How Much Heat Will An Underfloor Heat Transfer Plate Emit?

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.

"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.

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.

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?

I've done some experimenting using a mockup of my above floor heating system and thought I would share what I found.

I will be routing grooves in the existing 1-1/8” plywood subfloor for above floor PEX. The information I was looking for was a performance comparison of .024” X 6” wide heat transfer plates to .016” X 4” plates. I had already ruled out an under floor installation because the subfloor is fairly thick for the amount of BTUs per SF that it will have to be emitted to keep up with the loss on a -14f design day. I had also ruled out extruded plates due to the greater difficulty of installation and cost.

The mockup is 3’ long and 16” long. It is stacked up of 2” (R-10) of extruded polystyrene with ¾” of plywood with another ¾” of plywood cut into furring strips screwed down to allow grooves for the tubing. There are two, parallel 2’ long sections of aluminum heat transfer plate. The centers of the plates are spaced at 8”. One plate is omega style, .024” thick and 6 inches wide. The other is C style, .016” thick and 4” wide. ½” PEX is inserted in the plates. This is topped by ¼” plywood, screwed down, and .220” laminate flooring.

Examples of the heat transfer plates. The wider one is painted, brown on one side, white on the other. The narrow plate is shiny mill finish on one side, shiny anodized blue on the other.

Under the area not covered by the plates is .039 inch/1mm foam underlayment. My rationale for this is that it will act as a thin furring strip and prevent small waves in the floor where the ¼” plywood is pulled down tight by screws.

Although it’s not in the mockup, there will be a layer of .039”/1mm foam underlayment between the ¼” plywood and laminate flooring.

At the ends of the two lengths of ½” PEX tubing the flow is split by tees to keep the flow through both the same. The water source is a temporary connection into the recirculated DHW line going to the house. Temperature varies between 113 and 129 degrees as the water heater cycles on and off. Since my interest was relative performance rather than specific temperatures, the temperature swing isn’t a problem. The PEX is snug in the omega style plates and fairly loose in the C style plates.

Out of curiosity, I put a thin bead of silicone caulk between the plate and PEX on the second foot of each transfer plate to see if it made any difference.

There are six sensors in the mockup. On measures ambient air temperature, one is placed next to the PEX where it enters one of the plates, and each plate has two, placed at the inner edge and just under the plate. They are spaced at 6” and 18”. I've found the electronic thermometers to be pretty consistent - they are within .2 degrees of each other under 100 degrees and within one degree above that. Although they touch the plates (or for the entry sensor, the PEX) I’m not convinced the heat transfer to the sensors is optimal.

Here is also a FLIR image take on the surface overlain on an "Xray" image of the plates.

The FLIR image by itself.

Location of the sensors under the edge of the plates all the relevant sensor temperatures at a point in time.

What I’ve learned –

The wider and thicker plates give a more consistent temperature across the surface. And, of course, the heated area is 33% wider. The narrower plates are hotter above the PEX, apparently because the heat isn’t being transferred through the plates as effectively.

The thin layer of silicone caulk does make a difference. Where it is used, the sensor at the edge of the plate is about two degrees warmer than where it isn’t used, i.e., there is better heat transfer from the PEX to the plates. This is apparent from the sensors and the FLIR image for the wider plate, but less evident for the FLIR image for the narrower plate. It appears to be worth the extra effort and cost for the caulk for this type of installation.

The omega style plates fit into a ¾” groove. The grooves for the C style plates needs to be about 1/16” wider.

The temperature difference between the water in the PEX and the outer surface surprised me. Around 20 degrees.

These dual electronic thermometers are dirt cheap off of Amazon. I power them with an extra 12vdc wall wart. I plan to incorporate several of them into the boiler room.

I’ll need to pilot drill and countersink the holes for the 1” drywall screws in the ¼” plywood. If I don’t, the screws break when they try and make their own countersink.

A question - The heat transfer plates will be clamped firmly under the ¼” plywood. Is there any reason to use staples/nails to hold them in place?

Both sides are parallel through bull headed tees. I watched them from cold start, but they had been running for several days before I took the measurements that I posted. Exactly the same flow wasn't necessary for my purposes, but I made the lengths and fittings as close to identical as I could so I expect the flow to be pretty close between the two.

Accurately measuring delta T of the water across the two feet of tubing with heat transfer plates is beyond the capability of the tools I have. The mockup was at the beginning of the DHW loop. Delta T for the entire 40 foot DHW loop is only 4-5 degrees. Before I break it down I want to make measurements of surface temperature at various points at right angles to the tubes, water temperature, and ambient temperature, just for the record. I don't have a mixing valve on the water heater. If I can get one installed before I break it down I'll recheck the temperatures when there is less of a temperature swing.

I saw pretty much what I expected to, but there is enough contradictory and self-serving information out there that I wanted to see for myself before deciding how much to invest in heat transfer plates. There were a couple of things that I didn't expect - the temperature difference between the water and outside surface of the PEX and the effect that silicone caulk had on the temperatures of the plates.

Thanks again for all the comments and suggestions.

Xpress track looks interesting. However, with most floor coverings I see that they they recommend 1/4" plywood or cement board on top of it. I'd have to raise the kitchen counters for the dishwasher to still fit. It's only 5/16" tubing, so more loops and manifold connections would be required. I couldn't find anything on the manufacturer's site giving thickness of the aluminum, just one individual on a blog saying it was .020 inches thick.

The point of checking with the manufacture before using silicone caulk or similar is well taken.

I first used thermal paste to put power transistors on heat sinks over 50 years ago and last used it a few months ago to install a a solid state relay. It's way too expensive and messy for this application.

I came across an Ebay seller who advertises "12oz tube aluminum thermal conductive sealant" for use between PEX and heat transfer plates. In theory, aluminum in the mix should improve conductivity. I couldn't find anything else about it or similar products on the web. I wonder if it is a special product and how it performs? Cost is about 50% more than caulk. With one exception, descriptions of "aluminum" caulk say they are aluminum color or gray but don't say they contain aluminum. The only one I found that said it contained aluminum in a quick search was electrically conductive and cost over 30+ what standard caulk costs.