO-3 manual. Could someone explain to me why it specifies the 12" height difference between the cold water feed to the coil and the cold water feed to the mixer?

MikeSpeed6030

Probably to help prevent gravity flow of hot water back into the cold water supply.

kcopp

The older mixing valves being exposed to the super hot water would tend to shorten their life and cause warm water to migrate out. The newer asse rated valves have check valves in them. However it is still good practice to do so.
That my 2 cents.
In all honesty tankless coils in steam boilers are really energy hogs and I try to get customers to not have me install them. That being said I just did one this week....

JohnNY

Once in a while you've got little choice but to reinstall a tankless coil but I usually push for a storage tank, which can be an electric water heater (wired or unwired), and pair the two with a good shuttle pump.

That 12" drop is called a thermal trap or a thermal loop. It's there to help prevent heat migration. Of course, if you're using a recirculation line, the loop doesn't help much.