I'm in the planning stages of a switch to hydronic heat for my home
which will replace my existing forced air heat. The system will
include a mix of radiant panels and staple-up under-floor tubing. The
heat source for this system will be my current Takagi T-KD20 tankless
water heater. It is rated for 20 to 185 MBTU input and 85% efficient.
The conceptual problem I'm running into is this: If, for instance, I
wanted to heat my master bedroom and bath on one zone, the calculated
sensible loss is 6,032 BTU/hr. If this were the only zone running, it
would come in significantly under the 17,000 BTU minimum (20,000 BTU *
85% efficiency = 17,000 BTU) the Takagi can put out. Wouldn't I run
into a thermal runaway situation? If there's more heat going into a
system than what is coming out it would seem so. Am I missing
something? Do each of my zones have to emit at least 17,000 BTU to
prevent this situation?
I've thought about adding a 20 to 40 gallon storage tank to the design
and a three way valve controlled by an aquastat. While each zone is
calling for heat the circulator(s) would run and, only if the water
temp in the system was below a certain threshold, would the flow be
routed through the Takagi.
I've seen some prefab plumbing designed for the application using a
thermostatic mixing valve offered by some online vendors. I have two
problems with that design though. First, I'm going to need water temps
approaching 180 degrees in order to get enough output in the coldest
winter months to meet the loss. Second, with a mixing valve it would
seem those kits are just bouncing back and forth between the hysteresis
of the flow sensors in the tankless heater. I would guess that would
cause frequent cycling of the tankless. That's where my storage tank,
three way valve and aquastat idea came from - similar concept to the
thermostatic mixing valve but extended cycle period.
Anyone have any thoughts?
Thanks in advance!