[Greenbuilding] [BULK] Furnace Efficiency Question

[Lawrence Lile]

>I have a furnace that was built in 1978.  It has an input rating of
75000 
but no output rating.  What do you suppose it would be for a forced air 
furnace of that vintage.

I don't know, but I'll put it this way: if that number were your highway
speed, you probably wouldn't get a speeding ticket.  If it is better
than 65% I'd be surprised. 


>The reason I am concerned is that I have done a lot to tighten up the
house 
and add insulation but have calculated with HVAC-Calc that I have a heat

loss of 51K.  If I assume anew  90% furnace and no safety factor I get a

minimum furnace of 57K which would in the real world be a 60K Btu
furnace.

>Now if I assume the old furnace is operating at 70 % then it's output
would 
be 52.5 K. This doesn't seem like enough for this house before I did any

work to it but I have lived here for 16 years and the furnace has never
run 
for more that 20 or 30 minutes at as time.

Heat loss calcs assume design conditions and no internal gains (lights,
people, mass, solar)  they are sort of a worst case situation.  It's
likely the furnace is well matched to the real conditions you are
encountering.  I certainly would not put in one that was smaller.  


--Lawrence