he was stating that he didn't want it to run contiusly. by changing efficeincy your getting more btu's in the house. example 80% furnace for every $100.00 spent on fuel $80.00 goes into the house & $20.00 goes up the chimmney, with a 95% every $100.00 spent $5.00 goes up the chimmney and $95.00 goes in the house. so a 80% 100,00 btu and a 95% 85,000 btu in the same house will cycle the same. I am not saying how to size it by cycling. a load on the house is whats needed to size properly. as long as it sized to design temp it's actually more efficient to stay running then on and off. ( same as your car, better gas milage on the freeway than stop and go).
Okie, I can sorta understand what he was trying to say but was equally as concerned.
Fact is, more efficient is more useful Btu's and less waste. BUT If the furnace was grossly over sized in the first place, can you assume 80 or 105 will be ok. What if a 45,000 input at 95% is the right choice. You say your stuck with two choices and that tells me your not looking at the right brands or your trying to buy out of the back of a truck :eek:
GET A LOAD CALCULATION
For me to give any other advice than that would be careless and give a false sense of security with your decision.
Think of it this way, get it wrong and you pay WAAAAAAAAAAAAAAay more than it would cost to get it sized right the first time.
And hey, what do I know!
I barely do any hvac work, almost all service, just a handful of replacements.
I certainly have more options than an 80 or 105, those are just the closest to the existing furnace in the brand that my preferred supplier sells.
I have been using http://www.hvaccomputer.com/startpagea.asp and am very happy with it:D My area now requires an approved load calc before a changeout or new install...
No simple formula unfortunately though we have all beeen told them at one time or another...
400 CFM Per Ton of Cooling
500 SQFT Per Ton of cooling
The rules of thumb may work 75% of the time but that will make you wrong the other 25%. Completly throw standardization out the window (no pun intended) when it comes to loading new construction...
Arn't furnace/boiler heat calcs done for coldest degree days, like maybe 2% of the year. Should'nt a furnace run steady when producing heat for those coldest days, I think thats when you get the full 80 or 90%. Most of the year a furnace is oversized. Anything mechanical does not like to cycle.