Anyone here using a 2-stage / multi-stage air conditioner? I'm curious, do you find it a benefit? Or are we just better with a single-stage AC paired with a multi-stage furnace fan?
So, here is my setup - two furnaces, two air conditioners.
Setup 1) This one had a load calculation done by an HVAC engineer, as required by the city for our building permit. Main floor, 2nd floor, and basement. Total living space 2835 square feet. Estimated volume 25515 cubic feet based on a 9 foot average height. (Lower ceilings in basement, but cathedral ceilings on 2nd floor.)
Cooling requirement is 34983 BTU - Got a Lennox 14ACX-36, which is a 3-ton single-stage AC
Heating requirement is 51057 BTU - Got a Lennox G71MPP-36B-70K, which is a variable stage 26500 to 66000 BTU furnace (70000 BTU input).
Installed about 7 years ago. After updating the ducting, this seems to run reasonably well, with the fan usually a fairly low setting. Does not short cycle in the summer and on the hottest days doesn't need to run all the time. Humidity controlled in the 40%-50% range in the summer IIRC. Also have an HRV, but it doesn't seem to kick in much. I'll have to play with the settings.
Previous to this install, I had a 100000 BTU furnace and a 2.5 ton AC. Supply and return duct sizing was too small and while the place was fine in the winter, the basement ducts going to the 2nd floor got very warm, but by the time it got to the second floor it wasn't so warm, so I guess it was way oversized to overcome the small ducts. Plus it was on full blast when it came on, which was really annoying. Summer was a problem though, with 2nd floor always hot in summer.
While the AC might have been slightly undersized, probably more of the benefit of the new install was the updated duct sizing, correct? Current AC sizing is about 945 square feet per ton, including the basement. Previous AC sizing was about 1134 square feet per ton.
Setup 2) This one was installed before I was smart enough to demand a full HVAC load calculation. This is around 2100-2200 square feet, with no basement, just a 4 foot tall crawl space, and includes a converted garage, with a bedroom over top of it. The two bedrooms have big windows and windowed patio doors, and the main floor is covered in windows and has two sets of windowed patio doors. In other words, the main floor is almost like a green house on two sides. Guy originally came in and spec'd a 2.5-3 ton AC unit, but then came back one day all worried and insisted we up the tonnage to 3.5 ton. I was thinking that by a rule of thumb estimate, a 2.5 ton would have been OK but he convinced me that due to all the windows and doors, 3.5 ton would be safer. Cost premium was not much at all, so I believed him and had the 3.5 ton installed.
Furnace is pre-existing 60000 BTU Carrier 58MCA060-12, single-stage I believe, with 56000 BTU output.
AC is 3.5 American Standard / Trane 4A7A4042B1000AA 3.5 ton single stage.
Furnace installed probably about 15 years ago. AC installed 9 years ago.
The second setup is actually in a later build with somewhat better insulation, but the windows and doors are a significant source of loss. The ducting to the main floor and main second floor bedroom is probably fine but the ducting to the converted garage and the bedroom on top of it is not. Plus I'm not sure how good the insulation is there. In the winter the converted garage was always cold, so I installed electrical baseboard heating for on-demand heating. In the summer the garage is perfect, but the bedroom above it is often very warm, so I got a window AC unit for it (although so far I haven't used it).
I'm thinking the AC unit here is oversized especially considering the converted garage doesn't get adequate air flow, but interestingly the AC doesn't seem short cycle either, even though the furnace is single-stage (I think). (It doesn't run as long as setup #1, but setup #1 has a variable stage fan.) The humidity is in the acceptable range. On the hottest days it's in <50% range, and although I've noticed IIRC the humidity can hit the low 50%+ range on an humid day when it's cooler outside and the room doesn't need as much cooling. So, while the humidity seems to be in the acceptable <55% range in the summer (with rooms cooled to 22C to 23C), it does seem the humidity does run a little bit higher than setup #1.
Nonetheless it's comfortable. I like 45-50% humidity the best it seems. Would a 2-stage AC + 2-stage furnace be best in this context? Or would a 2-stage / multi-stage furnace with single-stage AC be sufficient, like setup #1? Judging by my results, I'm thinking 2-stage 2.5-3 ton AC might work best in setup #2 (although I would lean heavily to just getting a load calculation done by an HVAC engineer).
Cost premium of 2-stage AC units is a consideration, but not a huge concern.
I know it's early since my AC is only 9 years old, but nonetheless I've been thinking about this, and want to be prepared should the AC and/or furnace conk out one day. I'm thinking the furnace may die first as it is likely 13-15 years old now, so I'd get a multi-stage furnace to replace it, and then at a later date when it comes time to replace the AC, I'd consider a 2-stage.
BTW, do all furnaces have the option to be installed sideways? As mentioned, that has no basement, just a 4-foot tall crawlspace. So the furnace is installed there horizontally.