fresh air from the flue

January 11, 2007 6:07 AM

I try to protect my health inside my abode by allowing access to outside air at all times, even in extreme temperatures. When my fireplace is not in use, I keep the flue open anyway, for that purpose. My question is: If the fireplace *is* in use, will the open flu serve the same purpose, or do I need to open a window as well?
posted by theskillspool to science & nature (6 comments total)

Depends on how well designed the flue/fireplace is.

In most cases the fireplace draws air from the room and pulls air from the flue (thus why most fireplaces are terribly ineffecient at heating the space they are in.)

Honestly opening a window in weather cold enough to use a fireplace will further negate the heat transfer from the fireplace. I wouldn't worry about the fireplace beyond making sure it has had a good cleaning (especially if you don't know if it has been cleaned before.) Creosote building up in the chimmeny stack is a bigger health risk (by increasing risk of house fire) than the possible shift in air quality resulting from using the fireplace.
posted by JFitzpatrick at 6:13 AM on January 11, 2007

To directly answer your question, you do not need to open a window when using your fireplace. Now, on to the long winded version: leaving your fireplace flue open is just sending heat up the pipe. you're not getting "fresh" air with this method, because the flue draws air (and heat) out, not in. That air is simply replaced by other air within the building. All buildings have a certain rate at which they draw in new air from outside. In that sense, your indoor air is "fresh". It's not as if you live in a sealed environment and are breathing your own stale air. If you truly want "fresh" air, just open a winow occasionally. (I will not not pass judgment on the idea of "fresh" air being somehow more healthy.)
posted by cosmicbandito at 6:27 AM on January 11, 2007

Depending on the age of your home, and its fireplace, I could disagree with both JFitzpatrick and cosmicbandito. Newer homes, with energy efficient features, are often fitted with fireplaces which draw combustion air from outside the home, via a second vent, which looks like a dryer vent from outside the home, and avoids the problem of older fireplaces drawing room air up the flue. Furthermore, most homes built in the last 10 years to energy efficient standards have been Tyvek wrapped, and heavily insulated with joint treatments in areas such as the sill plate, primarily to eliminate "leakage" air. This to the point that radon infiltration is a significant issue in some areas with suspectible geological formations, which form and trap radon gas at higher rates. In these homes, leaving the flue open does very little, if anything, to increase fresh air infiltration, as the cold sink rate of outside air is greatly offset by the rise of indoor heated air, but the heated air column that would normally form in older homes is greatly impeded by the limited pressure differential than can form in a house that is well sealed, insulated and weather stripped.

If you're concerned about fresh air for your health, carry your concern through to real radon and environmentals testing, and look at the right way of providing positive air exchange, that works with the HVAC system of your home to intake, clean, and distribute outside air in a seasonally appropriate manner.
posted by paulsc at 7:00 AM on January 11, 2007

The only time you would get air in from your chimney is if you had some powered fan (large stove exhaust fan for example) de pressurizing your house. This is actually a dangerous condition if have any gas fired appliances as it can lead to CO poisoning. Any time your flue is open you will be losing heated air out the top, even more so if a fire is burning. The air is replaced from all the cracks in your building envelope (openings around doors, windows, electrical boxes and other penetrations and even right thru building materials like gyproc).

To your question: if your fireplace isn't drawing well or you are noticing smoke in your room you should crack a window otherwise the infiltration is enough to maintain fresh air.

cosmicbandito writes "(I will not not pass judgment on the idea of 'fresh' air being somehow more healthy.)"

Besides the radon risk paulsc mentioned houses with air change rates less than about a 1/3rd an hour are unhealthy. Outgasing of materials and mould growth become a health hazard. It's pretty easy to seal a house up tighter than that and if so a Heat Recovery Ventilator should be installed to bring the air change back up to 0.5 per hour
posted by Mitheral at 7:21 AM on January 11, 2007

Mitheral, I won't argue with you about poor infiltration being a health hazard. I just don't think that air coming directly from outside is any healthier than air that infiltrated via normal methods. theskillpool doesn't mention what sort of building s/he is living in, but I'd bet they don't have it sealed up tight enough for infiltration to drop below the threshold of healthiness.
posted by cosmicbandito at 7:57 AM on January 11, 2007

I'd bet they don't have it sealed up tight enough for infiltration to drop below the threshold of healthiness

Seconded.
posted by chrisamiller at 9:58 AM on January 11, 2007