Why run a whole-house exhaust fan in winter?

I have a single-floor 2-br townhome, full finished basement, gas furnace, all other appliances are electric, located in metro Mn., built in 2007. The house gets dry enough in winter that we use a small humidifier in our bedroom at night. The builder’s instructions say to leave the whole-house fan running all the time, but I question the need to do so, especially in below-zero temps. Blowing hot air out the roof seems wasteful. Opinions?
Furnace was replaced 3-4 years ago, and we have the installer inspect, clean and tune it every fall.

Whole house fan? or Air exchanger?
Whole house fan I have no answers, air exchanger I could explain.

Eelpoutguy wrote:

Whole house fan? or Air exchanger?
Whole house fan I have no answers, air exchanger I could explain.

Okay, I think I figured out the air exchanger. There’s a control box on the furnace set to run the furnace fan for 10 minutes every 30 minutes. That recirculates the air inside the house. There’s also a fresh air intake about 8″ diameter running between two joists into the furnace room ceiling. No fan in that. The whole-house fan is in the main floor hallway, exhausts out the roof, with an on-off switch on the wall.

I got rid of our air exchanger.
Ive got TnG in most of the house and if humidity is not correct it shrinks and like you I am running humidifiers to keep it correct. You need to watch condensation on windows however during really cold snaps.
I find it very comical that extra money is spent to seal homes so tightly to “save energy” that people tell you to use energy to bring fresh air into your house that also need to be heated/cooled.

I wouldn’t use the whole house fan even if it had adequate make up air which I’ve never seen.
The 8″ fresh air intake is for the gas furnace which I’m guessing is atmospherically vented.

House built after 2000 needed to be mechanically ventilated, That’s where airexchangers came in. (there were other ways to make code but difficult and costly).
Because homes were now being built “tight” without changing the warm moist air for dry(er) air homes were rotting from the inside out.
It is a balancing act in the winter between the dry (air exchangers) and humid (humidifiers).
The best advice is to get a humidistat and monitor it that way and make adjustments daily especially when it gets this cold. The newer units do this for you.
I go by the “when I wake up I should see about a pencil size bit of condensation on my windows”. Anything other than that I make an adjustment.

HVAC contractors can probably add to this. Or take away.

I totally agree with Mark, but that is what the “experts” tell us.
BTW – an air exchanger has 3 filters that need to be cleaned regularly for the machine to operate at peak performance.
My Daughter just bought a 20 year old home and I opened up the unit. Had to pry the cover open and when I finally got it open I could tell the filters have never been cleaned.

Buy a digital humidistat off Amazon and leave it on the counter somewhere. I regularly check mine and make sure I’m between 30-40% humidity (wife likes it closer to 40%). Currently I have my air to air exchanger unplugged

Eelpoutguy wrote:

The best advice is to get a humidistat and monitor it that way and make adjustments daily especially when it gets this cold.

During the last cold snap around Christmas, I noticed a fair amount of frost/condensation build up on the edge of a few of my windows. So I tried your humidistat monitor thing, which was about 15 bucks at Menards.

Turns out the amount of frost or condensation on the windows I have was nothing really to be worried about, because my humidistat reading was normal.

Thanks EPG

Are you sure they didn’t mean to leave the furnace fan on to circulate air through the home? This is what we do. Fan hasn’t been off for 5+ years now other than to change filters. Does a great job of evening out hot and cold spots in the home. Got rid of the humidity in the basement to.

I have condensation on my windows most of the winter. When it’s really cold it gets bad and I need to wipe them or wait for the ice to melt first. My window trim is getting slightly black and rotten in spots but that is after 20 years. In the summer I run a dehumidifier in the basement. My air exchanger is also unplugged year round.

Lindy what type of air exchanger are you running? Hrv or erv ? The air exchangers are designed to remove excess humidity from your house.

lindyrig79 wrote:

I have condensation on my windows most of the winter. When it’s really cold it gets bad and I need to wipe them or wait for the ice to melt first. My window trim is getting slightly black and rotten in spots but that is after 20 years. In the summer I run a dehumidifier in the basement. My air exchanger is also unplugged year round.

Well plug it back in.

Mike W wrote:

Are you sure they didn’t mean to leave the furnace fan on to circulate air through the home? This is what we do. Fan hasn’t been off for 5+ years now other than to change filters. Does a great job of evening out hot and cold spots in the home. Got rid of the humidity in the basement to.

Bingo^

Relatives in HVAC both commercial and residential. They claim everyone should run the fan on their furnace non stop. We also run ceiling fans a ton to keep air moving.

Our home is brand new, and I have a thermostat upstairs that I watch the humidity levels on (I shoot for ~40%) and downstairs I have a humidistat that I leave laying in the laundry room and check periodically.

I’ve always been told that if in the dead of winter you have a little humidity on your glass, that’s what you’re shooting for. Ideally the sun and a little warmer temperatures into the afternoon should take care of most of it. If it’s running or dripping down you need to adjust.

If you have large spans of wood floors or tongue and groove you want to keep a bit more moisture in the home.

Eelpoutguy wrote:

<div class=”d4p-bbt-quote-title”>lindyrig79 wrote:</div>
I have condensation on my windows most of the winter. When it’s really cold it gets bad and I need to wipe them or wait for the ice to melt first. My window trim is getting slightly black and rotten in spots but that is after 20 years. In the summer I run a dehumidifier in the basement. My air exchanger is also unplugged year round.

Well plug it back in.

It’s the kind of condensation Bucky described above. By afternoon most of the time it is gone and not typically touching the wood trim. Only when it’s -20 we get the ice deal and I wipe them. Looking at my humidity level right now is 32. And we have lots of wood in the house including some real nice wood floors. Not sure what the answer is.

Don Carlisle wrote:

Lindy what type of air exchanger are you running? Hrv or erv ? The air exchangers are designed to remove excess humidity from your house.

It’s an HRV. I’ve not noticed any improvement with humidity levels while it’s running.

I like 30% in the winter, anymore and ice builds up on the north facing windows, cold snaps like we are in now, I turn off my whole house humidifier off and will turn it back on this weekend when temps get above 20.Its a love hate thing.

Exactly. I just came to the conclusion that there were bigger battles in life to worry about and so just unplugged both the air exchanger and whole house humidifier. I guess each house may be slightly different in how it breathes, how much humidity you put in the air with cooking, showers, etc.

lindyrig79 wrote:

Exactly. I just came to the conclusion that there were bigger battles in life to worry about and so just unplugged both the air exchanger and whole house humidifier. I guess each house may be slightly different in how it breathes, how much humidity you put in the air with cooking, showers, etc.

The only thing that could create an issue is if you have bathrooms that are vented ONLY with air exchangers. The pro’s and con’s of doing it this way are debated and specific to different scenarios (from what my pea brain can understand). Some say that only exhausting bathrooms with traditional fans depressurizes the house essentially exploiting other air leaks bringing in cold air.

Other perspectives say that exchangers don’t get the humidity out of the house quickly enough in places where showers happen regularly.

We have large bathroom fans in the x2 baths on the main level, but just utilize an exchanger in the basement guest bathroom that sees substantially less use (showers). I don’t unplug it, but keep the exchanger set at a medium setting and remind guests to just click the button on or even gap the door an inch when they shower to help

Mike W wrote:

Are you sure they didn’t mean to leave the furnace fan on to circulate air through the home? This is what we do. Fan hasn’t been off for 5+ years now other than to change filters. Does a great job of evening out hot and cold spots in the home. Got rid of the humidity in the basement to.

This is one of the greatest features of our new furnace. The fan has a recirculate setting that runs the fan at low capacity periodically.

There should be a humdistat on the hrv to control the humidity level. Are you keeping the filter clean ?