Heating, Air Conditioning, Fridge, HVAC/New Furnace and AC
Thanks for volunteering and sharing your expertise. My 21 year old furnace kicked the bucket, I now have the opportunity to replace it (and the 21 year old AC) with a new complete system. Here is some info on my situation:
- Brigham City, UT (84302)
- Home constructed in 1946
- 1100 square foot single story w/ full basement
- All brick construction
- Silver aluminum roof
- Two 50' trees on west side
- One 50' tree on east side
- All supply registers are wall (6 up, 2 down)
- Two returns (30" x 6")
- All ductwork is metal, main duct tapers as branches come off, air flow at registers is high
- Supply plenum has a 90 degree "no elbow" at the top
- Return stack has a 90 degree "no elbow" at the top and "non fitting" at the filter/bottom
- Had a blower door test two weeks ago to see if I qualified for free insulation via rebate and didn't qualify. Home is quite well sealed.
- Current AC is 2.5 ton 10 seer
- Current furnace is 75,000 BTU 90% efficiency
Hope that wasn't too much info!
On to the question(s) - I have had a few quotes, one 2 ton, some 2.5 ton. I am leaning towards the 2 ton. I'm thinking 13 SEER, single stage. Does it benefit me to get a 2.5 ton coil or should I just get a 2 ton coil? Any reason I should be looking at 16 SEER?
The furnace is even more confusing, I've had quotes from 60k to 80k BTUs. The one contractor's "basic" Manual J stated a 43k BTU loss (but was based on -5 degrees outside design temp, making a 75 degree temperature difference, and also entered my wall insulation as R-13 where its brick with a 2-3 inch air gap). The furnaces I am looking at are 46k BTU and 69k BTU both 95% efficient. I can get them in a single stage multi-speed blower, two stage multi-speed blower, and two stage variable ECM blower. I am leaning towards the two stage variable furnace, but have no idea which size (because of mixed info from contractors) or if I would be better off with the single stage. Also, not sure how this impacts things, but currently only about 400 square feet of the basement is heated, but I plan to add 1-2 more registers there in the future as kids get older and we need the space.
Thanks again, I look forward to any advice and assistance you can offer me.
Never buy more capacity than you now have. If you have rooms that are too hot or too cold, that's due to unresolved duct problems, not insufficient capacity. Go back to the guys proposing 2-1/2 and offer them the opportunity to rebid at 2. 550 SF/Ton should be fine with your favorable blower door test results. And you don't have enough returns for 2-1/2 tons (stud spaces are limited to 200 CFM, you have no more than four).
The only reason I like larger coils is for better dehumidification, but since you're in an extremely dry climate, that's a non-issue. You'll want to operate the new system at 450 CFM/Ton because of your climate; as you've seen, you'll need more return to do that.
When I replaced my International Comfort Products heat pump at age six years due to a botched coil manufacture, I went to 14.5 SEER. I wanted more than base efficiency, but after studying the economics I realized the incremental savings weren't worth the incremental cost of 16 SEER/two stage. Then there's the increased repair risk of high SEER equipment: When it breaks, it'll be more expensive to repair. You might rethink 13 in your climate (Salt Lake City AP summer design at 94F), but I wouldn't go to 16.
Winter outdoor design for Salt Lake City AP is 11F, so it sounds like the Manual J heating calculation may be overstated (Happens all the time) by as much as 25% or so. Then there's whether the guy took the time to measure everything (I saw one guy take every window in a home as 15SF, regardless of size). You don't need 80MBH, 46 may be enough.
As to two stage, it depends where the wall outlets are: If they're low on outside walls, two stage is good; if they're elsewhere, stick with single stage. At issue is getting the heat to the floor: High outlets may not project the heat down far enough on low fire/low flow; inside wall outlets may not project the heat to the outside walls, ditto.
Multispeed is fine, so long as you remove duct restrictions; ECM will be quite expensive to repair when it breaks.
When you check my website, do read up on high resistance duct fittings.
The basement will never be comfortable until you zone in separately: There's virtually no cooling requirement, and its heating requirement is incompatible with that of the first floor.