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Old 10-01-14, 08:08 PM   #27
Snail
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Default Air stratification and ceiling radiant heat

Hi AC, This is a bit late, I just noticed this thread. My 2 cents worth:

The temperature of the air in contact with a flat ceiling radiator will have virtually no effect on the heat output of the radiator. Air is basically transparent to infrared and thus does not affect the amount of heat transferred to the solid objects in the room below. Hot air stratified at the ceiling or suspended radiator-face will totally prevent any convection of course but you wouldn't want any up there anyway.

The output of the radiator per unit area is proportional to (Tr4 – Ts4), where Tr4 is the fourth power of the radiator face temperature and Ts4 is the weighted average of the fourth powers of the surrounding solid surface temperatures in the room. (Note that these are absolute temperatures, i.e. in degrees above absolute zero, so, for example, 20degC is 20 + 273.15=293.15 degrees Kelvin). I have seen published claimed figures of about 1kW per square metre radiant heat output for electric ceiling panels operating at 110 to 115degC, which by my calculation would imply less than a 10th of that for low temperature hydronic systems.

Note that wall “radiators” actually deliver most of their heat by convection. This makes them more efficient per unit area at getting heat into the room. Against that, a lot of the heat goes straight to the ceiling! Also, radiant heat delivery seems to be generally accepted to be more efficient than convected heat for delivering comfort. Radiant floors presumably deliver quite a bit of convection too. For a given area and temperature this would result in a greater output than a ceiling radiator. I imagine calculating the proportion would be difficult but I imagine that the figures in the various programs you use will already have taken all that into account? Do any of those programs handle ceiling radiant heating AC?

The following is speculation by me, based on my understanding of the physics only. I’d be interested in what others think. Compared to other forms of radiators, ceiling radiant has the following:

• Somewhat lower output per unit area for a given radiator temperature. This may be at least partially compensated for by the fact that the ceiling is unencumbered by furniture etc., as floors usually are;

• The floors will be warm. If you use low temperature floor heating, the floors will usually be below blood temperature, so if high conductivity flooring is used, as recommended for efficiency, they will actually feel cold to bare feet. For ceiling radiation, carpets on floors actually improve effectiveness, the reverse of the situation with radiant flooring.

• Some people dislike heating directed more at the head than the toes. On the other hand, that is the most efficient way to reduce body heat loss and others prefer it.

• For timber frame houses, it is impractical to add thermal mass to the ceiling. However, there seems to me to be no great disadvantage to having a high thermal mass floor or wall that is indirectly heated by a radiator, compared to heating the floor directly? You can of course have the thermal mass in a storage tank instead, possibly at greater cost?

Last edited by Snail; 10-02-14 at 01:15 AM..
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