Fire in the Fireplace
February 26, 2011 12:26 PM   Subscribe

Please help my S.O. and I settle a debate about the fireplace.

One of us believes making a fire in the living room fireplace makes the rest of the house colder.

The other believes that because there is a vent at the bottom of the fireplace, it draws the cold air from there, and not the rest of the house. Therefore: a fire would not make the house any colder.

Who is correct? Or are we both wrong?
posted by The ____ of Justice to Science & Nature (24 answers total) 2 users marked this as a favorite
I've always heard that an open flue will wick heat out of the house naturally. It makes sense, what with heat rising and you having a hole in the top of your house, basically.

One thing you can be sure of is that if your thermostat is in the room with the fireplace (the warmest room, being closest to the heat source) that your furnace will never kick on.
posted by nevercalm at 12:30 PM on February 26, 2011

It depends on the size of your fireplace. When we bought our house, a chimney sweep told us straight out that a fire in our small fireplace would make the house colder. I live in the South and don't care too much, so we do have fires anyway.

I would think it would also depend on the location of your thermostat - i.e., if it's near the fireplace, the heat wouldn't kick on as much, meaning the rooms farther away from the fireplace would stay colder.
posted by something something at 12:31 PM on February 26, 2011

The answer is "it depends on much more than the placement of the vent."

For example, because of the stack effect, you could be drawing cold air from outside (e.g. through cracks and crevices) into the house and into the fireplace. Chimneys have to be designed right so as not to create too much stack effect in your house -- and they are very often not designed correctly, because of aesthetic decisions.

In other words, you could be both right and both wrong.
posted by Cool Papa Bell at 12:31 PM on February 26, 2011

there might be a hole in the top of your house... but there's this nifty ball of gas heated to hundreds of degrees sitting under it.

If the fire is close to the thermostat your furnace might not turn on letting the rest of your house cool. If your walls near the fireplace are leaky it might draw a little cold air in that doesn't immediately go up through the flue.

I don't see that the fire can make your house noticeably cooler but it might feel cooler in comparison after sitting in front of it.
posted by mce at 12:36 PM on February 26, 2011

Thanks for the answers so far...

Incidentally, we're looking at the temperature issue independent of our furnace, which is being kept off anyways.
posted by The ____ of Justice at 12:59 PM on February 26, 2011

The stack effect link from Cool Papa Bell should give you the basic answer. A properly designed fireplace and chimney should draw air out of the house all the time, fire or no, unless the damper is closed. My opinion is that, given the cold air vent (which I hope runs outside and not just to the basement), you will have a net heat gain when there is a fire burning and a net loss if the damper is left open when the fire is out. But, as others have said, there are lots of variables - one of which is the shape of the firebox. A poorly designed firebox will not reflect much of the fire's heat into the room.
posted by Hobgoblin at 1:13 PM on February 26, 2011 [2 favorites]

Thing is, you have to leave the damper open even as the fire dies, to prevent smoke from coming into the room. So with an almost dead fire, all the interior air you spend good money to heat gets sucked up the chimney all night. I put glass in front of my fireplace to seal it, and fed air from outside via the ash pit below.
posted by weapons-grade pandemonium at 1:32 PM on February 26, 2011

It's going to depend on factors specific to your house, as people have said. We have one point of anecdata above for a small fireplace that makes the house colder; in my apartment, on the other hand, our gas fireplace is our main form of heat during the winter and it definitely warms the place up nicely. So it's going to depend on other factors. I would probably try to measure the actual temperature both with and without the fire running on several nights and see if I noticed any difference.
posted by Lady Li at 2:12 PM on February 26, 2011

Just to complicate things, all the answers above are dealing mainly with the effects of convection, but if you have a nicely burning file (as opposed to a bed of glowing coals) there is some degree of radiant heating going on as well; how much depends on a number of variables specific to your room/house, the temperatures inside and outside, and the specific fire (even the type of wood and the way it is cut and stacked in the fireplace; some produce hotter fires than others). The other modes of heat transfer, evaporation (phase change) and conduction, are unlikely to be of major importance in your scenario. For some reason the link button isn't working for me right now, but the article on heat transfer in wikipedia is a good starting place. (This stuff is important in the operating room, especially with babies, because it can be difficult to keep an anesthetized patient warm and all sorts of bad things happen if you don't.)

Short version: there are a ton of variables involved and it might be easiest just to get a thermometer and measure under different conditions.
posted by TedW at 2:39 PM on February 26, 2011

Bottom line, other than the radiant heat you get from being close to the fire, the air flow is from the house into the fireplace and out the chimney. That is, of course, air you paid to heat.

In this case, the house is being kept warm by the furnace. The answer to your question is, if the furnace can put out enough heat to overcome the heat lost through the fireplace, your house will not cool.
posted by tomswift at 2:55 PM on February 26, 2011

One more effect....

In MY house, one of the three thermostats (zoned heat) is in the same room as the fireplace. When we operate the fireplace, and the room gets warm, the thermostat is satisfied, thus, it does not 'call' for heat in that zone. Oddly, that particular zone is both the first and second floor in that section of the house, hence, the upper rooms suffers from cold. We could compensate via bumping up the setting, but the fireplace output varies and the net effect is not that predictable.

Anecdotally, even though this is the BEST fireplace without an insert I have ever seen in terms of radiant heat, and even though it has an outside air supply to both sides of the fireplace, the air requirements of the fire apparently swamp the thermal output of the fireplace, and the rest of the house suffers. It's mighty toast next to the fire, and the atmosphere is nice, but we run the thing with the sure knowledge that it's inefficient and a luxury/waste of energy.
posted by FauxScot at 3:13 PM on February 26, 2011

If it's warm enough to keep the furnace off anyway, why not experiment?

You'll want to use temperature gauges in the various areas of your house, since "it feels warm" and "it feels cold" won't be reliable after sitting in front of a bunch of burning matter.
posted by galadriel at 4:02 PM on February 26, 2011

the air flow is from the house into the fireplace and out the chimney

Not necessarily. A well-designed chimney uses a cold-air intake that is presumably outside air. So, no net loss necessarily.
posted by Civil_Disobedient at 4:14 PM on February 26, 2011

Fireplaces aren't actually very good at heating houses (and never have been; that's why stoves were invented). There are some systems to recover the heat that's going up the chimney, such as heat exchangers in the grate or in the chimney, or heavy masonry stoves.
posted by hattifattener at 4:17 PM on February 26, 2011

You can eliminate the argument by getting a fireplace stove insert. Your fireplace will then heat up the house a lot more than it does now and definitely provide a net gain in heat.
posted by eye of newt at 4:48 PM on February 26, 2011

What you want is a wood stove. Way more efficient than a fireplace.
posted by lohmannn at 4:53 PM on February 26, 2011

Unless the fireplace was specifically designed to draw make-up air from a specific place (like it's own vent to the outside), the "makes the house colder" person is right.

When you say there is a vent right at the bottom of the fireplace, where is it? Venting to the room? Venting in from the outside? If it is pulling air in from the room, the room is then pulling cold air into the rest of the house. Even if there IS a vent to the outside right in the fireplace, if you have the doors open, the chimney will suck some percentage of the air from the rest of the house. The chimney creates negative pressure, and the higher pressure outside will ooze in at varying percentages through any crack or crevice.

The only way to have a fireplace that won't affect the rest of the house is to have one where the firebox is functionally "outside". You load the fire, close the doors, and then it draws from and vents to the outside. The inside gets heated by radiant and convection off the sides of the fireplace, and sometimes by a little fan that draws air around the outside of the box (which is functionally inside) and blows it out to the room.
posted by gjc at 7:17 PM on February 26, 2011 [1 favorite]

FWIW, we heat a 1600 sq ft house with a wood stove which has an optional fan installed. I'm almost through 3 cords of wood, the stove's been going since October with few breaks. Between my neighbor and I who have similar homes, he thinks that my stove - which is in the room - does a much better job than his insert, which he says loses heat up the chimney, whereas mine is a 500lb slab of hot metal in the living room giving most of it's heat into the house.

Of course, none of this answers your question, but it might be useful advice....
posted by nevercalm at 7:21 PM on February 26, 2011

They did this on Mythbusters and the other rooms got cooler.
posted by a humble nudibranch at 8:03 PM on February 26, 2011

Thanks for the answers guys, aside from the poor respondent who seems unable to understand the point of askme.
posted by The ____ of Justice at 8:14 PM on February 26, 2011

My fireplace definitely makes my house colder. I never use it.
posted by stinker at 7:11 AM on February 27, 2011

My parent's fireplace has glass doors that you can close, along with a fan that pumps air underneath the hot coals and then out a vent in the front. If you have a setup anything like this, the debate is pretty much over - you are absolutely heating your house.
posted by chrisamiller at 7:39 AM on February 27, 2011

Conventional wisdom is that fireplaces make the house colder. They're very inefficient at producing heat, and there's a big hole sucking up air from the house (which is replaced by cold outside air).

But if your partner isn't buying these arguments, a thermometer is very cheap and easy to deploy. Shouldn't take you more than a few hours to get a definitive answer to your question. Make it fun and bet each other the price of dinner on the results!
posted by ErikaB at 11:22 AM on February 27, 2011

As others have said, it really depends on your fireplace. I've seen several gas "fake" fireplaces that draw air only from outside the house and so heat the house very well. I've also seen some real fireplaces that burn really hot but end up just sucking hot air out of the house.
posted by sninctown at 12:59 PM on February 27, 2011

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