I spoke with the salesman today about the choice to go with an 80,000btu furnace (natural gas) for the 800sq' area. The single pane windows are 2' x 6', and this is in western NY State. There is existing duct-work that is quite large and old (there is no AC).
The salesman said that the heat loss showed the need for 59,000btu's and that a 60k btu furnace would only output 55k. Therefore he said that he would rather use an 80k unit to ensure that there were no problems on very cold nights.
He said that the cost of the units was very similar, but I am worried about the operating costs. The existing unit is working, but I am trying to reduce operating costs.
I do not mean any disrespect to the salesman. He has been doing this for 20+ years. I am only confused, because 80k btu is larger than I expected.
Thanks,
Bill
Unless one of those windows is litearally stuck open or the ducts run outside on the roof , have no insulation, and have big holes in them, there's no
way an 800' apartment would have a heat load as high as what he came up with. A lot of old-schoolers have their own sizing methodologies, but even most of those guys with the BTU/ft coarseness never use anything more than 35BTU/square foot of conditioned space, even in buildings with literally NO insulation, (and no heated apartment below them.) Even if he used the record low temp in your area as his outside design temp (rather than the
ACCA 99% value ) it's on the face of it an insane heat load.
His heat loss calculation is simply wrong, and not by just a little bit- even an 800' tent might not have that big a load. A crusty old-schoolers 35BTU x 800' only comes in at 28K, and is nearly guaranteed to be higher than reality. A heat load of 59K it's more than 2x THAT number, and the 55K output number is nearly 2x that.
The crude heat load calc, assuming a 23 x 35' floor aspect, 10' tall exterior wall, 10 square feet per window, and a 0F outdoor, 70F indoor design temp:
Windows:
6 x 10'= 60' of U0.5 window(minimal double-pane or single-panes + storms) you get 60' x 70F x 0.5BTU/hr-degree-ft= 2100 BTU/hour
plus
2 x 10'=20' of U1 window (single-pane), you get 20' x 70F x 1 BTU/hr-degree-ft= 1400 BTU/hour
total windows:
2500BTU/hr
Ceiling, assuming 3.5-4" of fiber insulation, 20% framing fraction (it's probably a lower fraction) you buys you R10 for whole-assembly R. R= 1/U, so the U-value of that ceiling will be 1/R10 or U0.1.
70F x 800' x 0.1=
5600 BTU/hr.
Walls: total area is 10' x (23 +23 + 35 + 35) - (8x 10' windows)= 1080' of U0.1 wall so:
70F x 1080 x 0.1 =
7560BTU/hr.
Your conducted losses are then 2500 + 5600 +7560=
15,560 BTU/hr, plus infiltration losses.
Are we then to assume that you have a wind-tunnel blower pumping air through the place that the infiltration losses add up to another (59,000 - 15,560=) 43,440 BTU/hr?
That's what they're asking you to believe, with that 59K number. I'm betting it's more like 18-20K, depending on how leaky the apartment and ducts are, and you can probably heat the places with a hot water heater or a 1.5 ton mini-split.
[edited to add]
BTW: Is the furnace in the apartment, or somewhere else, (like the basement of the building or something?) Being in a remote un-conditioned space there will be other losses, but it's typcally not more than a 20% load adder. But sealing the ducts would be in order. A baseboard (or fan-coil) heating system running off a loop from gas-fired hot water heater would have far lower distribution losses than old leaky ducts outside of conditioned space too. Unless the ducts are well designed, pressure-test at very low leakage, and insulated the net efficiency of an 80% water heater + baseboards could yield a higher net system efficiency than a 95% AFUE furnace on a leaky duct system when the ducts are outside of conditioned space.