Need help with new furnace - sizing issue
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
I would go with the modulating set up as described bc a 60k 2 stage in first stage will be oversized 90% of the time. I would spring for a modulating stat if I were you so it can work as designed, but if you don't want to dish out the extra cash the way you described will work as long as its set up properly. Good luck with your install.
Sounds like you need a smaller furnace. Otherwise you are buying a very expensive single stage unit that runs on low most of the time. Also when it does go to high, like recovering from a setback, it could be noisy. I'd think the heat loss from the HRV could be a smidge high. Maybe a furnace closer to 45-50K input would be better. Rheem's RGRM-04 is 45K input 2 stage. If I were buying a mod, I would want one that responded to a thermostat, not a board guessing at what to do.
Thanks BaldLoonie! My research indicates only Rheem, Amana / Goodman and Armstrong Air have 45K two stage units (I could not find specs for Lennox and Bryant, while Carrier / ICP only have 40K input). Could you suggest other brands / models in this size?
Originally Posted by BaldLoonie
Thanks in advance,
Lennox has a 2 stage I comfort enabled 45K 2 stage variable model EL296v.
Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced
So am I the only one here that would go with a 60K furnace in this circumstance? In Portland the design load is based on 24 degree outdoor air temp, yet we do often hit below that and if he is putting in a 42K BTU furnace at a 42K BTU heat load, the furnace is 95% so you are actually outputting 39,900 BTU's assuming its running at full efficiency from now until 20 years from now. I know that a manual J has a slight fudge factor built in but that is because people make errors! You mention the ventilation load, is that assuming the HRV working at a certain efficiency, because on a windy day the ventilation load will change from a calm day. You also would NOT be able to have any setback on your thermostat which many people like. If you have a 42K bTU loss day and a 39K BTU furnace and your thermostat sets back 3 degrees at night you will never heat up the house again. There is a time for sizing at the load(cooling IMHO) and a time for putting in a slightly larger system(heating).
I like the modulation furnace option but the IAQ stat will not properly control it. So why not use the IAQ just for ventilation and run a sensing relay off the blower so that the HRV turns on with the blower but the modulating furnace gets used properly by a modulating thermostat.
In my area we rarely see heat loads over 20-35K BTU's and all we have available is a two stage 60K BTU 95% furnace and they work just fine, I prefer a modulating furnace here but most are still very happy and comfortable with a two stage with that low of a heat loss so in your case I will go against the grain and say go with the 60K BTU two stage, or put in a modulating with a modulating thermostat.
Rheem's output would be right on the button. There's some fudge in the manual J. If I could have, I'd rather have a unit that run full bore on the coldest mornings rather than what I have, a 60K with low shut off. Just my view.
Thanks! I understand everyone's dismay at the thought of using a mod furnace with a non-mod thermostat, but that's the only way I can achieve a lower BTU output in 1st stage, and consequently longer run times in 1st stage (confirmed promotional price identical to a two stage 60K).
Another question - is the VisionPro IAQ capable of going from 2nd to 1st stage or does it simply turn off heat when satisfied? My salesman thought ALL thermostats do this and none is capable of "down-staging".
And lastly - there is no 42K BTU input model, only 45K input / 42K output. This is almost ideal for the heat load, unfortunately the contractors I like the most only install Carrier / ICP so they don't have a 45K (Carrier smallest is 40K input / 38K output). Would a 38K output be too small given the safety margin built into the calculation:
- 11,016 BTUh assumes 120cfm running 100% of the time. My VisionPro IAQ only ventilates about 35% of the time (to meet ASHRAE 62.2)
- also 11,016 BTUh assumes no heat recovery (in reality a min. of ~50% is recovered)
So with the above figures, I would estimate the ventilation loss to less than half of the calculated value - less than 5,500 BTUh. That subtracted from 42,674 BTUh leaves me with a real load of 37,174 BTUh (which a 40K input / 38K output perfectly matches).
In addition, I will most likely close off the make-up air supply and the chimney once I also replace the water tank, and that would reduce the loss even further
What do you all think - am I crazy and should go by what everyone is telling me?
Call a Lennox dealer. Get 2 stage variable speed and i comfort communicating wi fi stat.
I prefer to stay with my Vision Pro IAQ and don't need wifi control. But thanks for the suggestion!
Originally Posted by Freezeking2000
I'd look really hard at the 2 stage 45,000 Rheem Furnace. Get the 90%+ model for your climate. Best comfort for the $$$ IMHO. For the coin you're dropping on some of your other prospects you could even get the Rheem variable speed blower.
Would it be terribly wrong to go with a single stage, variable-speed blower in this situation? Inexpensive and it'll do a great job. Just MHO.
Thanks for the suggestion to look at Rheem, unfortunately I have not found a GOOD installer for them. There are 2 or 3 in the area, and was advised to stay away from them all
I currently have a single stage, and have read so much about two stages and VS blower that I'd really like to get one