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Heat Transfer Plates: Do they make a difference in heat transfer?
Copyright (c) 2013, Robert Bean, R.E.T., P.L.(Eng.), www.healthyheating.com and content contributors

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Readers are advised the following is a snap shot of just one third party evaluation and I recommend reading "Plateless in Radiantville" and "Shakeout Time" by John Siegenthaler, P.E. accessible from this website.

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Results from a finite element analysis (FEA) study below comparing fin effectiveness using thick extruded aluminum plates versus thin stamped aluminum plates. The parameters of the study are;

120F water in tubes, 70F air above and below floor assembly, 8" o.c. tube space, 1/2" PEX, heat transfer coefficient of 2.0 Btu/hr/ft2/F, 8" x 1/32" stamped plate and 4" x 1/16" extruded plate, non disruptive contact between fin and sub-floor.

Note on FEA studies: the results only reflect the combination of parameters stated above and readers are advised not to draw conclusions based on parameters not studied. Nor should the results be indicative of transient conditions such as start up nor do they account for disruptive contact (i.e. air gaps due to twisting and buckling of the fin (inherent characteristic of thin plate systems). Additionally, the model does not account for the 3D transfer along the pipe, i.e. how much energy is pulled out of the pipe along its length.

Message: Any number of combinations of parameters' not studied will render different results than what is shown below. Also, given the choice between plates and plateless readers are informed that a plateless system, particularly systems where the pipes are mounted to the sides of joist or suspended in the air between joist is the most inefficient method of heating a floor. In the case of the latter it would be akin to to heating a pot of water by holding it off the burner. In all but the warmest climates and/or most efficient homes such as R2000 or PassiveHaus, the side mount or suspended or plateless system will literally destroy the efficiency of a boiler due to the necessarily higher temperature required due to the ineffective installation method.

In addition to evaluating the use of plates for heat transfer, users should also evaluate plates for sound amplification potential due to pipes expanding and contracting. Methods of installation unique to the pipe manufacturer should be followed to ensure a quiet and effective conditioned floor.

Figure 1. Profile of the extruded (top) and stamped (bottom) aluminum heat transfer plates used for sub-floor radiant cooling and heating systems.

 

Figure 2. Graphical results of study. Due to the increased contact area and conductivity the extruded plate delivered higher surface temperatures over points b to b' in comparison to the thin fin. This represents an approximate width of 2.5".  Approaching the edge of the extruded plate at c and c', note the thermal degradation from points b to c and from c to d and b' to c' and c' to d'. This is due to the narrower profile 4" (c to c') compared to the stamped profile 8" (e to e') where c to d and c' to d' represent the surface temp over unplated subfloor.

Note with all other factors equal, the higher peak temperature delivered by the extruded plate also implies a lower return fluid temperature which improves the boiler efficiency. From this study using the parameters stated, the extruded plate would be a good choice for narrower joist bays with higher resistance flooring and where there is a value for a lower return fluid temperature. A more detailed study with different combinations would provide further comparative results.

 

Figure 3. Assembly details for extruded 1/16" x 4" aluminum plate. Make note of the center line chain of conductance of tube, plate and wood. With this profile the plate facilitates the conductance from the pipe into the wood more effectively than the stamped plate shown in Figure 5 and Figure 6.

 

Figure 4. Color painted isotherms from the finite element analysis (FEA) for the extruded 1/16" x 4" aluminum plate. Note the thermal intensity around the extruded plate in comparison to the stamped plate shown below in Figure 6. This type of plate is more effective at drawing the heat out of the pipe resulting in lower return temperatures which improves boiler efficiency.
 

Figure 5. Assembly details for extruded 1/32" x 8" aluminum plate. Note the reduced area and conductivity at the centerline for this specific stamped plate, this results in the lower surface temperature (b to b') shown in Figure 2 in comparison to point to the temperatures shown from a to b and a to b'.

 

Figure 6. Color painted isotherms from the finite element analysis (FEA) for the stamped 1/32" x 8" aluminum plate. Due to the wider profile for the 8" plate the distribution of the heat extends consistently further outward but with less intensity across the entire plate as shown in Figure 2 from e to e' in comparisons to c to c' of the extruded plate.
 


Comments: for this specific study, it can be shown that both plates are effective at drawing heat out of the PEX pipe and distributing it via conduction into the wood sub-floor. As with many components it is not a matter of one being better than an other rather one needs to look at the application and choose which one is best suited for the task. The thicker extruded plate would be effective in narrower joist bays with higher resistive flooring assemblies and anywhere where lower return temperatures are of value, whereas the thinner stamped plate would be effective in wider joist cavities with less resistive flooring and anywhere where lower return temperatures are not of value relative to the capabilities of the extruded type.

Note: Further research would show additional useful information based on a more comprehensive study of additional combinations of spacing, insulation, air spaces (gaps), plate configurations, flooring materials and flooring systems. Readers are advised the above is a snap shot of one evaluation and I recommend reading "Plateless in Radiantville" and "Shakeout Time" by John Siegenthaler, P.Eng. accessible from this website.

Regardless of the thin stamped or thick extruded, we do not recommend the ventilated fin types that are fastened around the pipe for suspended tube systems. Suspended tube systems are the least effective out of all options available and fail at returning the lowest temperatures back to the heating systems.

If your organization would be interested in funding further FEA studies on this and other topic please let us know. We have a library of FEA models using FLEXpde plus we work with a number of industry colleagues who also specialize in conductive and radiant heat transfer from surfaces within the building sciences.

It should be noted that many jurisdictions regulate the design and installation of hydronic radiant cooling and heating systems either by requiring professional design or in the absence of professional designs by good practice documents published by the RPA, ACCA, CIPH/CHC, HRAI and TECA.

Suggested Reading

  1. Alberta Standata 06-BCI-012 for Hydronic heating Systems 

  2. Maximizing efficiency: Computer modelling of different radiant floors using finite element analysis (FEA).

  3. Should you fully insulate under concrete slabs on or below grade?

  4. Comparison - below grade heated basement slab

  5. Calculating downward heat loss - design tools
     

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