Adding furnace combustion air vents

(Warning – before you start putting in combustion air vents: Wear gloves when working with sheetmetal. Trust me!)

A view of the furnace with the new vents in place.
A view of the furnace with the new vents in place.

Before we started the basement project, our furnace had one four-inch combustion air vent that drew air in from the outside. When I got my building permit, I learned that I’d need to have two six-inch combustion air vents coming into the furnace room.

The first thing I needed to figure out what how to cut the holes in the side of the house for the wall vent caps.

I looked into buying a six-inch hole saw, but decided, unless I’m building corn hole sets, I’d probably never use that tool again. I figured I could freehand it using the jigsaw.

Luckily, there was already a hole in the side of the house where the smaller vent exited. So all I had to do was remove the old vent, place the larger vent over the existing hole and trace around it. Next, I drilled a quarter-inch hole at the top, bottom and each side of the circle and then starting cutting with the jigsaw. The jigsaw cut through the siding and the insulation, but didn’t quite make it through the 2×6. For that, I had to bring in the reciprocating saw. Once I got it all cut out, I inserted the vent and made a couple minor adjustments.

The holes in the house. The wire is a sprinkler control wire that a previous owner ran through the vent hole.
The holes in the house. The wire is a sprinkler control wire that a previous owner ran through the vent hole.

I positioned the other vent so that the vent frames didn’t overlap and repeated the cutting procedure. Luckily, the hole was right above the sill plate, that’s something I should have taken into account.

I bought two 6-inch vent caps with dampers which weren’t really what I needed, but I got them to work. This vent cap with screen is what I needed, but Home Depot didn’t have it in stock.  I made the others work by removing the damper and installing and gluing some pest screen.

With the outside work pretty much done, I headed back inside to plan out the rest of the install. Because the vents are running perpendicular to the floor rafters, they have to run underneath the rafters. The holes I put in the house are above the foundation. So, the vent and the pipe don’t line up. To solve this, I bought two adjustable elbows for each vent and made an s-shape of sorts to get the pipe to line up with the vents.

The wall vents in place.
The wall vents in place.

I also bought five sections of 5-foot-long duct round metal duct pipe and some duct strapping. I measured the length and cut one section of duct pipe so that a 90-degree elbow would fit on the end closest to the furnace. Attaching the duct to the floor boards was a bit of a challenge with just one person, but I accomplished it by screwing the strapping in loosely on both sides, and running the pipe through the loop. On the other end, I screwed one side of strapping in before inserting the pipe and with the pipe hanging on the loose strapping, I screwed the other side up. It took some adjusting to get it tight. Once tight, I screwed the elbows directly into the furnace’s hot-air return.

On the other side, I screwed one of the elbows that made up the “s” shape to the ducting inside. Then I headed back outside and pulled the vent and the other elbow out. I screwed and taped that elbow to the wall vent and routed them back through the hole. Back inside, I screwed the two elbows together and taped up all the seams.

Code says that one combustion air vent needs to be within 12 inches of the ceiling and the other needs to be within 12 inches of the floor. So, I left one of the 90-degree elbows, and attached a cut-to-length piece of duct to the other. I bought these tin snips and they are working great.

An inside view where the vents exit the house.
An inside view where the vents exit the house.

Unfortunately, in the middle of this project, we got a round of cold weather and snow. These big pipes bring in a lot of cold air. I stuffed a towel in one, which kind of helped, but it’s still cold in the basement. This is usually a big no-no, but I figured it was OK in this case because there are no walls in the basement yet, and the four-inch vent has been working up to this point. Because of all this cold air coming in, code requires the furnace room be insulated with minimum R-13 in the walls and R-19 in the ceiling.  It also requires solid, weather-stripped doors be installed.

All said and done, I think the parts and tools for the bathroom and furnace combustion air vents cost about $150. I am reusing one six-inch elbow I had laying around and the old four-inch combustion air vent pipe and elbows in the bathroom, so that saved a little bit of cash.

Up next, I’ll be installing the venting for the bathroom fan and then adding some registers to heat the other basement rooms.

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