Radiant heat

The sun is beating down hard today, and it reminds me that I got asked some questions about radiant panels at the last BOS meeting.

Brewster Hall has electric resistance heaters, most of them baseboard convection units.  Mr. Houseman described a situation in the Town Manager’s office where the units are obstructed by filing cabinets.  A contractor recommended radiant ceiling panels and Rob advised that they are considering them.

Selectman Silk asked about the effectiveness of a radiant panel on the ceiling because “heat rises”.  Rob deflected the question to me as I was conveniently in the audience behind him.

The short answer is that they can be effective.  The popular belief that “heat rises” is founded in the observation that hot air will rise, because hot air is less dense than cold air.  Radiant heat, heat energy, is nothing more than low frequency light – infra red light.  So it works pretty much the same as light from a bulb.  It radiates out from the source and heats (illuminates) what it strikes. Think of your oven broiler.  That’s a radiant panel.

Like light, the intensity goes down exponentially with distance, and to be effective, must be in a line of sight of the object (person) to be heated.

I was asked if it was the best solution.  That’s a different question from “can it work?”  Offhand, I don’t think it’s the best technical solution, but the budget is tight so I’m not sure if better solutions are on the table.

Mr. Bowers asked if the temperature can be regulated effectively.  I replied something like “It doesn’t work like that”.  To understand the concept of radiant panels, consider yourself outside on a cloudy spring day where the temperature is 65 degrees.  You have a thin shirt, have been sitting for awhile, and are getting a little chilly.  Suddenly the clouds part and Ol Sol beams down on you.  You fell marvelously warm, yet the air is still 65 degrees.

That’s how radiant panels work.  A body at some given temperature (like 98.6F) radiates heat.  If a body receives more radiant (and conductive) heat than it emits, the body feels warm, and visa-versa.  So effectively controlling radiant panels has to do with balancing the amount of radiant energy vs. the air temperature and outside wall temperatures.  Simple on/off controls will probably result in less than satisfactory performance.

Radiant floors work in much the same way, but they typically also cause the air above to be heated by convection, and that’s the primary way they work in homes.  In places like churches and halls, the radiant component is more dominant.

On the energy side, if overhead radiant panels are controlled effectively such that they radiate appropriate amounts of heat, they can be more economical than trying to heat the air, particularly when the walls are uninsulated drafty brick.  On the other hand, it’s electrical resistance heat, and still one of the most costly sources of heat available.

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