Planning the Intake

Today, I further document the efforts to put heat into this old farmhouse. First, Hapy New Year. I absolutely recognize that it is past Winter Solstice and we have not had central heat since last spring. We have been getting by with small oil-based space heaters, as opposed to the ones with bright orange electrical coils and blower fans. Except for the sub-freezing cold snaps or windy days, the space heaters have been up to the challenge. On those sub-freezing days and nights, we have closed off sections of the house and hung blankets over doorways, etc. It has been an adventure. A couple of weeks ago, we had that which had once been thin gross cellulose insulation in the attic removed. So, what little heat that grot held in was now escaping to atmosphere. If being cold were not enough of a motivator before, it just went up another notch

In my last post, I could see the finish line. The furnace had been moved, mounted on a stand and angle-confirmed for safe exhaust. The conditioned air side had nee disassembled, cleaned, re-assembled and insulated. The gas line had been reconfigured to feed the furnace, with a "T" fitting so we could tap into it for the stove later. The electrical had been completed to the furnace, with an integrated worker-safety switch. The thermostat control cable had been wired into the furnace. So, what remained? Connecting the thermostat to the control cable, the exhaust vent and a cold air return. I started with the cold air intake, but shifted midway once I concluded that we could get the furnace running for a short time without a cold air return, drawing air mostly from the crawlspace.

Best Laid Plans

completed box

My original plan for the cold air intake was, again, to reuse as much of the HVAC stuff that I had removed from the furnace as I could, and add what was needed. When I looked at the pieces above the plenum, though, I decided they were not up to snuff. Whoever built that intake restricted the airflow considerably. Consider that the plenum is 16x20. The intake grate and filter are also 16x20. In between, they built a 90* turn using a 15x15 box. No wonder the furnace made so much noise when it was running; it was begging for more air. Simple math: 16x20 = 320 square inches. 15x15 = 225 or over 30% smaller. When discussing this with our electrician, he indicated that the restriction was probably not as bad as I thought. I never sought to be an HVAC expert; I just want heat and a quiet running system.

My plan was to install the original intake-side plenum and simply reverse it back on top of itself and then route over to the hole in the floor where the furnace used to sit. In theory, this was a great idea. There's an old Yogi Berra quote about theory and practice (In theory there is no difference between theory and practice - in practice there is). I think it applies here. The intake plenum was nearly as big as the conditioned-side plenum, and once attached it took up more than 50% of the space between the floor joists above and the vapor barrier on the ground. There would be no doubling-back on itself. For a moment I thought about having it turn on it's side, but that would route it where the exhaust vent is. We can't have both things in the exact same space.

I could route the cold air intake around the chimney and under the main beam of the house twice to leverage the existing hole where the furnace was. I started to take measurements and discovered that the space between the bottom of the beam and the top of the concrete curb underneath where the furnace used to be... was a little over 12 inches. I can't run a 16x20 rectangle through the 12" space. I could neck it down, but that would decrease air flow, create noise and potentially shorten the life of the fan since the reduction would be, like 25% (16 to 12). We have already suffered a 30% reduction before, and all of the downsides with it. Now, I could create a pass-thru that's like 12 x 26 so we don't restrict air flow, but when I took a minute and considered how long that intake would be, I figured it was time for a new course.

Before I got too wrapped up in the next step, I installed the intake plenum, sheet-metal screws, tape and all and then suspended it from the floor joists with webbing. With the plenum in a fixed location, I could take some measurements and make some plans.

New Plan, New Hole

simple floor grate

I started by determining where the joists were and marked them on the floor above. Since the joists are 16" on-center apart, the usable gap between the joists is about 15". While obviously that is less than 16, it is greater than 12, so I could put a hole anywhere without touching a joist and the neck-down would be significantly better than running it in a big circle around the chimney in the crawlspace. I also figured that the shorter the intake, the fewer opportunities for leaks. Consider that the area of the house above the furnace is our emerging kitchen. We had been making plans accommodating an intake along one wall, making an allowance for a space without a cabinet for that intake. Now that the intake will no longer reuse that big hole, we can reconsider the cabinet plan there, and have that floor fully repaired by our flooring guy (Thomas).

Both Boo and I have lived in older houses before and we both recall having air intakes in random places in the floor. For whatever reason, they seem to usually appear in hallways, so you get to walk on them a lot. Since there doesn't appear to be much reasoning other than have them centrally located, We are going to make the intake run as short as possible, setting the intake alongside the chimney. This will allow for a simple 90* turn at the end of the plenum straight up to the floor. I will add a little bit of ducting to align between the floor joists, but it will be quite short.

Cutting a hole in a perfectly good floor is hard to bring myself to doing. I ultimately asked our general contractor to do it. I figured he could get the cut straight and along the top of the joists on his first try. In the picture on the right, here, you can see the floor grate I got on Etsy from a guy (Doug) who hand makes these in Minnesota. Big fan of Etsy's handmade stuff.

Return

building the box

With the hole cut, I had a clear target for building the rest of the cold air return. My plan was to have a simple straight down, 90* bend to the plenum. At the point where the venting turns, the air filter sits and it is replaced from above, after removing the air return grate. To help hold the air filter in place, I planned to add a small rib an inch to the plenum side of the 90* bend. Last, I added a flip-down tab above the filter so that after the filter was set in, the tab would rotate down to hold it in place. Back to that Yogi Berra quote, the reality, however, was not simply applied theory. I mean the applied Yogi Berra theory would be there's no difference between what you plan and what you build until you start building. In that spirit, I got after it with a stack of 24" by 36" HVAC sheeting.

I started with a simple box that lined the sides of the joists down to the level where the plenum was. The top of the plenum is about 1 inch below the floor joists, so the box was little more than a 3-sided rectangle. With one sheet, I bent the side furthest from the furnace into a wide U, with the center section 22" across. I pre-drilled holes to connect additional pieces and screwed it into the floor joist level with the top of the joist. Adjacent to the shorter sections of the "U", I added 10-1/2" and 12-1/2" sides (also 24" long). The wider of the 2 sides was bent 90* for the final 2-1/2", and that short stretch runs along the furnace-facing side, reducing the aperture from 22" to 20" to align with the size of the plenum. I left an extra 1/2" of HVAC sheeting so I had something to connect the next pieces to. Along that 4th edge, I ran an 8-1/2" strip of HVAC, covering the floor joist plus an inch. That last half-inch was angle bent rear and upwards to provide something to attach to. Once I got this far, I shifted gears over to the exhaust vent so we could get the furnace tested while our electrician was available.

I am going to stop here since things got moving really quickly around the electrician's visit. Thanks, as always, for following along-