Preheating tankless WH or combi with solid fuel boiler

GroundUp

Super dumb question: will a tankless or combi boiler recognize the hot inlet water and resist firing if the water was preheated with a plate HX in a wood boiler loop? I do this all the time with heat-only boilers, with the TT interrupted by an aquastat on the OWB supply line, but with a tankless or combi operated by a flow switch I'm not sure what the proper channel is. Has anyone done this? The application in question is a Lochinvar Noble 110, but I can't imagine there would be much difference with any other tankless. TIA!

STEVEusaPA

Why wouldn’t you just dump the hot water from the boiler into a storage tank?

GroundUp

Because he doesn't want a storage tank, he wants a combi.

hot_rod

I feed my Cadet and Nobel pre-heated water, either from the solar or wood boiler. If the pre-heat drops below 110°F the Nobel fires up.
The flow switch always "makes" when it sees .5 gpm or more, but the burner will stay off on temperatures.
If you feed it say 90°, the combi kicks on low fire to make the difference to 120° or set point.

I have heard of excessively high temperature from a wood boiler to the mod con, boiler side, depending on piping tripping a high limit. See if yours has an automatic, or manual reset high limit.

Larry Weingarten

Hi, Tankless gas heaters don't often do well with preheated water if they have thermostatic controls. Electric tankless can do better. A problem happens when the supply water is close to the desired temperature because tankless gas cannot heat it just a little. Something with a big burner and a small tank (like the HTP Crossover) would probably do better.

Yours, Larry