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Q & A: How can I run added duct work from my house furnace to heat my attached garage?

house furnace
by www78

Question by John L: How can I run added duct work from my house furnace to heat my attached garage?
I ran ductwork from my gas furnace through the main plenum, across my basement, through a cement block wall into my attached garge. I also installed a booster fan to help pull the heat into the garage, but have very little air flow into the garage. The other ducts that branch from this main duct (plenum) have excellant air flow. Where did I go wrong?

The new duct measures: 8″ X 31/2″. I had to choose this size duct as I wanted to fit flush or above the floor joices. Please advise?

Best answer:

Answer by face9202004
Your furnace may not be big enough to handle the additional load. It may also be as simple as closing a few dampers in order to increase the air flow.

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4 Responses to Q & A: How can I run added duct work from my house furnace to heat my attached garage?

  1. Terry N

    Get a separate heater for your garage. The vent size that you ran is big enough to carry 70 cfm of air which is enough for a small bathroom. In some areas you may be violating codes. Most areas do not allow ducts to vent into a garage from your house duct. The reason being is you have the ability to get fumes from you garage directly into your house. If you have exhaust fumes, gas fumes and etc. they may easily get into you duct system. I wish you luck! They make regular small heaters for your garage.

  2. rethinker

    You may need to provide for return air flow. As a test try opening a window in your house (and a window or door in the garage if it is sealed.) (Or just open a door between the house and garage.) If the air flow to your garage improves then that means that you may need to install another duct from your garage to the house or to the furnace intake to equalize the pressure between those two areas.

  3. TB28

    You probably don’t have a large enough unit to support the additional ductwork, especially if it’s a fairly long section of ductwork. The cfm output probably isn’t great enough to support the volume of ductwork you have. You might just have to get a through the wall heatpump for the garage, if possible. Or if you’re in a real cold climate…a wall A/C and maybe a natural gas wall heater.

  4. tonalc1

    Close off one or more rooms’ dampers to increase the air flow to the garage.

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