This is a further installment in my two-dead-heat-pumps, gonna cost me $50K and up to fix it, saga.
Today’s punchline. My 1959-vintage first-floor HVAC ducts are, objectively, way too small to work with a modern heat pump. The main duct is roughly one-third the size (cross-sectional area) it needs to be.
We could put the best ground-source heat pump in the world at one end of those ducts, and the kitchen at the other end of the air duct would still freeze in the wintertime.
If feasible, we’re going to replace (one of) our dead ground-source heat pump(s) with a couple of ductless mini-split air-source heat pumps. Just bypass the grossly undersized ducts entirely.
Sounds like a fundamentally stupid thing to do. But not so, in this case. I think.
Never make fun of the size of a mans ducts.
I finally got the bright idea to measure the size of my first floor ducts. The ones that barely function. Admittedly, guessing about it was more fun. And even if I knew the dimensions, figuring out the “right” size is an engineering black art.
But I had a hunch that a quick ballpark answer would be good enough. The main duct measured out at 0.75 square feet in cross-sectional area. The first floor of the house is about 1500 square feet. Per two on-line rules of thumb, the original 1959 ducts are about one-third as big as they need to be.
That squares with the rest of it. Not just their abysmal air delivery, but just by eye, the cross-sectional area of the main duct is about a third that of the plenum to which it is attached.
I can easily believe that the folks who originally installed my ground source heat pump installed a super-duper ground-source heat pump, then blithely hooked it up to grossly undersized duct work. It’s of-a-piece with the rest of the shoddy retrofit they did before selling the house.
But the ducts themselves appear to be much, much older. They’re behind plaster walls, for one thing, and I’ll swear that plaster has never been disturbed. They are in an unusual configuration, with both ground-level ducts, and ceiling-level ducts that must be fed by long risers. The guy who built this house seemed to build pretty good houses. How’d the original builder manage to put in such goofy undersized ducts in the first place?
I now think that these first-floor air ducts were originally designed and sized for use with a gas-fired hot air furnace. The air coming out of one of those is very hot, and so quite energy-dense, compared to the lower-temperature air you would typically get with a heat pump. Not only would you have to move less air to heat an area (thus requiring smaller ducts to move it), you probably got a considerable “chimney” effect in the vertical risers that serve the many ceiling-level vents. (Vents that, in the current system, seem to do absolutely nothing.)
In the end, it doesn’t matter. A few simple checks all tell me that they are, in fact, just way too small for use with a modern HVAC system.
Twenty years ago, they cut a major corner in the original ground-source installation. For 20 years, system performance must have been sub-par as a result. For sure, for 20 years, the kitchen has been freezing cold every winter.
It’s time to fix that as best I can.
Rule number 4: Yes, they really can be that stupid.
A buddy of mine once gave me a little laminated list of rules for life. Rule number 4 was as stated above.
At root, my biggest problem so far with this two-dead-heat-pumps fiasco is forgetting Rule #4. Because, when I bothered to check, sure enough, the folks who retrofit this charming home with a super-expensive ground-source heat pump system then proceeded to hook one of those heat pumps up to grossly undersized ductwork. Which made the entire point of installing an efficient heat pump almost completely irrelevant.
And so it has remained for two decades.
And now, completely contrary to the conventional wisdom, it makes sense to replace a worn-out ground source heat pump with an air-source heat pump. If for no other reason than to bypass the undersized ducts.
Addendum: Or duck the ducts.
I finally got it. The story ends … and you can’t replace the duct, because a properly-sized main duct would stick down too far in the basement. So not only didn’t they replace the ductwork, they couldn’t replace the ductwork without losing standing headroom right down the middle of the finished basement.
This situation is no-one’s fault. It is what it is. Deal with it.
Winter approaches. 
