Beware RFRoT. Stick With BTNRoT

Fair warning: All my rules of thumb have a black fingernail where I hit it with a hammer. In my experience, most rules of thumb about house construction are devised by an engineer, not an architect. If you ask an engineer how strong something needs to be to keep if from falling on the occupants, he’ll tell you triple what’s really necessary. Can you blame him? He doesn’t want to get sued, and he knows that whatever he specifies is going to be installed by a guy whose lips move when he reads street signs. If you ask an architect how strong something needs to be to keep it from falling on the occupants, he’ll first wonder aloud why anyone would worry about the occupants, and then tell you to ask an engineer if you’re that interested.

I do know how to do things correctly. I can follow the building code to a T, and have. I can ladle customer money all over a budget with triple what’s necessary to get the job done. But unfortunately, I’m the customer, and a budget is some sort of bird that looks like a canary, I think. I need poor people rules of thumb. I need Better Than Nothing Rules of Thumb (BTNRoT).

Designing an HVAC system from scratch required many, many rules of thumb to be concatenated and then basically ignored. Plumbing and heating guys are maybe the most nuts about spending your money to amuse themselves. If you’ve ever watched This Very Expensive Nominally Old House That Is Definitely Not Going To Get Any Cheaper, you’ve seen their HVAC homunculus standing in what looks like the engine room of a nuclear sub, explaining why you NEED a $175,000 heating system. You know, to save money.

So while I’m forced to use rules of thumb, I’ll be shaving points off them like I’m a power forward on a college basketball team with money riding on the game. I’m going to make it work, not make it work plus 200%.

Now, the heat pump air handler says it puts out 1,000 cubic feet per minute (CFM) of handled-air goodness. The compressor is rated for 36,000 BTUs. Both of those numbers are exactly half what the RFRoT (Rich Folks Rule of Thumb) say they should be to heat a house in western Maine. And that’s after I’ve shaved off the fudge factor. I’ve seen 100,000 BTUs mentioned here and there. Yikes.

You can buy a larger versions of the heat pump/air handler we purchased. They’re terrifying. They have frightening price tags and use horrifying amounts of electricity just to increase the output from 36K to 48K. And you couldn’t lift the condenser without mid-80s Schwarzenegger to lend a hand. And even if our electrical service could handle one (it can’t), we couldn’t possibly afford to run it. So I used my ultimate rule of thumb: If I can’t afford it, it doesn’t exist.

So rules of thumb say if I have a 1,000 CFM air handler, and maybe 36,000 BTUs, I can heat half my house. I know I can do better than that using BTNRoT.

So how do we make the CFM work? By making it easier for less to do more. We’ll put the shortest possible duct runs into the house, instead of running them all over the place to achieve all sorts of weird rules of thumb that matter to the RFRoT HVAC homunculi but no one sane or poor. We won’t use anything but smooth, round, metal ducts, instead of the moderately less expensive insulated flexible duct, which is essentially a slinky with a garbage bag wrapped around it.

Everyone uses it nowadays, and everyone needs more CFM because of it. Air doesn’t like passing through a bendy, crinkled, corrugated tube, so you have to push harder. We’ll use a lot of these instead:

Speaking of crinkled, this is a photo of a delivery during a somewhat amusing interlude where the Orange Place tried sending things without boxing them first. The delivery drivers treated them pretty badly, but honestly, no worse than the average HVAC journeyman would while taking them off the truck and throwing them down the bulkhead stairs. Short, straight runs with smooth metal tubes can almost heal up that black thumbnail on your BTNRoT. And we’ll be avoiding elbows like the plague, because they slow down airflow a lot, and they cost a relative fortune.

Of course you’re going to need plenums, coming and going. If you’re not familiar with moving air in a house, you have to understand that big fans don’t matter if you don’t take as much air out of the room as the amount you’re trying to put in. That means you need cold air return ducts going back to a cold air return plenum, and a hot air plenum to distribute your warmed air back to the octopus of ducts you have serving all your rooms. Ours looks like this:

Sheet metal work like these plenums is amazingly expensive if you hire it done. I bought a return air plenum kit to cheat a little. I had to modify it a lot, but it was still a lot easier than making one from scratch. It acts as a filter box and a stand for the air handler. The hot air plenum on top I made from sheets of galvanized metal. It’s not that hard with a few cutting shears from Harbor Fraught and a box of bandaids. It’s bigger than it looks. Part of it extends out over the back to accept large pipes nicely.

In the picture, you’re looking at two hot air ducts, and one cold air return (it goes down under the air handler to be pulled through a big filter and up into the air handler to be, you know, handled). There’s another very large cold air return hidden behind the cabinet. The floor grate in the dining room floor has a filter box I built slung under the floor (the tan thing), which has a pipe leading straight down to the back of the filter box. Two big ones in, two big ones out. You know, like a date at a Sizzler.

[To be continued]