I've been reading this forum for more than a year now, and learned a lot so thanks to all who take the time and share their knowledge! I am currently in the process of selecting a furnace replacement - also selecting the contractor (as learned here that's the important thing), but still need to choose a few equipment options...
I'm in Ottawa, Canada and live in a townhouse end unit of ~1800sqf attached on one side. I had a proper heat loss calculation (not by installation contractor but a certified HRAI tech) and the numbers are as follows:
Heating Load - 42,674 BTUh (@ -13F design temp). This includes a load of 11,016 BTUh (25%) due to 120cfm of ventilation (HRV operated by a VisionPro IAQ). From speaking with the tech who did the calculation there is significant margin built into the heat load:
- software has some built-in margin
- HRV is operated only ~35% of the time as the VisionPro IAQ determines ventilation needs
- HRV recovers some of the heat (an estimated 60% - 70% efficiency)
Now the trouble begins... Most furnaces come in 60K BTU or higher, also most have first stage at ~70% of max - therefore for a 60K this equals ~42K BTUh, almost bang on matching my heat loss at design temp. This means it will almost never go into second stage. To be fair there are a select few two stage units that come in 45K input (43K output) so these would be ideal. However most techs I spoke with strongly suggest I go with a 60K unit (which I am very hesitant to do after lots of reading about proper sizing here)
The second option I came up with is to install a modulating that can use a two stage thermostat (my VisionPro IAQ) and reading the tech manual from Comfortmaker it seems like the furnace can operate in one of the following ways:
1. adaptive (not sure how this works)
2. tstat stage 1 turns on Low Fire (~40%) and stage 2 turns on High Fire (100%)
3. tstat stage 1 turns on Intermediate Heat (~70%) and stage 2 turns on High Fire (100%)
modes 2 and 3 are identical to a two stage furnace EXCEPT they have different Low Fire BTU outputs. For a 60K BTU unit that means roughly 24,000 and 42,000 BTU respectively. Mode 3 is identical with a regular two-stage and its Low Fire value matches my heat loss, so will effectively work as a single stage furnace (main reason why I DON'T want a 60K BTU two stage). But mode 2 puts out only 24,000 BTU in Low Fire and would have longer run times at higher-than-design temperatures. Once heat loss exceeds 24,000 BTU it will stay continuously in Low Fire and start cycling in High Fire (which is exactly how I'd like a two stage furnace to work). I realize it won't run continuously in High Fire at design temperature or lower (which is less then ideal) but I think it's still better than a regular two stage.
Before anyone says I should get a modulating thermostat with option #2, I will mention I can get a modulating very close in price to a two stage due to a promotion in the month of December (plus I don't have to pay for a mod tstat)
I'd very much appreciate experts' opinion on the two options I'm exploring