Hydronic loop sizing


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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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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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.

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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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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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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snipped-for-privacy@gmail.com wrote:

I am looking at a website right now that is heating the slab with 83F input water. The exit temp of the water is about 71F. Flow rate thru each zone is about 3GPM. Total flow rate is about 9GPM. Slab temp is in the upper 60s with outside air temp at 31F. The heat source is putting roughly 30,000BTUH into the slab
The owner's manual for the Tagaki tankless that I am reading now says that it will monitor the temperature of the water it outputs and adjust the burners to LIMIT the output water temp to 120F. You can set the output temp to be as low as 105F.
Seems to me that setting the flow rate thru the radiant loop to 3GPM, and setting the tagaki to 105F, the Tagaki will intermittently fire to keep its output temp near 105F, until your slab temp sensor turns off the circulator pump. Even setting the temp to 120F, the factory setting, will accomplish the same goal.
After all, you really want your slab temp near 70F, not 90F. Restaurants in Korea put a shot of hot water thru the slab where you sit down. This ONLY warms up the slab to the upper 70s, and then it cools as you sit there.
I agree with your critics, you have focussed on the heater characteristics without considering the safety factors built into the heater. You have focussed on the characteristics of the radiant loop without considering the characteristics of the loop circulator pumps.
You need to expand your view!!!
180F temps are POSSIBLE out of the tagaki, but the unit would be defective if it permitted such a thing to occur
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But is the Takagis flow rate, or gpm enough. It has a 3/4 or maybe 1" piping, My Bosch Ng Tankless is 3/4, Boilers are not that small, my boiler is 2" output- input pipe
I lived with Radiant heat, never were the floors Hot as you think is necessary. Never should they be hot. But if you are trying to save on utilities you wont.
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We're getting ready to build a new detached shop and we're going to "tube up" the slab for radiant heat. Could you give me the links to the sites you are looking at? I need an online supplier for the tubing and as much education as possible. We are going to do this ourselves. Hopefully we'll drive it with solar panels in the future.
--
Steve Barker



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Steve Barker LT wrote:

www.ourcoolhouse.com
Radiant loop is powered by a ground source heat pump
The issue with solar is daytime use only. Detached shop OK. Maybe not out there every day. Probably not out there before sun up, and probably back to the house before or shortly after sun down.
Be sure to check out insolation rates for where you live and size the number of collectors to be able to meet a target 70F approx slab temp in winter.
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Robert Gammon wrote:

and to the OP
The Tagaki is likely to fire for 30 seconds to get the water temp up to 120F and then shutoff while you continue to circulate the water. After 30 to 90 seconds of circulation, the water temp has dropped low enough that the Tagaki will fire again. Remember the Tagaki detects BOTH flow and temperature of its outlet water.
Keep in mind that the circulator pump will want to move the water much faster than 1 GPM.
You will use ALOT of gas to do this, even though Tagaki does document the application in their guide.
As a radiant heat source, I think a tankless will work better as the heat source for driveway deicing than for keeping your slab warm.
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...
That's exactly the type of info I was looking for. Thanks! I though it may work like that, but needed confirmation.
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Thanks for the info. We're not trying to heat the place per se, but just enough to keep the slab above freezing as to prevent heaving and cracking. It'll just be a storage shed / occasional truck work type shop for private use.
--
Steve Barker




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snipped-for-privacy@gmail.com wrote:

As noted you need more info, study more.
When you run hot water through any kind of 'radiator' its temperature drops, losing its heat to the air. If it cannot lose its heat, the water simply stays hot, the boiler[wh whatever] will only heat it to the set point then it will shut off.
Again, not being rude, I installed radiant tubing, but had a pro do the design. I know enough to know that you do not know enough yet. Yet.
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