Starting to Solve for the Furnace Conditioned Air

If it feels like this furnace saga is unending, it definitely feels that way on my end. Every passing day, the weather gets colder, but I will not short-cut this work. It being correct is too important for health and safety. I do want to stress that our electrician, Gary, owned an HVAC company and did hundreds of furnace installs. He consulted on the plan and will be reviewing my work / confirming everything when he connects the electrical at the end. So, we keep going. Honestly, the hardest part is putting on cold clothes in a cold house to get into an even colder (and, frankly, filthy) crawlspace.

Anyway, today, I am focusing on what I refer to as the "conditioned" air. This is the air that is leaving the furnace, presumably warmer, but on those days we are only running the fan, it will just be filtered. For simplicity, and since this was the way I did it, we are working from the furnace exit to the floor vents. Sadly nothing was as easy as it seemed in my head, resulting in this expanding to cover many weekends. This is just the first part of it.

Furnace Mounted

checking pitch

Before I shift to the new topic, I had a couple of final things I needed to do with the furnace: confirm the pitch and attaching it to the stand. To confirm the pitch, I found a straight bit of hard plastic tubing that was over a meter long. At exactly 1 yard (36 inches or 3 feet), I attached a 1 inch thick piece of scrap wood. Recall the minimum pitch for the exhaust from a furnace installed on it's side is 1/4 inch of rise per foot (or 1 inch of rise per 4 feet). I figured that if I made sure my work was at 1 inch per 3 feet I would have exceeded the minimum and have some wiggle room for the furnace or the ground shifting. I set the plastic tube jig on top of the furnace and set the level on top of that. Bubble between the lines? Yes, so somewhere along the way between my level crawlspace patio and the top of my furnace the world tilted. Glad it pitched the right way for me. I had planned to shim the exhaust-exiting end. Do I want to know why it shifted? Yes, but I think because the furnace is offset rearward (you can see the stand in the lower right corner of the picture), the weight of the burner is causing the tilt.

Feeling fortunate, I grabbed a longer run of that plastic tubing to simulate the exhaust from the furnace to the chimney and set the jig on that. We have good angles, though I will need to add a brace near the chimney end to make sure it doesn't relax downward over time because the exhaust needs to enter the chimney near the top of the hole. To make sure the rest of my efforts don't cause the furnace to move, I sent sheet metal screws through the now-bottom of the furnace into the stand so it is fixed-in-place. I re-checked the pitch, and it is still 1 inch for 3 feet even after sending screws through. On to the conditioned air!

Plenum Pablum

building a plenum

The first thing the conditioned air enters is a box called a "plenum". It acts as a singular junction box for all of the conditioned air, but it does more than that. Consider what happens when your furnace kicks on. The fan starts and very quickly air pressure increases at the furnace outlet. To cushion the system from that large pressure increase, there is the big box (plenum). Secondarily, this box allows the air to flow smoothly and evenly into the ducts. If the box is too small, or nonexistent, the air would not enter all of the vents evenly, leaving some spaces with too much airflow and others getting an old microbus heating system experience (virtually nothing). I am over-simplifying it as there are air dynamics about back pressure from the branch lines, etc. that I really don't understand.

The original plenum to this house was just that. The original plenum, as in it was the plenum when the heat source was something other than gas, we believe. I think it may have been coal based on the soot we have found and then oil since there's an old oil-tank shed in the back of the house. Still, airflow is airflow and if the thing that is pushing the air is heated by coal or wood or geo-thermal fanciness, and the venting beyond the plenum is the same, I would expect the plenum should be relatively the same. Of course, 15 years after the house was built they added 2 rooms to the back, so maybe the plenum has been the wrong size since. I suppose, this could be a recently-added bit, but none of the other metal venting looks remotely new.

furnace entry

The plenum was almost a meter tall and about 2 feet square. From my research, this is excessive, but was unable to arrive at a clear answer as to what size it should be, like with a calculator or simple rubric. The most basic advice seemed to be "make it a little bigger than the outlet of your furnace". I am fairly sure that would lead you to a too-small plenum and poor circulation impacts, but I defer to experts. Regardless, I took all of this and decided that I needed the plenum to fit in the space I had, so if it was smaller, well... we'll just deal with it and worst case I'll make another one later. I simply built what would fit in the space without digging again. 

Plenum Fab-lum?

So, what did I do? The original plenum had the air enter from above and then it routed air sideways out 2 rectangular vents and one round one. In order to keep the orientation of the rectangular outlets, I needed the top covered and a new entry added on one of the other "horizontal" sides. In order for the 1-meter-tall plenum to fit into my not-quite2-feet-high crawlspace, I needed to shorten it too. So, I measured and then cut (with the death wheel) 9 inches down from the top in each of the 4 corners. Then, I folded the sides in like a cardboard box. With a hammer and dolly, I squared the newly folded lines. Content with the shape, I drilled and then pop-riveted the new top in place (upper image). Last, I sealed all of the edges with high-end foil tape.

Plenum added

So, I have a box, but no entry for the furnace. Onto the side which had a circle-vent, I applied blue tape and then measured the dimensions of the furnace exit. Half an inch inside this rectangle I marked the actual cut line. That extra 1/2" will be the lip which will press against the exit "flange" on the furnace. I cut the hole with the death wheel, turned the lip with some pliers and cleaned up the fold with the hammer and dolly again. The circle-vent used to feed the bathroom, which does not currently have a vent. If we re-introduce a vent into that bathroom, I can re-integrate it into the system.

I lowered this new plenum into the crawlspace, army-crawled it past the furnace and attached it to the furnace. It barely fits between the ground and the floor joists, but it does and it did not disturb the angle of the furnace. Once it was in place and the edges sealed, I was ready to look back at the main trunk / vent lines (the big rectangular ones).

As often happens when I get to posting about something, this got very long. This feels like a logical transition point so, I am going to stop here, and pick it up next time. Thanks, as always, for following along-