All folks are saying is that the difference is similar in the heating between a conventional oven and a microwave oven. Both take energy in and make things hot, the difference is the microwave doesn't the make air hot, only the objects. If a system isn't heating the air, it takes less energy overall to make things feel warm.
Is it myth or is it real?
It was an interesting topic so I started looking into it. Please correct anything I got wrong, but depending on the assumptions it looks like it can help, but not in a big way.
A tealight candle outputs about 35 watts, so at 10 watts/sqft heating you'd need ~41 to heat both the air and objects of a typical 12x12 room.
Experimental evidence says a tealight candle would heat the surface of a terracotta pot to around 270℉.
This is quite a bit hotter than the Radiant systems @Supervstech mentions, so the infrared wavelengths would be shorter.
Infrared heaters have filaments that operate at about 1830℉ for MWIR and 3,270 °F for NIR (even shorter wavelengths). | |
The conversion from a terractta pot wouldn't be very efficient as most of the heat from the system would still be convective. But the theory is, every percentage of the energy converted into radiant heat would heat the objects rather than the air.
How warm is Comfortable?
Being "warm enough" is complicated. Skin temperature is usually over 92°F and comfortable room temperature is around 72°F, so most folks are
comfortable when losing heat with a ΔT around 20 to 25°F. When it's 62°F the gradient is >30°F and bodies start to lose heat faster than they are comfortable with.
With radiant heat, the room's air temperature can be lower because in addition to the body's ΔT loss, the body gains heat from radiant energy.
Over time, the air will heat up as objects convect heat into air. So in a completely closed system,
it doesn't make a difference. In reality, there is air exchange with cold outside air, so less heat in the air means less heat escapes the system. In a forced air system, it's worse than you might expect as the fan's operation causes a partial vacuum inside the house drawing in more air than you'd like (it's why radiators are more efficient than forced air systems). I've heard that the average air-exchange in a house is 1 to 2x per hour.
If the primary benefit is not having to heat the air, how much energy does the air take to heat? What are we saving?
We know that Q = c x (m×ΔT) where c is the specific heat, Q the energy, ΔT the energy change, and m the mass of the object.
Using that, we can calculate the amount of energy needed to heat just the air to see if it's worth it.
The specific heat of dry air is approximately 0.24 Btu/lb°F and for water vapor is 0.45 Btu/lb°F. At 40% humidity and 60℉, it's about 0.4.
Air's mass is 0.0765 lb/cu ft at those conditions.
With an 8' ceiling, a 12x12 room would have roughly 1152 sqft of air, which would be a mass of 88 lbs. To raise the temperature of the 12x12' room 1 degree F would be Q = .4 Btu/lb°F (88 x 1) = 35 BTUs (about 10 watts).
With the assumption of 10 watts/sqft heating is required, 1 air exchange per hour, and ΔT of 30°F between inside and outside, then heating the air alone would require 300 watt hours or about 10 of the 41 candles calculated earlier. But, given the terracotta pot isn't 100% efficient at converting to radiant, you'd only be able to get rid of some fraction of them.
Summary - TL;DR
In a closed system, energy is energy and it doesn't make a difference between radiant or convective heat. But homes aren't closed systems and with 1 to 2 air exchanges per hour and not heating the air can save energy. Terracotta pots can change some convective energy into radiant energy, so probably marginally better than burning a candle directly.
