CO2 from ductwork, HVAC and gas tank in attached garage

Question

We live in older house in northern US with annual wildfire smoke and have exposed ductwork, water heater and HVAC unit in attached garage. No wall or room separating it from rest of garage.

How do we make sure exhaust fumes, CO2 and other dirty air don’t go in to house? Is a exhaust fan cut in wall of garage adequate?

Reference: https://basc.pnnl.gov/resource-guides/no-ducts-or-air-handlers-located-garage

https://ventilation-system.com/series/co2 Commercial parking garage guide - http://www.kb-sales.com/uploads/nKFe6quV.pdf

Answer 1

In answer to the question you didn't ask:

I looked at the government article you linked. The issue is the HVAC system drawing air from the garage. This air is potentially contaminated with CO, paint fumes, etc, from normal life inside a garage. It's got nothing whatsoever to do with air leaking directly into the house from the garage, which is what properly sealing the garage door would prevent.

In response to those concerns, the article recommends sealing the HVAC system from the rest of the garage. To my reading the key items are in points 3 & 4 on the Description page:

3. If any natural draft or power-vented equipment (HVAC and/or water heating equipment) is located in the closet, provide sufficient combustion makeup air through a ducted outdoor air intake to prevent back-drafting of the flue gasses...

4. If ductwork must be located in the garage framing, seal all joints and seams in the ductwork with mastic paste to the appropriate thickness... Then encapsulate the ductwork and framing cavity in closed-cell spray foam...

i.e.

3. Make sure the system has external air to breath (point 3). If it's pulling in fresh air from outside, it's not sucking in any potentially dangerous things from the garage.

4. Make sure the ducts are properly sealed (they advocate spray foaming once the joints are sealed with mastic - belt and suspenders, but go for it if it makes you feel better) so that no potentially dangerous chemicals get into the ducts.

For point 4, remember that if there's a leak in the ductwork, it's going to blow hot or cold conditioned air out into the garage, it won't suck it in because the inside of the ducts are pressurized when the fan's blowing. When the fan isn't blowing, it's possible that air from the garage could get through a leak into the duct, then be blown into the house when the fan kicks in, but really, if there's no pressure differential between outside the duct and inside the duct, there won't be much in the way of air exchange through a tiny air leak.

To my reading (and this is purely my opinion, take it for that), the recommendation to build a hermetically sealed chamber around your mechanical systems that are already installed in the garage, then supply outside make up air and seal all ductwork with mastic then spray foam amounts to belt and suspenders, duct tape, rope and staples overkill.

• Follow their advice to seal up the mechanicals.
• Make sure it's drawing make up air from outside.
• Make sure your ductwork is properly sealed.
  • This is a good idea anyway, as it ensures that the money you've spent heating/cooling it makes it into the house where you want it.
• Install detectors in the garage and inside the house.
• Stop worrying and enjoy life.

Answer 2

Typical measures to prevent garage air from getting into the house include the following:

• weatherstrip and put sprung hinges (auto closing) on the person door into the house
• caulk and/or expanding foam (might as well use orange firefoam) where ducts/wires/etc pass into living space
• negative pressure (ie, pulling air from the garage and pushing it to the exterior) is certainly an option, though relatively uncommon

If you’re concerned about carbon monoxide (CO), you can install a CO detector. Check the packaging to make sure the detector isn’t ruined by car exhaust. (Some smoke detectors don’t want to be installed in a garage.)