Old 05-26-2002, 06:04 PM
Moe Moe is offline
Join Date: Mar 2000
Location: Berlin
Posts: 2,768
Heat from candles

How much heat do candles contribute to a room? I'm at a dinner gathering with friends when one suggests we light some candles for ambiance. Another objects that the room will get too hot. We're talking approx. 6 candles in a room about 15 X 12 X 7 on a cloudy spring day, about 56 degrees outside.

I personally didn't give much thought to the issue until the one person objected. She suggested I post it, so that's a-what I'm-a-doin. So do you think the candles would contribute a significant amount of heat to the room (i.e. significant in terms of the comfort level of the room).
Old 05-26-2002, 07:22 PM
Straight Dope Science Advisory Board
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OK, I will take a WAG with part of it, since I searched and searched and cannot find the heat output from a candle.

A candle has an efficiency of about 0.1 lumen/W heat input (PE Handbook). Paraffin wax has about 44,000 kJ/kg. Typical burn rates for candles varies from 0.15 to 0.3 oz/hour. Let's say 0.2 oz/hour (0.005669 kg/hour).

We have:

44,000 kJ/kg * 0.005669 kg/hour = 249 kJ/hour = 69.3 W.

BUT...I also see that a "typical" candle puts out 12.6 lumens of light. At the efficiency above, that means that:

(12.6 lumens/W) * (1 W/ 0.1 lumen) = 126 W

So, somewhere between 125 and 250 W per candle of heat/energy. I'll let someone else take it from here...
Old 05-26-2002, 09:21 PM
Join Date: Oct 2000
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(12.6 lumens/W) * (1 W/ 0.1 lumen) = 126 W
126 W X 3.4 (B.t.u/H)/W = 428.4 Btu/hour per candle.
428.4 Btu/hour X 6 candles = 2570.4 Btu/hour.
(250 W per candle is too high an estimate, as a 250 watt light bulb, unlike a candle, will melt a lot of wax)

A typical gas stove burner puts out ~12,000 Btu/H, so we're not talking a lot of heat. On the other hand, a standard small window air conditioner is good for 5000 BTU/hour. Six candles would use up half its cooling capacity.

If we look at it in terms of people:
A person using 2500 Calories (Kcal) a day puts out 2500/24 = 104 Kcal/hour
104Kcal/hour X 3.98 Btu/Kcal = 414.5 Btu/hour per person.
So lighting 6 candles would have about the same effect as asking 6 more people into the room.
(1 person = ~1 candle)
That's not likely to be a problem when its 56 degrees outside.
Old 05-28-2002, 05:41 PM
Moe Moe is offline
Join Date: Mar 2000
Location: Berlin
Posts: 2,768
Well, I'll admit that according to those calculations the heat was more significant than I'd imagined, but I wonder if I'd be pushing my luck here if I asked for someone here to control for certain variables (or just simply comment on them):

For instance, the candles were placed on candle holders probably about 5.5 ft. off the ground attached to the walls. It seems to me that much of the candle heat, highly concentrated and travelling straight up, would be lost through the ceiling, while human body heat would spread more evenly throughout the room.

Well, I guess that's the only detail I can think to add to the question, except perhaps oxygen levels and their effect (lost by candle vs. breathing), but I wouldn't think that would be too significant with a few windows open.

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