I’m right at the border of choosing a 60k vs. 80k input furnace and need some advice. I’ve already asked a few questions here and appreciate the very helpful input.
I have a 58-year-old house in Arlington, Virginia, with a 20-year-old addition. The original house has an inch or two of rock wool batts in the walls, with the same in the attic with a layer of fiberglass over the top.
In my “worst case” load calculation, I factored in “poor infiltration,” no wall insulation because the old certainly isn’t the R-11 of new 3.5 inch fiberglass. I put R-4 on the crawlspace first floor duct even though only 33% of the house is over crawlspace, the rest is over a conditioned basement. (HVAC-Calc doesn’t seem to have a way to take partial crawlspaces into account for the ducting.) And I put in R-19 for the attic, even though I may upgrade to R-30 or R-38 in the near future. For my “best case” calculation, I factored in R-11 for the wall insulation and “average infiltration.”
I’m having trouble accounting for infiltration. I currently have original single-pane windows with leaky storms, but I’ve already signed a contract to upgrade to top-of-the-line Marvin windows. I plan to replace the attic insulation and seal any leaks through wires or piping holes. The three exterior doors are well weather-stripped or have a modern storm door. Would you rate this as average? Due to the age of the house, there is no Tyvek or even tar paper wrap. (The addition has a tar-paper wrap.) HVAC-Calc calculates an ACH of .7 for average, while the revised tables for the 8th Manual J for my size house at “average” put it at .32. This alone is a 5,600 BTU difference.
Under my “worst case” scenario with “poor infiltration,” I get a heat loss of 56,629 BTU. (The worst case already accounts for the new windows.) The “best case” is 45,497 BTU. I figure that the insulation is probably about halfway between none and R-11, and the infiltration is halfway between poor and average. Averaging these results in a heat loss of 51,063 BTU.
My current furnace is 32 years old, with a 100k BTU input and a 80k BTU bonnet output. I’m considering a Carrier Infinity 96 as a replacement. The output of the 94.1 AFUE Carrier is 57K BTU. I assume the 32 year old furnace is about 60 AFUE, so the actual output is very close.
I don’t feel that the current furnace is undersized, but the new one could be if my guess at the AFUE of the old furnace is off and my load calculations are wrong. Would you feel comfortable going with the 60k input versus the 80k? Do you use a safety factor?
I figure that by replacing the windows and redoing the attic insulation (R-19 to R-38 would save 741 BTU), as well as by sealing the previously leaking attic trap door, putting gaskets on the exterior outlets and switches I could feel safe going without the 15-25% safety factor that the HVAC-Calc author recommends if my current furnace is really about 60k BTU output.
If I’m cold next winter, I could also add more insulation around the crawlspace ducting (1100 btu according to HVAC-Calc) or upgrade the fireplace door (1400 btu). But if I installed the 80k, I bet it would never kick into high except for setback recovery on a very cold day as the low stage is 49k BTU. Would you go with the 60?
One other question: When calculating wall heights, what about the 10” of joist, flooring, and drywall between floors? That is about 10% of the wall area, which for 25k BTUs for the whole house, is a difference of 2,500 BTU.
Thanks again for your help.