10 Things to Consider When Buying Aluminum plate for floor heating

What are Heat Transfer Plates And What Do They Do?

Heat Transfer Plates are used in so called “staple up systems” where the floor is warmed by placing heating tubes underneath the floor.

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Aluminum material wraps around the heating tube and then spreads out and attaches to the under side of the heated floor.

Heat transfer plates perform three important functions.

There are differences of opinion about how much aluminum should be used, how thick it should be and even if aluminum is necessary at all.

Here, we will offer to you the benefit of our research and experience so that you can either understand our recommendation better, or make up your own mind.

RADIANTEC HAS CAREFULLY RESEARCHED THE PROPERTIES OF ALUMINUM PLATES TO HELP YOU MAKE THE BEST DECISIONS.

HEAT TRANSFER – The purpose of your radiant heating system will be to make heat in the form of warm water (in an efficient manner, we hope) and then transfer that heat to your floor. Some process must take that heat away from the tubing and apply it to the underside of the floor so that the floor can heat the area above. Aluminum is a material that transfers heat exceptionally well but it is expensive, and its use should be carefully balanced against other methods of doing the same thing, and its application should be optimized for performance and cost effectiveness.

NO ALUMINUM vs. FULL COVERAGE OF ALUMINUM – Controlled experiments have shown that covering the tubing with medium/thin gauge aluminum plates causes the system to put out about 60% more heat when operated at the same temperature.

If you use no aluminum at all, you must either raise temperature of the fluid in the tubes to a very high level, use more tubing, or both. It is not desirable to operate plastic heat exchanger tubing at high temperatures for reasons of safety, efficiency and service life.

DIRECTION OF HEAT FLOW – Heat that goes in the wrong direction is called “BACKLOSS”.

The aluminum heat transfer plates have a unique and generally unappreciated property. Aluminum has what is called “low emissivity”. The emissivity of aluminum is .05 vs. .95 for ordinary materials. That means that when aluminum is warm, it emits radiant energy at a much lower rate than most other materials (only 5% as much).

These two images show how aluminum plates can significantly affect the performance of a “staple up” type system.

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The image on the left is an ordinary photograph of the underside of a floor that has radiant heating tubes that are fully covered with aluminum heating plates.

The image on the right is an infrared thermograph of the same situation. Yellow and red colors indicate higher emission of heat energy while blue indicates lower emission of heat energy. All materials are about the same temperature in this case (110°F).

The aluminum plates, which are about 110°F are emitting heat in the downward direction (backloss) as if they were only 60°F. This property has a very positive and generally unappreciated effect on the overall performance of a “staple up” system.

SHAPE AND THICKNESS OF THE ALUMINUM PLATE – Aluminum heat transfer plates are available in any number of configurations and a decision must be made about which is ideal. Aluminum is expensive and the cost of installation is important as well. These photographs show some options.

A thicker aluminum stock will transfer heat faster than lighter stock. The extruded tubing groove fits tighter to the plastic tube for better contact. Overall, the thicker heavy gauge aluminum plate is 6% more efficient at transferring heat than the lighter gauge pre stamped aluminum plate. However, the heavy gauge material is four times as costly as the lighter gauge material. Also, it must be pre-drilled and screwed to the sub floor and the overall cost is much higher than that of the lighter material. The smaller size means that the low emissivity property of the aluminum will be less.

The lighter gauge pre-stamped material is economical enough to be used more thoroughly. It is thick enough to provide good heat transfer and thin enough to be stapled into place instead of drilling and screwing. The thickness is a little thicker than premium flashing but it is pure aluminum and it undergoes a heat treating process that makes it more malleable (called dead soft) so that it can be worked with easier.

The flat stock is for special applications. The material is malleable enough to be bent into custom shapes.

Heavy duty aluminum foil, available from the supermarket is at the other extreme. However, it is not really thick enough to transmit heat well and is little better than nothing.

RESULTS – Our data suggests that the options of no aluminum plates at all, heavy duty aluminum plates, and aluminum foil can generally be ruled out. The real decision is between a continuous covering of the tubing with a medium/light gauge aluminum stock, or an intermittent covering of half of the tubing. We have arrived at the following recommendations:

If heat loss downward (backloss) is entirely wasted as to a crawl space or basement, cover the tubes entirely and insulate well. If heat loss downward is useful to another space, then a reduction in aluminum by half should be considered.

If the flooring material is thick or carpeted, the tubing should be covered entirely.

If low operating temperatures are desired, as for solar heating applications or for very high efficiency, or for other reasons, cover the tubing entirely.

If you want to learn more, please visit our website Aluminum plate for floor heating.

Disclaimer, I'm not a heating professional. I am in the process of my own radiant heat and boiler install, it is possibly the longest running project in history.

First you should do the heat load calcs to see how many BTU's you need to heat the space. Then look at the heat output of the different combinations of plates, tube spacing, floor temperature, and water temperature and pick an approach. I ended up hiring a radiant heating consultant to design our system.

If you need heavy extruded aluminum plates to get enough btus, take a look at thermofin from
Radiant Engineering . I bought direct and save substantial bucks.

For lightweight plates I like the ones from Northeast Radiant Technology . The tubing actually snaps into the plate, unlike most of the lightweight plates I have seen that have a very loose fit.

As far as noise goes you need to be concerned about installation details and your control strategy. For example, if you use a mixing valve and outdoor reset control, your floor and plates will stay at a more consistant temperature and that should lessen expansion and contraction issues. Extruded plates are supposed to grip the pex tightly enough that you can use regular pex and not get a lot of expansion and contraction noise. With the lightweight plates you might want to consider pex-al-pex. Pex-al-pex expands and contracts much less than regular pex but might be harder to work with if you have to thread it through joists.

Don't take any of this as advice, just some things to think about and research.

in hot water said: the extruded plates do move the energy better for two reasons. First they grip the tube tightly. As the most powerful means of heat transfer is conduction you want a tight tube fit.

Also the heavier gauge moves the energy to the edge of the fin better. Thin flashing gauge plates get cool before the heat gets to the outer edge. You really do get what you pay for with extruded plates. Plus lower operating temperatures.

Thin flashing plates are prone to making noise. It's called oil-canning and it sounds like those oil cans that you push the bottom. One manufacturer suggested you fasten only one side so the plate could expand without poping. But that defeats the purpose of a heat transfer plate if it is not touching. They finally pulled the thin flashing plates from their offering.

There are several versions and grades of extruded plates. I feel the "lite" version works fine, Radiant Engineering manufacturers the best they are sold under various radiant brands so shop for price.

With extruded plates you can insulate right against them, no need to leave an air space as the energy transfer is all conductive. Spray foam is a good way to seal the joist end spaces, around the rim joist. with fiberglass batts you want R-19 under radiant.

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A couple of questions... I've seen one suggestion (with a jig to make them) that one can make plates from the surplus ends of aluminum sheet generated by the seamless gutter folks (apparently they get 10-15' of waste every time they have to re-thread their machine for a different color, or change rolls) Seems like that would be better than the flashing weight plates, while still less money than the extruded plates - what do you think? The same site (I think it was "Builditsolar.com, but I'm not certain) also suggested using a thin layer of silicone caulk between the plate and tube - claimed better heat transfer and also reduced noise because the plate was glued to the tube.

The other question is what about insulation if one is going over a floor? In my house I am thinking that the first floor can be staple-up because it won't be a huge problem to pull down the sheetrock ceiling in the basement, and obviously insulation is not a problem there, but I can't pull down the ceiling in the living spaces to do staple up under the second floor, so I was thinking in terms of laying the tube above the subfloor - using Thermofin U or something along that line, but I don't see any good way to get insulation under it - since I'd be over a heated space, do I really need to worry about that? (If I do, how would you suggest handling it?)

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