Efficiency of Radiant ceiling panels vs in-floor hydronic heating
Hi all, I’m hoping for a discussion on the relative efficiency of delivering heat using hydronic radiant ceiling panels vs in-slab hydronic, cost aside. That is, assuming the same BTU output, which is more efficient at making the occupants comfortable?
Some details of the project:
-Zone 4 marine
-2-story 2400 sqft single family residence for family of 5, tight home, well-insulated with 2.5” thick slab with r-20 sub-slab insulation. Heat loss was estimated at 18k btu using BetterbuiltNW HVAC sizing tool.
-Ceilings are 10’ high, exposed timber frame beams with 2×6 doug fir decking (the panels would fit between the beams).
-We plan to install a Sanco2 heat pump water heater (110 gallon tank), leveraging the extra BTU’s with either a Taco x-pump block heat exchanger or the Harvest Pod to add the hydronic system.
-1st floor would be in floor hydronic. 2nd floor would be low-temp radiant ceiling panels in each bedroom. There will be a single mini-split for cooling, and back up heat. There will also be a small wood stove for ambiance and power outages.
My concern is that in-floor hydronics on would have obstructions (carpet, furniture, shoes) that would reduce the comfort compared to ceiling panels, which do not have obstructions. On the other hand, they are farther away (10’ ceilings). My understanding, from John Seigenthaler’s articles, is that radiant ceiling panels are in an ideal location to radiate all the surfaces we tend to touch…tops of beds, chairs, couches, etc. It seems the floor would only radiate to bottoms of things, or the ceiling, or people as they pass by. I appreciate your thoughts!
Adam
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I thought the Sanco was limited to a lower output then that heat loss?
From their literature:
1. Maximum heating capacity must be less than 8,000 BTU/h.
2. Minimum design ambient temperature must be above 27°F.
3. Tank capacities of the 83 Gallon or 119 Gallon tank are required for performance.
4. DHW usage – minimum usage of 25 gallons per day.
Thanks Paul.
I’m near Seattle where the design temp is 27.
Yes, The folks at Foursevenfive have told me that using the Taco x block pump I can get an additional 8k btu, so I included a mini-split for back up heat.
But if I use the Harvest Pod, I’m told I can get much more.
We’re a family of 5, minimum use should be no problem we will be using the 119 tank.
But, assuming all this is not an issue, I’m interested in your thoughts about ceiling panels vs floor heating.
Adam
I would suspect that either would have extremely marginal comfort improvements but the best way to evaluate them would be to search for an Airbnb or something similar that has it so you can experience it yourself since it’s so subjective.
The ductless option is odd: if hydronics is the most comfortable solution why compromise with a ductless unit over a true air to water heat pump (not sanco2)? Or if the ductless provides comfortable heating then why bother with the complexity?
Fair question, i think we’re just trying to minimize the need for forced air movement due to comfort issues. Granted, if we want cooling there will have to be some air movement. Thus the ductless is more of a necessity.
IMO your concerns are not relevant to a high performance building. If you were struggling to provide the BTUs to heat a space with the available linear foot of tube in the floor, then things such as furniture and carpet would be limiting factors.
However this will not be the case in a high performance building. You will have either have so much tube that you would never notice that the floor is warm, or be able to put tube where it is unlikely to be obstructed
One of the advantages of radiant floors is that heat rises, or rather hot air rises, and the floor being the bottom, the things it heats create hot air which rises.
A room with a radiant floor is less likely to have a large temperature gradiant, IOW, 80 degrees at the ceiling where no one cares and where it provides a larger Delta T to the outside roof.
While obviously radiant ceilings work, I just don't understand the efficacy of heating your ceiling to 110 degrees or so when 12 inches away it is 10 below zero.
Idronics 23 goes over hydronics in great detail including radiant ceilings: https://idronics.caleffi.com/magazine/23-heat-transfer-hydronic-systems
"Efficiency" really isn't the right word when you're talking about hydronic systems, all of the heat they produce is released into the house, they're 100% efficient. I think what you mean is "effectiveness."
"Radiant" isn't really the right term either, at the low temperature that you typically see with floor and ceiling heat there isn't much radiant heat transfer, most of the heat is transferred by conduction to the layer of air adjacent to the heated surface and then transferred to the room by convection. Radiation works the same in all directions, but convection is highly directional, and the result is at the same temperature a heated floor produces about three times the heat output of a heated ceiling. The widely accepted figure is that a floor produces 2 BTU/hr/square foot for every degree of difference between its temperature and the surrounding air, John Siegenthaler says for a heated ceiling the number is 0.71 BTU/hr/SF/degree. I don't know where he gets the two digits of precision.
So for the same output you're looking at three times the area, which is probably going to end up as three times as expensive. There are a few benefits of heating the ceilings instead of the floors though. One is that typically you're able to heat more of the ceiling, you don't have things like cabinets in the way. And you don't put things like rugs and furniture on the ceiling, so you don't have to worry about them blocking the heat. Drywall tends to have a much more predictable fastening pattern than floor coverings like hardwood, so your chance of puncturing a tube is less. And if you need more output you can crank the heat up on a ceiling much more than on a floor.
I'm mainly reiterating what's already been said. The amount of radiant energy coming off the ceiling panels is NOT going to make surfaces like bed tops feel warm. Heck, I have radiant heaters in our bedrooms that generate orders of magnitude more radiant energy than that and you can't feel it on surfaces. You can barely feel it on your bare skin. You're not going to feel anything even on your bare skin with those radiant panels. Think of it this way, can you feel the heat from someone standing a few feet away from you? Because that's a similar amount to what's going to come off the ceiling panels.
The whole concept of "radiant" heating providing more comfort is outdated. It's a factor in a poorly insulated, poorly air sealed house. In a well insulated and sealed house, the amount of heat needed from the system isn't going to bring a noticeable warming effect.
>The whole concept of "radiant" heating providing more comfort is outdated. It's a factor in a poorly insulated, poorly air sealed house.
Exactly. I like to say an ounce of insulation is worth a pound of hydronics. And I'm a hydronics fan-boy.
I appreciate the feedback from everyone. It sounds like I shouldn’t expect a warm and toasty feeling from hydronic heat in a tight, well-insulated house regardless of where it’s located. But I can expect to be comfortably warm, and not have warm air blowing on me I suppose. Now it’s starting to feel like an expensive luxury, although I’ll be installing the distribution portion myself.
I think I’ll just be practical and avoid ceiling panels downstairs where it would cover up the nice doug fir, and floor heat up stairs where the beds would obstruct.
By the way, the slab ill be installing is actually an earthen clay floor, which “should” work nicely with hydronic heat.
Hydronics is 100% luxury! But if you're paying, good!
Not sure why a clay floor would work any better (possibly worse) than a concrete slab. Big thermal mass means slow to change temp, which is a bit of an issue at times.
Yep. And what you really want in a heated floor is high conductivity, not sure how clay rates in that regard. And because of that high conductivity you really need insulation below the floor.
Looks like clay has a similar Thermal Emissivity to concrete (>.9) so I guess it will store plenty of heat. But it’s only a 2” slab, so my hope was that it would be just enough thermal mass to even out the swing in temp at night.
I can appreciate now what your response would be to this, rightly so, that there shouldn’t be much of a swing in a well insulated tight house. Hmm.
I’ll say this though, clay, like concrete, or marble countertops, feels cold to the touch even at room temperature. So I worried that without hydronic heat it would be uncomfortably chilly on the feet. I’ve accepted the fact that it won’t feel warm on my toes, but paired with the “leathery” feel of clay it could be dang comforting.
The emissivity is irrelevant, a surface at the temperatures typical for heated floors or ceilings loses little heat through radiation. What you care about is conductivity and heat capacity.
You want a surface with high conductivity, so it warms evenly, and with low heat capacity, so it is responsive to changes in your thermostat.
And you really want insulation between it and the ground, otherwise you're paying to warm the ground.
I have heated ceilings in seven rooms of my own house. They feel like ... nothing.
I can't tell when they're running, they're silent and the heat isn't noticeable. The temperature inside the house is incredibly even, it's unlike any house I've ever lived in. I have to keep an indoor/outdoor thermometer next to the front door because I can't tell from the inside whether it's 30F or 70F outside.
A big part of this is the house, it was built in 2022, better than code in terms of insulation, air tightness and windows. So there just aren't cold spots.
DC,
I wonder whether comfort (maybe that's not the right word) can be entirely defined by lack of sensation? There is pleasure in feeling heat from things like sitting in front of a fireplace, or in a sunny bay window. Maybe comfort is the lack of negative sensations, as opposed to no sensations?
Malcolm, you raise a very good point.
The whole point of HVAC -- the whole point! -- is comfort. I mean, I guess there's some utility in not having your pipes freeze, but the reason we heat houses to 70F and not 40F is because it's more comfortable. And comfort equals money: how comfortable do you want to be?
And the science of comfort isn't entirely understood. Some aspects of it, like temperature and humidity, are well-understood and easily modeled. But when you start getting into intangibles like the feel of moving air or sound level there aren't good tools for modeling.
"Radiant" heat has always sat at the edge of science and pseudo-science. I put "radiant" in scare quotes because the name itself is a misdirection, radiation has little to do with it. In the early days there was a frequent claim that heated floors saved energy over conventional heating systems because they delivered superior comfort with the same amount of heat, or the same comfort with less heat -- and it's all about comfort. As far as I can tell those claims have never been proven and perhaps have been debunked. And when people start making wild claims for "radiant" heat, pseudo-science fellow-traveler words like "emissivity" and my personal bugbear, "thermal mass," can't be far behind.
DC,
I specified radiant cove heaters in one house when everyone was talking about them here. I wouldn't do it again.
I have a Taco XPB supplying heat from the DHW tank to my Warmboard system. It is setup for outdoor temperature reset to lower the output from 130F to 80 to 95F. Very comfortable. The house is micro zoned with each room being a separate zone. Typically the two bathrooms heat the second story. They are set to 72F, while bedrooms are 69 F. Similar pattern on the first floor, the porch and foyer supply heat to the rest of the house. The tile floors in the bathroom are warm, because the bathrooms are functioning as stage 1.
I would have your installer run a LoopCAD simulation to project your surface temperatures. A simulation may help quantify the comfort vs. cost design tradeoff.
I have a very similar setup -- heated floors in two bathrooms, outdoor reset and staged thermostats where the bathroom floors come on first. For at least half the winter the bathroom floors are the only heat on in the house. Second stage is the ceiling heat, third stage is the air handlers.
DC brings up a good point, I have my air handler circulate for 10 minutes every hour during winter. It is not staged, but the house is more comfortable. Robert Bean suggested this in Healthy Heating.