A house is made up of many components, with many overlapping and at times even conflicting goals. Its most essential purpose is as a shelter from the elements. That means making it shed water and retain warmth (or cool) and in the modern day that means a properly functioning HVAC (heating, ventilation, and air conditioning) system.

The HVAC system may well be the most complicated mechanical system in most modern houses. In our climate the HVAC system must heat the home evenly and efficiently, and cool it in the summer as well. It must maintain comfortable humidity levels in all seasons. It needs to distribute air well enough to avoid hot / cold spots while also blowing slow enough to avoid drafts. With a building as air-tight as ours is meant to be, there are concerns about stagnant air, mold, and other things we used to never worry about. And you also might like it to not be too loud (something we were acutely reminded of in our rental where the furnace sounds like a 20 year old garbage truck just pulled into the garage.)

The HVAC system is also the hardest to accommodate in the structure. The systems themselves take up space, and the ducts that carry air can also be quite large. They are far larger and more sensitive to bends and sharp direction changes than plumbing or electrical wiring. And you can’t just go cutting 10 X 18″ holes all around your house to accommodate trunk ducts wherever is the most direct route.

This is an area of the design where I may have gone just a little overboard in my research and development…

My first forays into what makes for a good system in a modern, tightly air-sealed and well-insulated home did not inspire confidence. It was clear from many different sources that modern homes called for radically different HVAC design but that most contractors were not evolving. Many energy conservation groups, the federal government, and even the Air Conditioning Contractors of America (ACCA) trade group were instructing, imploring, and even begging HVAC contractors to please update their methods and do proper sizing of HVAC systems. And many of these articles and slide presentations were full of stats about how essentially none of the contractors actually followed the rules.

It turns out most of them just look at the square footage of the house, pick the largest possible furnace and A/C unit that would ever be needed, and sell you that. A larger system costs more, requires more ducts which is also more money, and will fail earlier, earning them more money. Nobody ever complains if a system produces too much heat or cold, it just fast-cycles on and off which they don’t know enough to worry about. So I decided I’d do the math myself.

I went on a deep dive that included watching hours of youtube videos on basic science, a long saga of inter-library loan to acquire the industry manuals for designing systems, and hacking (yes, illegally breaking into…) an Excel file lifted from the internet to use as a Manual J template. In the end I calculated that our house needed about 35K BTU of heating and about 1.5 tons of cooling. I also calculated the air flow required for the house, as well as to each room of the house in both heating and cooling seasons. (As a side note, you learn a lot of interesting things going through this process. For example, the two large windows in our stairwell generate more cooling load on the house than all of the walls of the home combined. Also, having too many houseplants increases humidity and raises your cooling requirements!)

When I went to get bids for our system I mostly confirmed the sad state of affairs described in the industry papers. None of the four companies I talked to asked for details about how the house was to be made or the system design goals, they just asked for a copy of the floor plan and came back with bids for a huge furnace (95 – 130K BTU) and A/C (4-5 tons). The ones that did any detail work at all insisted on running huge trunks from the basement to the attic so they could put all their ducts in these spaces, in direct contradiction to the recommendations of their own trade association and engineering design documents. When asked why they said “its too much trouble to try to run ducts in the main part of the house. Much better to just take ’em up to the attic where everything is wide open.”

That might be true in your average construction project, but in a world of unicorns and rainbows where you could design the entire house and all its mechanical systems all at the same time, it wouldn’t be so hard. And that happens to be the world I am inhabiting for this project. So I designed the structure and framing of the house to accommodate the HVAC system from the ground-up. All the ducts would be small, and routed through the main living spaces of the house. The chase that was designed to carry them is a lowered hallway ceiling (8′ instead of 9′) which serves an important aesthetic goal of varying ceiling height which we were committed to doing anyway. This will drastically shorten duct runs, maintaining velocity in a low-flow system. It also minimizes temperature losses in cold attics and basements, which improves both efficiency and comfort. Ducts should be sealed well anyway, but if any air does leak out, its just leaking into the same conditioned space it was destined for anyway, rather than leaking into an unused basement or attic.

Since I couldn’t find an established contractor who wanted to do an HVAC system this way, I figured I’d have to strike out on my own. As I soon discovered however, that’s easier said than done. For a fairly unusual system like mine you need a few specialty pieces of ductwork and a lot of varied sizes. I can’t get all those parts from Menards (where most of my house is coming from, to be honest). It turns out that there are two specialty suppliers here in town, just blocks from one another. One has a giant sign posted to look like a traditional STOP sign and it says “contractors only.” They politely refused to serve me. They were the nice place, while the other one had a single guy sitting all alone in an empty store. He never even got out of his chair, as he could be a rude so-and-so without having to get off his backside.

With that avenue denied, I fell back on the old HVAC repair guy who we’ve used for over 15 years now. He’s not the quickest, but he’s honest, and he’s willing to be flexible. He and I are working the project side by side now. He installed the furnace while I ran all the gas lines and exhaust pipes. He’s buying all the duct materials and installing the trunk lines, but leaving me to run all the individual small duct runs to each register. He gets paid by the hour and takes a markup the the materials, but I’m still saving a lot by doing half the labor myself. Between us we’re comparing notes on modern high-efficiency home systems vs. his 30 years of practicing experience in the “old school” ways of HVAC. Its a good partnership.

Oh, and just the other day he said of another project, “I usually just look at the square foot of a house and multiply by 40 BTUs to size the furnace. That’s basically like a house with no insulation at all.” Thus confirming once again the standard practice. We calculated our new house at less than 15 BTU per square foot. Quite a difference. In fact, if you use his rule of thumb number of 40 BTU per sqft you get 104K, pretty much dead center of what our professional bids were. Sigh.