At this time of year the temperature is perfect in South Texas…for about a week anyway. I sometimes open the windows, turn off the a/c, and just turn on the fan. I have the idea that that circulates air and brings air in from the outside. Is that true or am I fooling myself?
It does circulate air but it does not bring in air from outside.
Well, if it’s a combustion furnace (natural gas, propane, oil) then it must be vented to the outside or else you’d die.
So, if air is blowing outside, then air must be being drawn from the outside. I believe you are correct in your circulation theory.
ETA: Wait. You may not even HAVE a furnace. What a strange concept.
I’m in a rental house. It was built in 1936 and the central heat/air was added… I don’t know when. It’s all electric.
Even if the fan doesn’t bring air in from the outside, if I have the windows open does some sort of circulatory-air-exchange-magic happen?
Nah. I have a forced-air electric furnace too and running the fan would do nothing to bring in outside air. It sure feels nice though because it does circulate the interior air. I have a basement though, so cooler basement air gets moved upstairs. I don’t suppose you have a basement?
The fire side air is totally separate from the space air. And on every furnace I have seen separate fans.
I have no idea what this means. Furnaces I’m familiar with have one fan.
No basement.
Mine has two. One fan is for circulating air in the living space. The second fan is for combustion. This fan is the only fan that has anything to due with the combustion process.
The fan USUALLY is between the combustion chamber and the ducts.
It has ZERO to do with the flue - if it was blowing air up the flue, it wouldn’t be real useful, would it?
(my furnace has a small blower inside which blows a bit of air out the flue to ensure it is open before lighting the gas).
The fan gets air from the ‘cold air register(s)’ - open grills usually in the floor of older homes, now on the wall below the furnace in newer homes.
It will NOT bring in fresh air unless it is in the basement - in which case, the air in the basement may count as ‘fresh’ - it is usually cooler.
Under the OP’s scenario, you might be getting some fresh air from outside. If there’s a nice breeze a-blowin’, you would get some natural cross-ventilation through your house just from that.
But if the air outside is still, then it’s probably rather still inside too, and you might get only a minimal amount of random mixing of new outside air coming in at the windows.
But if you have a fan running inside, then you have air moving around inside. Just from the turbulence of all this air moving around, you’ll have increased mixing of inside air and outside air at the windows. So you’ll get some mix of fresh air coming through your house from that.
attic fan
window box fan blowing inside
Harem girl with big feathers
It will bring in fresh air due to the pressure difference and the open window. In the place where the air is sucked into vents that will be a negative pressure so will suck in air from outside. The far rooms will have positive pressure and will have air escape out the windows. If you close those doors, build up more positive pressure in the far rooms (and also more negative pressure close to the sucking vent), you will get more outside air mixed. It can be pretty effective in doing what you desire.
When I open the door to the outside to my utility room with the furnace fan running there can be quite a inrush of fresh (cold) air for this reason.
I guess this is kind of what I was thinking. Thanks.
You are circulating air within the house, with some exchange between inside and outside air through the open windows. This can help even out temperatures in a house, but only if there is an open passage between every outlet and a corresponding inlet. If you have upstairs bedrooms with outlets that depend on airflow through the doorways to a central intake, they aren’t going to show much effect from running the fan unless you leave those doors open.
In general, I’d say there’s no benefit from running a central HVAC fan by itself. There might be a few limited cases where it circulates hot air out of some rooms and corners, but overall I’d say you’re just wasting the 'lectric to run the fan. Any benefits are likely to be imaginary - you hear the fan running, which is always associated with increased comfort, so it makes you feel good.
Remember, too, that fans do absolutely no good running in an empty room. Their only benefit is moving air past your skin to cool you through convection and evaporation. People who have a houseful of running fans with no one near them are simply wasting juice and adding heat (from the motors) to the house.
Respectfully disagree. If the windows are open, then air from outside is drawn into the house and recirculated through the return vents along with whatever air is already in the house. I’m not familiar with a system like you describe in the first sentence, as I was under the impression that there have to be return air vents throughout the house. Ours has one upstairs and one downstairs. But I’m not a heating guy.
I can’t see outside air being more than a negligible contribution to the mix. HVAC is a closed-loop system in most homes (and offices): you suck in X cubic feet, you blow out X cubic feet. Losses and inefficiencies aside, it’s a balanced system. Unless the intake is near an open window (which it’s probably not: bad system design) and that room has restricted outflows (ditto), the only air that’s going to come in from outside is pretty much whatever would drift/blow in without the HVAC running.
Open the windows and let the air in, by all means. But running the HVAC fan adds next to nothing in most cases. (Except expense, noise and some heat.)
All the previous posters are wrong.
The fan circulates air in your house, sucking air in through the intake and exhausting it through the vents. When you open windows, you cause the house air to exchange large air volumes with the outside.
So with open windows, parts of the house have now exchanged large volumes with the outside of the house. And with the fan running, those cold/hot air regions of the house get mixed together by the forced air.
Not if there’s nothing driving the exchange. Opening windows on a hot, still day will do very little to “freshen” the inside of the house.
And if there is any thermal or wind force mixing inside and outside air, running the HVAC fan will do little to add to it.
I mean, criminy, folks, this isn’t the Middle Ages, with “good air” and “bad air” we have to cope with.
You’re probably correct. Our vent is indeed near a sliding window and located pretty much directly above the furnace room. Like I said: not a furnace guy.