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Posted by on October 14, 2006, 3:08 pm
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!
Stan
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Posted by m Ransley on October 14, 2006, 3:23 pm
I dont think the pipe can take more than around 115, you dont want 180f
under the floors. Your heater can put out 185000 btu but to just heat
water for hydromic it might only use 50000 btu. You need to research
this with the vendors and reading.
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Posted by on October 14, 2006, 4:43 pm
> I dont think the pipe can take more than around 115, you dont want 180f
By "the pipe" I'm guessing you're referring to PEX. Standard PEX (ASTM
877/876) can take 180 deg at 100 psi. PEX-AL-PEX (ASTM 1281) can take
180 deg at 125 psi. My 180 degree max is based upon the rating for the
radiant panels (which generally use copper pipe, not PEX). But none
the less, the issue of my target temperature was not what I was asking
about. The design temperature is irrelevant - or rather only relevant
for the density values used in calculations. This is a heat-in vs.
heat-out situation.
> under the floors. Your heater can put out 185000 btu but to just heat
> water for hydromic it might only use 50000 btu. You need to research
> this with the vendors and reading.
Believe me, I've done plenty of research. If I wanted to be told to
"do more research" I would have posted this in alt.hvac! :-)
To "show my work" so to speak, using the 6,032 BTU/hr loop mentioned
above, if I design it with a 180 deg entering temp, and 20 deg nominal
temp drop, it would need to flow 0.62 gal/min. That is actually below
the Takagi's turn on threshold of 0.75 gal/min. If I go with 1.0
gal/min I get a resulting drop of 12.4 degrees. When the water returns
to the heater, if it is getting 17,000 BTU minimum (I'm assuming that
rating is in BTU/hr) put back into it, the resulting temp increase
would be 34 degrees, thus an exit temp of about 201 degrees. If this
cycle simply continues there would be thermal runaway.
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Posted by Edwin Pawlowski on October 14, 2006, 5:32 pm
> To "show my work" so to speak, using the 6,032 BTU/hr loop mentioned
> above, if I design it with a 180 deg entering temp, and 20 deg nominal
> temp drop, it would need to flow 0.62 gal/min. That is actually below
> the Takagi's turn on threshold of 0.75 gal/min. If I go with 1.0
> gal/min I get a resulting drop of 12.4 degrees. When the water returns
> to the heater, if it is getting 17,000 BTU minimum (I'm assuming that
> rating is in BTU/hr) put back into it, the resulting temp increase
> would be 34 degrees, thus an exit temp of about 201 degrees. If this
> cycle simply continues there would be thermal runaway.
>
I think your work needs work. Why would there be a thermal runaway? Does
the Takagi not have an aquastat to cycle it off if heat is not needed? If
not, it is the wrong appliance to use for a heater.
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Posted by m Ransley on October 14, 2006, 5:57 pm
Even for you to consider 180f input is crazy, Think Cooking your feet
unless your in bed. I also wonder why you are doing this, you can get a
94% efficient furnace Vs your 85% takagi, a Vsdc blower motor that will
consume on low less than your planned pump. Can the 3/4 or 1" output
handle the gpm you need to pump through the house, If you think you will
need 180f something in your plan is wrong, radiant heat is to be under
115f, not cook you at high temps, to consider that 180 is the Max
rating as you say, well that is the maximum and not wise to continualy
stress the pipe, even if you wanted cooked feet for dinner. Do you have
pets, because unless they live on the furniture all day and night they
will be dead in a short time
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