What is the best speed for a fan used with a wood stove?
February 8, 2013 4:25 PM   Subscribe

Is heat transfer and room heating better with a low or high speed fan?

We have a clean burning wood stove that does a great job at heating the house. We have an oscillating "tower" style fan behind it in the corner of the room.

As the wood stove heats the air around it, the air immediately next to it gets very hot. Lets say 140 degrees. As you travel farther from the stove, the air is cooler.

Is it better to run the fan on low and blow a lower volume of very hot air into the room, or is it better to run it on medium or high to move a higher volume of "less warm" air (because of less time in close proximity to the stove)?

These numbers are only used for illustration.
Is it better to move air at 140 degrees at 500 CFM or...
Is it better to move air at 100 degrees at 1000/1500 etc. CFM?

My question is specific to the fan spreading the heat out by blowing hot air, not on the energy consumption of the fan itself.
posted by Leenie to Home & Garden (8 answers total)
Best answer: The amount of (heat) energy transferred from the stove to the air surrounding it will increase in some proportion with the difference in temperature between the stove wall and the nearby air.

If you wish to maximize the net transfer of heat from the stove into the room's air, you would want to run the fan at its highest speed, so as to minimize the temperature of the air in the immediate vicinity of the stove.

However, moving air at temperatures less than 98 or so degrees Fahrenheit can tend to make human beings lose heat more quickly, especially in conditions of very low humidity. So, the fan setting for optimal comfort may differ from that for optimal heat transfer into the room. That said, unless the fan is blowing what feels like cold air directly at someone, I believe that the highest setting is still likely to be the best for comfort overall.
posted by Juffo-Wup at 4:33 PM on February 8, 2013

Also, for what it's worth, the amount of energy consumed by a household fan is negligible compared to the amount of energy consumed/released in a household wood burning stove.
posted by Juffo-Wup at 4:34 PM on February 8, 2013

The faster you run the fan the more it will cool the stove. The fan isn't destroying heat, it can only move heat around. Therefore, the more you cool the fan the more you heat the rest of your house.

In other words, I agree with Juffo-Wup.

However note that Juffo-Wup's effect applies equally well to the exterior walls of the house and windows. The warmer the air around them, the faster you will heat the outdoors. So by spreading out the heat from your stove you may be decreasing the total average temperature of your house. But that's just an academic curiosity - what really matters is having more comfortable space in the building, so yeah, crank it up!
posted by aubilenon at 4:47 PM on February 8, 2013

Hm. I'm not a thermodynamics engineer, but I'm not sure I agree.

I don't think it matters. The rate of heat transfer from the stove depends on the fire, not the fan. Heat that isn't blown into the room is retained in the stove until the stove gets hot enough to radiate it. I think the only thing you're changing is how large your heated area is - do you want a small area near the stove to be very warm, or a large area to be kind of warm-ish.
posted by ctmf at 5:16 PM on February 8, 2013

ctmf: I don't think so; the heated air inside of the stove tends to leave via the chimney.
posted by Juffo-Wup at 5:19 PM on February 8, 2013

You may be right. It seems to me anecdotally that once I get the heavy metal and bricks of my stove heated up, it doesn't matter what I do with the fan; the house as a whole gets warmer at about the same rate. Just without the fan, it feels much warmer near the stove.

I actually prefer to not use the fan, because once I get a good burn going the recirculating air flow in there keeps the burn going much hotter. I feel like I get much more from radiative heat transfer that way. When I start the fan, the fire acts like I've dampered it down some and while the blowing air feels warmer, the stove itself cools off. Obvious, I know, but it also feels like the radiating hot metal vs. warm air convection trade-off is a wash.
posted by ctmf at 5:39 PM on February 8, 2013

If there is a second story involved, the strategy changes. In that case you would do best by putting the fan at the top of the stairs so that it pulls warm air up to that level. This works very well if you otherwise would be uncomfortably warm in the room with the stove.
posted by megatherium at 6:04 PM on February 8, 2013

Convection alone will carry away a lot of heat from the stove's surface. Often that heat hangs out up at ceiling level, and it's useful to use a fan blowing cool air towards the stove at floor level, to encourage a convective loop which in turn distributes the heat over a larger area.

Also, it's sometimes possible to take too much heat from the stove. Most of the heat liberated in wood burning is from secondary combustion (flame), which is the burning of gasses and smoke particles forced out of heated wood. Contact with cool surfaces can extinguish flame before all the fuel is consumed, so you end up sending unburnt smoke up the chimney. A fan isn't likely to cause such problems unless the stove is running at unusually low temperatures to begin with, but it's something to be aware of.
posted by jon1270 at 7:04 PM on February 8, 2013

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