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Study of Heat Transfer Plates

What are Heat Transfer Plates And What Do They Do?

lighter gauge aluminum

lighter gauge aluminum

heavy gauge aluminum

heavy gauge aluminum

half coverage with aluminum

half coverage with aluminum

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

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.

  1. They help to carry heat away from the tubing and distribute it through the joist space and along the floor. (Heat transfer)
  2. They support the plastic heat exchanger tubing.
  3. They greatly reduce heat loss in the downward direction (backloss).

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.
Radiantec Research Facility

Radiantec Research Facility

Careful Temperature Measurements

Careful Temperature Measurements

Recording and Measuring Performance

Measuring & Recording Performance

No Aluminum

No Aluminum

Full Coverage

Full Coverage

Half Coverage

Half Coverage

Heavy Duty Aluminum

Heavy Duty Aluminum

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.

SUPPORT – Aluminum plates support the plastic tubing well. Poorly supported plastic tubing will tend to sag over time, particularly when operated at high temperatures.

The tubing in the photo – installed without aluminum plates – has developed a sagging problem within a few hours, even at moderate temperatures and when supported with a fitting every 5 feet.

Sagging Tubing

This tubing installed without aluminum plates has developed a sagging problem within a few hours

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

If backloss is not controlled well, all of our other efforts in designing an efficient radiant heating system will be for nothing. The worst problem that we observed with systems that did not use aluminum plates was that nearly all of the heat went down instead of up. It is a common misconception that “Heat always rises.” This is not always the case. It is true that warm air or warm water will almost always rise in relation to a colder fluid because of the difference in weight and density. But radiant heat travels in all directions. Those who would say that aluminum plates are not needed because the heat is going to go up anyway are misinformed. It is true that this backloss can be reduced with insulation and reflective barriers but it will cost money and will not be entirely satisfactory.

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).

Ordinary Photograph
Infrared Thermograph

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

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.

heavy gauge aluminum

heavy gauge aluminum

lighter gauge pre-stamped

lighter gauge pre-stamped

flat stock

flat stock

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 predrilled 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.

 

Do not hesitate to consult with your Radiantec technician if you have any questions or special applications.