[Greenbuilding] [BULK] Furnace Efficiency Question

[Keith Winston]

Also, if you did your heat loss calc with 100 or, gawdforbid, 110F as 
your "design" temp, it'll be way off. The design temp for your climate 
(assuming you're close enough to Sacramento, the closest I found) is 90F.

Keith


Lawrence Lile wrote:
>> 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
>
>
>
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