Circulating warm air when above a crawl space

[soundofsilence]

Hi everyone! We are new to wood burning inserts, and we have finally have secured some dry wood for the winter here in Chicagoland by way of slab wood from a local mill. It was NOT cheap ($575 delivered, not stacked, for 3 face cords, but it is ~15% MC).

I am testing our new unit (osburn matrix 2700) on cold fall nights to try to get a handle on things before the real cold hits. Our insert is right in the middle of our 1.5 story ranch home, and I am hoping to circulate the warm air to mostly heat our home with wood over the winter.

I was thinking I'd run the furnace fan overnight, but after thinking about it I realized the air intake is drawing from the crawlspace, which seems like that would only introduce more cold air into the house.

Am I wrong in this line of thinking? (Edit: I could be totally wrong, I do have low and high vents in rooms, it seems the lower vents blow out air so maybe my return is actual fed from the higher vents, which would be the warmer air that I want to circulate?)

We have a ceiling fan on one side of the house (opposite the bedrooms), which I thought might help matters by forcing warmed air back downstairs.

I've got a room temperature monitor on order from Amazon, which will record the daily high/low temperature in a given room, so I'll update as I experiment.

Would love any input from others with a similar situation or experience.

 

[webby3650]

Running the furnace fan does very little to help. The best thing you can do to move air is by using a low speed fan on the floor blowing cold air toward the stove. Also be mindful of your plumbing. Sometimes your water lines require some heat from your ductwork to prevent freezing. I haven’t had this issue, but my crawlspace is sealed up right. Of your isn’t, keep an eye on it.

 

[EbS-P]

Is the crawl space conditioned? Studies have show a 5-9x increase in the leak rate of a house with the hvac fan on.

 

[stoveliker]

I do think it's highly unlikely your system pulls in cold air to heat it and distribute in the home. Unless the crawlspace is inside the insulated envelope.

Furnaces are normally set up to get air from the home, heat it, and put it back in the home.

Your furnace may get combustion air from a crawlspace, but that does not get blown into the home.

 

[begreen]

Is the crawl space conditioned? Studies have show a 5-9x increase in the leak rate of a house with the hvac fan on.

I'd like to understand these studies better to see if there were unusual conditions. A balanced hvac system should be pressure neutral and not sucking in vast amounts of outside air.

@soundofsilence is the system set up to provide makeup air from the crawlspace? Normally no air is pulled from that location. It can make for a musty smelling house.

 

[EbS-P]

I'd like to understand these studies better to see if there were unusual conditions. A balanced hvac system should be pressure neutral and not sucking in vast amounts of outside air.

@soundofsilence is the system set up to provide makeup air from the crawlspace? Normally no air is pulled from that location. It can make for a musty smelling house.

It’s all about duct leakage and differential pressure. New (less than 20 years old) homes are probably less built to a higher standard but old homes are super leaky companies to todays best build practices. Any leak inside supply duct must be made up by infiltration. 10-15% duct leak rate is average. Moving 1000cfm now we are sucking in 100-150 cfm of cold air.

There is good reasons running the HVAC fan does not help heat distribution. I’ll try and look some of this up some time. In the mean time HVAC school podcast is where I haves leaned the most.

 

[stoveliker]

This is only an issue (of this magnitude) if the ducts are running outside the insulated envelope (e.g. the attic).

 

[GrumpyDad]

Would they want to push the warm air toward the colder rooms? Naturally the stove wants to suck in air that will then go out the chimney, and the radiant heat is all in that room. If they put a fan blowing colder air toward the warm room, isnt that speeding up the process of the cold room getting colder as it has to suck in new air to equalize and that new air is likely coming from the crawlspace

 

[vbu]

Would they want to push the warm air toward the colder rooms? Naturally the stove wants to suck in air that will then go out the chimney, and the radiant heat is all in that room. If they put a fan blowing colder air toward the warm room, isnt that speeding up the process of the cold room getting colder as it has to suck in new air to equalize and that new air is likely coming from the crawlspace

No, cold air is denser and settles at the floor. It is easier to move denser air. New air will not get sucked in, it will be replaced with warm air from the area you're blowing the cold air into.

I've tried running the furnace fan, and I had no luck with it. As a matter of fact it seemed the whole house was colder. I suspect my ductwork is leaky, and I have a package unit outside so all the air is going outside before returning back inside, which doesn't help things.

I do question the claims of air leakage rate increasing 5-9x by running the furnace fan. It may sound very dramatic but that is not necessarily the case. 5-9x of what rate to begin with? 5x a very small amount is still a small amount.

 

[stoveliker]

Would they want to push the warm air toward the colder rooms? Naturally the stove wants to suck in air that will then go out the chimney, and the radiant heat is all in that room. If they put a fan blowing colder air toward the warm room, isnt that speeding up the process of the cold room getting colder as it has to suck in new air to equalize and that new air is likely coming from the crawlspace

you can do the test yourself. Put a fan on the floor in a doorway pointing to the stove room. Run it on low (important; so as not to create too much turbulence). Then tape some toilet paper to the top of the doorway. You'll see it blow towards the colder room, indicating warm air from th stove room is moving to the colder room to replace the cold air you just removed from there with the fan.