Tapping house furnace for garage

[JackOfDiamonds]

I'm framing in a workshop in my existing attached garage. I'm not looking to keep it climate controlled, but during the season extremes I need a little supplemental heat in the winter and cooling in the summer.

The house furnace/AC is located in this space. It pulls air from the return ducts in the house, heats/cools it, and then sends it back to the house. I'm thinking, why not just tap the trunk duct on the main house furnace and pipe one small vent into the garage shop? Why spend thousands on a mini split when the main house unit is right there and has some spare capacity? The only problem is that if I do that, I would be pulling air out of the house. There's no return path from the garage to the furnace.... unless I make one...

Is this a dumb idea?

 

[PoorUB]

Is this a dumb idea?

Yes! against code too!

Run a car in the shop and you will end up with carbon monoxide in the house.

There's no return path from the garage to the furnace.... unless I make one...

When you pull air out of the house, and that is what you will be doing, plus you are pressurizing the garage, trust me, fumes and carbon monoxide will get into the house. It is a bad idea.

 

[JackOfDiamonds]

Hmm I might be stupid but I guess I'm smart enough to ask random people on the internet before I do stupid stuff...at least sometimes...

 

[loganb]

As mentioned, bad idea due to significant safety concerns.

How big is the area being dedicated to workshop? is it insulated? What's the available electric capacity? For a basic "take the chill out" heater it's hard to beat the simplicity and cost of electric, especially for something not used a lot.

 

[whateg01]

I put up a ceiling mounted heater just to take the chill off but it is capable of keeping my 750 sf insulated garage comfortable.

https://www.menards.com/main/heating-cooling/heaters/electric-garage-heaters/dyna-glo-240v-forced-air-electric-garage-heater/eg7500dgc/p-1562567355851-c-6328.htm

 

[JackOfDiamonds]

It's going to be one normal garage bay. I'm not sure how much sq ft that is. I know heating will be easy.

 

[macs_rock]

Another reason it's against code is because garages are disproportionately responsible for house fires. An opening in the duct work that leads to the house is a pathway for smoke and toxic gases to enter living spaces. It could create an issue for your homeowners' insurance. I'd recommend against it, and instead look to alternative heat sources.

Rereading, you mention you're framing in a shop inside the garage? It's possible, that done correctly, your shop would not qualify as a garage space depending on what you're doing in there. If it's something like a pure woodworking shop, or something like that, code might view it as habitable living space and not garage space. It would need to be separated from your garage to a higher degree to meet other code requirements in that case, but you'd definitely need to check with your local building authority in that case.

If your furnace is gas, something like this could be relatively affordable to install:

https://www.northerntool.com/shop/tools/product_200664874_200664874

It's what I plan to put in my garage this winter, but mine's detached and already plumbed for gas.

 

[JackOfDiamonds]

I'm not going to truly convert the space to living area. I plan to leave it functional as s garage (at least in theory, if I moved all my things out).

 

[danski0224]

Is this a dumb idea?

Yes.

Even without a hole in the garage for some sort of ductwork, one can still get CO poisoning or die from the car running in the garage, overhead door open or not.