[Greenbuilding] Furnace Efficiency Question

[Drew A. Gillett, P.E.]

a double check on your calcs, is to take the usage x 90000 btu/gallon and 
divide it by the dd x 24 then multilpy by the max temp difference. this will 
give you the peak btu's on the worst night.

so  700 gallons times 90000 divided by 24 divided by 5000 dd x 70 delta t = 
36750 btu's per hour peak load.

you might get away with an even smaller new furnace say 40000 btuh input. 
although that won't save much capital or operating costs on the 94% 
efficient units.

your max duty cycle of 30 minutes per hour would indicate the furnace is too 
big for the current load as well.
----- Original Message ----- 
From: "Steve Houlihan" <sho11 at comcast.net>
To: <greenbuilding at listserv.repp.org>
Sent: Wednesday, October 24, 2007 11:31 PM
Subject: [Greenbuilding] Furnace Efficiency Question


>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.
>
> 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.
>
> By the way, I insulated the cathedral ceiling with 5 inches of Icenene and
> R19 batts under that and then new 2x6 t&g and reduced my propane useage
> from 1000 gallons last year to 700 gallons this year.  At that rate the
> payback on the insulation is only 5 years.  All the labor to put in the 
> R19
> and the t&g has an immediate payback. Good excercise!Can't beat that!
>
> Steve
>
>
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