How Much Heat Will An Underfloor Heat Transfer Plate Emit?

AlaskaDick

I have easily spent 50 hours going through Internet posts here (including some spirited discussions from 20 years ago) and elsewhere, Siegenthaler’s book, and seller’s information. A question that I’m still not comfortable that I can answer is how much heat I can reasonably expect to emit into a room from underfloor ½” PEX with heat transfer plates.

I found that there is pretty much universal agreement that heat transfer plates are desirable, if not necessary, but little is said about the variation in the style of plates or how their performance compares beyond extrusions being better than .016” stampings and a comparison between 4" x 1/16" extruded and 1/32" x 8" stamped plates. Some of the seller's claims can only be described as remarkable. The range of commercially available aluminum heat transfer plates that I have found for use with ½” PEX include –

• .062” thick extrusions, 3-3/8” to 6” wide
• .025” thick stampings, 6” wide
• Rauplate. .032” thick, 8” wide with two runs of PEX, one on each side
• .016” thick stampings, around 4” wide

According to the trial version of LoopCad, my worst case room heat loss is 27 BtuH in the living room. Room loses for about another 25% of the house is at 22 BtuH and 13 BtuH is the lowest room loss.

Floors are 1-1/8” plywood with plastic laminate, so about R1.9. Also according to LoopCad, on a -14 degree design day the worst case room would require an average floor surface temperature of 85 degrees with water temperature of 152 degrees. This is with 3-1/2" by 1/16" plates. Because of striping, maintaining an 85 degree average floor temperature could actually result in peak floor temperatures closer to 90 degrees.

I plan on 8” tube spacing and probably using a combination of plate styles, selected based on the room heat loss and ability of the plates to emit heat into the room. If I can’t get the needed heat into the living room I’ll supplement the in-floor heat by leaving in some of the old baseboard units operating at a lower water temperature, a panel radiator, or break out an electric radiator on the coldest days. I don’t really want to do any of these.

Can anyone suggest some good numbers to work from?

hot_rod

The amount 0f heat you can transfer into a space from a radiant surface, floor, wall, or ceiling depends on the temperature difference, period.

So a floor will give you 2 btu/ sq. ft per degree difference.

68° ambient air temperature with a 82° floor surface is 82-68 X 2= 28 btu/sq. foot of floor surface.

If you needed more btu/sq ft to heat the space, either increase the floor temperature or decrease the temperature in the space. So the high 20's btu/ ft is considered the maximum for a comfortable residential floor. Rooms with higher loads may need supplemental heat added.

Walls and ceilings can run a bit higher surface as you are typically not barefooting across them :) so their output is a bit higher. Here are the multipliers for walls and ceilings.

So the key is getting a much transfer from the tubing wall to the floor surface. this is whee you find some performance differences. The tube needs to be inntight contact in the plate and the plate needs to be tight against the floor material. This is why the extruded plates perform the best. The tube is tightly held, and the stiff plate contacts the floor without gaps.

We did some studies years ago with the RPA at KSU in Kansas.

Dale at Radiant Design has crunched some numbers also comparing various plates. a link to his results https://radiantdesignandsupply.com/new-page

The main way to show or compare various methods is wit FEA finite element analysis software. An example examples showing bare tube to transfer plates. The colors show the spread and heat intensity.

The floor temperature wants to be as consistent as possible to get that btu output. This is where the 12 or 18" on center jobs fail miserably. You have such a wide temperature spread. The other important aspect is the "available" floor space. Radiant under cabinets, appliances furniture with dust ruffles, etc has a penelity. Radiant energy is radiation, line of sight. Just like the suns energy coming through space. You only feel that when you are in the path or line of site.

So the decision usually comes down to cost difference vs performance difference. And this comes down to the amount of aluminum in the product. It's the material cost mainly, that seperates the cost of the various products.

I think the medium gauge ThermoFin are adequate. The heavier gauge are a tad bit wider also. Worth the extra$$? Hard to say.

I've found the Roth Radiant Panels with tube inseted 6" on center gives you the most consistent floor surface temperature. Warmboard comes close but is only 12" on center grooved.

Some testing I did many years ago in my shop.

In this IR pic, from left to right

1/2" pex tube in ThermoFin

Suspended pex, no contact

Staple up Onix rubber tube

Copper tube in ThermoFin

Warmboard

The back 1/2 of these 4X8 panels has carpet, no pad. Carpet reduces output, but does spread the heat across the surface a bit better.

Notice where I routered across the Warmboard and heat transfer stopped!

The metal staples are the hottest point in the rubber tube panel:) I call them heat transfer staples. Use a lot.

The 1-1/8" plywood and laminate is hitting you pretty hard if you need 157° supply?

DCContrarian

"The amount 0f heat you can transfer into a space from a radiant surface, floor, wall, or ceiling depends on the temperature difference, period."

This is so important that it's worth saying twice. It seems so obvious, but it wasn't to me, it took me almost 25 years to figure it out.

Bearing that in mind, I would like to recommend a different way of looking at designing your heat delivery. First, keep in mind that the entire purpose of a heating system is comfort (once you make sure the pipes don't freeze, I guess.) Sure you want it to be efficient, but the reason you're doing something more elaborate than a fire in the corner is for comfort.

In particular, floor heat is a very expensive way of heating your home. So if you're going to that trouble, design it to deliver the maximum comfort. In particular, do not assume that the design that delivers the maximum comfort will be the same one that heats your house on the design day. It almost certainly won't be.

Decide what floor temperature gives the comfort you want, for most people it's about 80 to 85F. Size your system so that on just about every day of the winter the floor can be at that temperature without the house overheating. Then add supplemental heat, whether it's radiators or convectors or whatever, for the coldest days.

If you try to size your floor to meet your heating design temperature — the 99th percentile temperature — with a temperature that is comfortable to walk on, most of the time in the winter the floor is going to be cool to the touch.

EdTheHeaterMan

How Much Heat Will An Underfloor Heat Transfer Plate Emit?

is the question, and I hate to state the obvious, but in my opinion it will transfer about one half as much as two heat transfer plates.

I like to look at things logically with an eye for accuracy. And I believe that @hot_rod may have the best answer, but a bit longwinded.

Hope this helps.

Mr.Ed

hot_rod

unless you have loads in the low teens, the floor should always feel slightly warmer than ambient temperature

A room with a 10 btu/ ft load would only need a 75 degree floor temperature. That is at design condition!

If the entire home has those low btu requirements, I don’t know that radiant floors will get you the “warm toes”experience Go with radiant walls or ceiling. Or panel rads,

prevent overheating by reducing the SWT with reset controls,

Any time the floor surface is 85 heat will be emitted. Up until the space is 85f

Hot goes goes to cold, always Another Holohan parable

AlaskaDick

Thanks for the comments.

Hotrod - I think you meant to include the photos in this post, I found them when I was doing a search today. I hadn't seen them before but it is good information. It appears that these were the ones that I couldn't see in some of your old posts because they had been deleted.

https://forum.heatinghelp.com/discussion/159018/an-infrared-look-at-various-radiant-installation-methods

"The amount of heat you can transfer into a space from a radiant surface, floor, wall, or ceiling depends on the temperature difference, period." Yes. In a case like mine where it is questionable whether enough heat can be passed through the subfloor, not only the average heat, but limiting the maximum localized heat in the stripes becomes important.

I decided to take a fresh look at things. The 1-1/8" subfloor is sound plywood that I installed when I did a major remodel and replaced all the subfloor 23 years ago. I see that 1-1/8 plywood is used in the commercial radiant panels, so why couldn't I do the same with my 1-1/8" subfloor? With some more research I found that Hotrod and Siegenthaler have both mentioned this scheme. Instead of putting the PEX and plates under the floor I'm looking at the option routing grooves in the 1-1/8" flooring and putting the PEX and heat plates on top.

The disadvantage is that I will have to remove, and possible scrap the current floating floor in about half the house. I know what's involved in replacing it - I was the one who installed it. I have two rooms in which the laminate hasn't yet replaced carpet so they can be a test bed.

Advantages are that I will have to buy less aluminum (with freight to Alaska) and half as much fiberglass insulation. I will also have a more control over tube spacing as I won't have to deal with joist spacing and should be able to keep the floor temperature more consistent in my worst case room to slightly raise the average temperature. A major advantage is that the installation would be above floor instead of in a 18" to 24" crawlspace.

Plan B is starting to look really attractive, especially the part about avoiding a couple of weeks of work of my back in the crawl space.

DCContrarian

"A room with a 10 btu/ ft load would only need a 75 degree floor temperature. That is at design condition! If the entire home has those low btu requirements, I don’t know that radiant floors will get you the “warm toes”experience"

I think we're making the same point.

Note that you're only at design condition for 1% of the time, 99% of the time you're below it.

10 BTU/sf for new construction built to the current energy code is completely reasonable. Of course it depends on your climate.

I would argue that the solution is to limit the square footage of heated floor. Only do the floors you're likely to be barefoot on, ie the bathrooms. Size them so that on an average winter day they can be on full blast. Then install a radiator or even — gasp! — a heat pump* to keep you warm at 3AM on that 99% day. Use a two-stage thermostat with the floors as stage 1 so the floors always come on first and the other heat only comes on if the floors aren't enough.

*(My current recommendation is to size a heat pump and ductwork for your cooling load, then add hydronics strategically to cover the difference between what the heat pump can deliver in heating mode and your heating load. Then prioritize the hydronics with a two-stage thermostat so mostly you get that sweet hydronic heat.)

AlaskaDick

I'm replacing 23-year old cast iron boiler and baseboards so I can replace it on my schedule rather than the boiler's. Decided to go with a mod-con and radiant for several reasons, including that the utility says they are going to have to start supplementing gas from local wells with gas shipped in on boats by 2027. Not ready for a heat pump yet, but radiant will give me that option in the future. Design temperature is -14, but I've seen colder than -20 a few times. Something that may skew the heat loss calculation is that when the temperature is zero or below it is usually dead calm. An open window and a fan takes care of cooling except on the hottest (about 80 degree) day. The house is fairly tight and well insulated. I did a lot of work a few years ago to get a 5-star rating. The reason my problem room needs so much heat is because two of its walls are exterior and there are quite a few windows. They are good windows, but they are still windows. Heavy drapes would help in lieu of the honeycomb mini-blinds, but I hate drapes. I may leave in some of the baseboards to cut in as supplemental heating for at least for the first winter to see how well the system performs. I'm not so much looking for a warm floor, but to get rid of a drafty feeling in some parts of the house (room air, not the floors) when the air temperature is 72 degrees.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1800536#Comment_1800536

It is a messy time consuming job, but certainly you could groove the 1-1/8" plywood. Is it true plywood not wafer type board?

I would do a 3/8" tube 6" on center. What flooring will go back over the "on top" system. You would need to deal with the .050 thickness of the plate, even if you routered in a U fin. The tighter the tube spacing, the lower the supply water required and that helps a lot with heat pump options. the lower the SWT the higher the COP.

I/m not sure if there is a 3/8 U fin, unless you go with the thin gauge flashing type plates. But those would need a thin wood floor nailed over them to keep then from moving and making noise.

The pic on the right is Warmboard, 12" oc

hot_rod

Maybe I misread this statement you posted. No way you can maintain a 85 floor temperature across the heating season? Low load rooms would have a small portion of the room as a radiant panel, that is not practical?

Decide what floor temperature gives the comfort you want, for most people it's about 80 to 85F. Size your system so that on just about every day of the winter the floor can be at that temperature without the house overheating. Then add supplemental heat, whether it's radiators or convectors or whatever, for the coldest days.

AlaskaDick

Yes, real plywood. My current thinking is to top it with 1/4" plywood. That would actually take the floor to it's original height when it had a low pile carpeting (I went to laminate to accommodate my wife's wheelchair) and would give a flatter surface for the laminate. One seller offers 6" x 22 gauge/.025". I was thinking 8" centers. What do you think of that for my worst room and probably narrower plates or wider spacing for the less demanding rooms? I'm open to 3/8" tube, but it looks like the choices for heat transfer plates for that size is pretty limited.

hot_rod

No problem getting 1/2" tube to 8 or even 6" spacing. The A pex seems to be the easiest. Although I hear PERT is very easy to work.

Yes the spacing could vary based on rooms with higher loads. LoopCAD should be able to allow you to play with spacing and see how SWT is changed?

.025 you could bury below the 1/4 ply without furring around all the plates. The extruded plates would be too thick to just plywood over without recessing them.

Do all you can to keep the maximum SWT below 120F, if you have a HP in your future.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1800556#Comment_1800556

OK, I'm going to try and explain. For the purpose of illustration, imagine a house in the land of my birth, Suffolk County, MA. Heating design temperature is 12F, cooling design temperature is 88F. The coldest month of the year is January, where average temperature is 29F with an average high of 36F and an average low of 23F. Imagine this house is 3,000 square feet, it needs 15 BTU/SF/hour at design temperature to hold 70F indoors, or 45,000 BTU/hr.

I'm going to throw out two straw men before my final proposal. The first straw man is to put heating under all the floors in the house. Ignoring for a moment the feasibility of putting heating under 100% of the flooring, to get 15 BTU/hr/SF you need a floor temperature 7.5F above room temperature, or 77.5F when room temperature is 70F. That heating system would be very nice — silent and invisible, like it's not there at all. (That's the way heated ceilings are, BTW). But it would also be really expensive, and you wouldn't feel anything, and you might say you want heated floors you can feel.

Straw man two would be to say I want floors I can feel, let's heat them up to 85F. At 85F they're going to put out 30 BTU/hr/sf, so to get 45,000 BTU/hr you only need 1500 SF of heating. Sounds good. But they're only going to be 85F when it's 12F outside, and that's only 1% of the year. On a typical January day, you only need 26K BTU/hr during the day when it's 29F out, and 36K BTU/hr at night when it's 23F. That works out to 17 BTU/hr/sf and a floor temperature of 78.5F during the day, and 24 BTU/hr/sf and a floor temperature of 82F at night. That daytime temperature is going to be right on the edge of perceptibility.

My proposal would be to shrink the heated floor area even further so that it can run more of the time at its hottest. Let's say we do 800 square feet of heated floor, at 85F and 30 BTU/hr/sf that's capable of 24K BTU/hr. The heating load of the entire house is 24K at 39F, so that floor can run at 100% for basically the entire month of January without overheating the house. At a floor temperature of 80F and 20 BTU/hr/SF that floor puts out 16K BTU/hr, which is the heating load when it's 49F, so the floor can be 80F or above for almost the entire winter without overheating the house.

But that floor is only capable of 24K when it's on 100%, so you need to come up with another 21K for times when it's below 39F out. There's lots of ways to do that. One way: this house is going to have air conditioning, summer design temp is 88F. So there's 58F of heating and maybe 13F of sensible cooling, throw in latent, solar gains and occupant gains, this house is probably going to have a 2.5-ton HVAC. I like the Mitsubishi M-series, something like this:

https://ashp.neep.org/#!/product/34583/7/25000/95/7500/0///0

which has a maximum heat output of 32K BTU/hr at 5F, it could pick up the rest of the heat you need.

But Massachusetts has really high electricity prices, and you're already installing a boiler, so my recommendation would be to get all of your heat from the boiler. One way would be to put a hydronic coil in the ductwork because you're installing ductwork too. Another would be to install radiators, and a third would be to install more hydronic loops.

But the key to making this work — always having those warm floors — is blocking the other heat source, whatever it is, from coming on unless the heated floors are on 100% and you still need more heat. That's where the two-stage thermostat comes in. This guarantees that in mild weather the heated floor is the only source of heat and can run full out.

The other key is being strategic about how you place those heated floors, places where you're likely to be barefoot but also spread around the house enough so it can be the only source of heat in mild weather. Bathrooms are a good place to start.

AlaskaDick

Thanks for everyone's comments and suggestions and with helping me to think this through. To summarize where I am -

For my problem room my goal will be to keep the floor temperature as consistent across the floor surface as possible, i.e., keep the hottest areas directly above the tube at the beginning of a circuit as close as practical to the coolest areas between the plates at the end of a circuit. This will allow the highest average floor temperature while avoiding uncomfortable hot spots.

• Use tube spacing and wide heat transfer plates to keep striping to a minimum.
• Use circuit length and flow to keep the delta t of the circuits relatively low so the average temperature of a circuit is kept close to the beginning and end temperatures of the circuit.
• Start the circuits along the exterior walls where the temperature can be a bit warmer but where no one is likely to walk.
• Put the tubing and heat transfer plates above the floor in routed grooves and take advantage of the (overbuilt) 1-1/8" plywood subfloor as a place to put it. This will give me a lot of flexibility in tube placement/spacing. It will also make the system more responsive by removing the plywood's mass and R-2 insulation value of it between between the tubing and floor surface.

One question - I told my son, who is a remodeling contractor and has installed quite a few floating floors, that I planned on putting 1/4" plywood over the plates and tubes. He asked why he couldn't use a thin layer of floor leveler without plywood. This would be his first experience with this type of installation. My suspicion is that the leveler is pretty inflexible and the flexibility of the plates and the temperature changes would be lead to the leveler cracking. Any thoughts?

hot_rod

You can pour gypcrete to a feather thin edge. But I have never tried it over aluminum plates. I'd expect some hairline cracking, so the final flooring would want to be a floating type I suspect.

Gyp would give you a small amount of mass, some fire and soundproofing if that matters. The psi of the mix has some effect also.

One of the counterflow serpentine tube methods helps even out the temperature spread. A bit more complicated to lay out.

The Uponor design guide shows some options.