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Old 02-22-2013, 10:15 AM   #1
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Furnace sizing question, lesson learned


Hi, HVAC gurus,

Just replaced my 19 year old Carrier 110K BTU 58WAV with 90K Lennox 2 stage EL296V, the carrier was way over-sized and did short cycles, I calculated the heat loss of my house based on hydro bills following the instructions on CMHC's web site, the result shows that I only needed a 40K furnace (with 40% buffer). The contractor shrugged it off and made me think I am an idiot to put in anything smaller than 90K based on my house's sq. footage (2300 sq. ft. 2 story). I gave in thinking they are professionals and treated the same way I would have treated my lawyer, doctor and even accountant. I was wrong. I now have a 90K BTU furnace that will never enter stage two even during recovery (the iComfort Wifi t-stat bumps up .5 degree each time so that the furnace recovers gradually), even on the coldest windy nights in January, the furnace will heat at low fire for less than 50% of the time (heat for 5-7 minutes, off 6-10 minutes), I paid the extra $$$ for a 2 stage 90K furnace, however, I ended up with a single stage 60K one (90x0.7x0.96=60), I could have bought a single stage 45K furnace and it would work a lot better and make my house more comfortable. My question is, why do contractors tend to oversize furnaces? Makes no sense to me, any thoughts?

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Old 02-22-2013, 11:10 AM   #2
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Furnace sizing question, lesson learned


First of all something is wrong with your heat cycles. The furnace should be off for at least 10 minutes or longer not 6-10. A 110 WAV I would replace with a stage 70,000 EL296V but not go to 45,000 and I do not believe anyone makes a 2 stage 45.000, 60,000 is probably the smallest in a brand I do not like. If you can get the cycles/hour down pat it should work a lot better, being oversized nothing I can do about it but it won't really shorten the life of the furnace. Contractors oversize furnaces because some of the salespeople are idiots and to be on the safe side they tend to go with a larger size so that if they undersize they won't be stuck removing it for free. The tstat may also be getting a draft thru the wall behind it so they should remove it and stuff the hole with fibreglass pink insulation. Drafts will definetly short cycle it and there should be no return air grills within 6 feet of it or that draft if you run the fan all the time can affect it. Do you run the fan all the time? I would with the V drive and it will make your house more comfy. I checked the install manual and there is no cycles/hr setting but the differential between 1 and 2nd stage can be set for 1-3 degF so a 1.5 -2 may be better. Need to get the installer to go into the setup menu to do that. I would get him to check the hole/draft behind it first and then increase the differential to give longer low fire and better comfort. Better yet get the proper sized furnace but that is another story. The bigger companies have comfort satisfaction guarantees or they will remove it so if all fails try that.

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Last edited by yuri; 02-22-2013 at 11:37 AM.
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Old 02-22-2013, 03:33 PM   #3
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Furnace sizing question, lesson learned


Thanks, yuri, I was googling for "furnace oversizing" and came across this essay by Mr. David Butler, it's eye opening, I think every HVAC professional who is serious about the business should go through it,

http://optimalbuilding.com/files/ele...n_the_room.pdf
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Old 02-22-2013, 04:30 PM   #4
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Furnace sizing question, lesson learned


Quote:
Originally Posted by lennoxel296v View Post
Thanks, yuri, I was googling for "furnace oversizing" and came across this essay by Mr. David Butler, it's eye opening, I think every HVAC professional who is serious about the business should go through it,

http://optimalbuilding.com/files/ele...n_the_room.pdf
Its a very nice article, but you have the audience wrong. That's how I differentiate my company from the "Bubbas".

However, I did sell my ductblaster machine because NOBODY WOULD PAY for the QUALITY.

So distribute that article to as many friends and fellow homeowners that you can and it will be appreciated..

Like last month, following up upon a job that I lost....homeowner said he went with the other company because he got a 60000 furnace when my quote was a 44000 (smallest I could get) after spending time on a MANUAL J8 (latest revision per your article) to determine his heatloss of 22000 on a 1000sqft space.
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Old 02-22-2013, 06:56 PM   #5
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Furnace sizing question, lesson learned


Many don't want to make the effort to do one, or spend the time. So they just over size them cause they know it will heat the house, and they get paid and move onto the next customer.
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