What size of high efficiency furnace should I buy for a 2-storey house?
Hi, I have difficulties in deciding whether to buy a 80000 btu or 60000 btu 2-stage multi-speed motor furnace.
House is in Zone 7A – cold climate built in 1973. 2 storey rectangular shape, detached
Existing furnace 45 year-old 110,000 btu 70% efficiency, single stage still working fine but have 2 cold spots that could be due to windows
main entrance of house facing NE but the backyard is facing NW with a baseball and a soccer field (no houses in the big open area)
always have people in the house including seniors
using my gas bills to calculate heat load. if I did it correctly, it seems to suggest a required output of 41409 btu/hour; with permissible oversizing by 1.4, it becomes 57973 btu/hour. Same period last year (Dec 2016 to April 2017) suggests higher numbers (46136 btu/hours or 40% oversize 64591 btu/hour). In my calculation, I assumed my old furnace efficiency is 60%.
Energy audits were done in 2010 and 2018 with air blower tests
2010 report – my home’s design heat loss would be 49003 Btu/hour (14361 Watts) and design cooling load would be 35826 Btu/hour (3 tons). attic insulation upgrade to R50 was done after the energy audit
2018 report – heated floor area 2261.5 sq ft although I calculated the heating area excluding finished basement and garage is 1800 sq ft.
Airtightness: 6.56 air changes per hour at 50 pascals
total 17 windows; mostly two pane energy efficient windows >10 yrs
60000 btu low fire output 40320 btu high fire output 57,600 btu
80000 btu low fire output 53760 btu high fire output 76,800 btu
alternatively I may choose Lennox 70000 btu but prefer Amana for better warranty and stainless steel heat exchangers. Supplier did not suggest Rheem which I am also interested.
supplier suggested 80,000 btu claiming I may feel cold with 60000 btu? both sizes are recommended by different retailers but none of them did a heat loss calculation.
Based on the above information, would you suggest 80,000 btu or 60000 btu? what should be the right size for my house?
Hope to receive some advice or let me know if I miss any other factors to decide.
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