anyone know of any sites for calculating a hot air furnace size ( btus )
also i would like to know if there are any fairly simple sites for sizing the trunk line , and return air ? Yes i know a good heating guy should know this however i am getting all diffrent answers from them ! One supplier did a heat loss and came up with ( 27,000 btu ) a contractor came and said ( 70,000 ) internet said ( 40,000 ) I want to just put a new unit on the first floor ( 1000sq ft ) insulation is good cielings are 7′ 6″ total glass area windows & doors is 157sq ft
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I don't know of any on line calculators. Even computer calcs require a fair amount of manual gathering of information/calcs (i.e. surface areas, U-values, etc.).
for 1,000 sqft fairly well insulated and assuming moderate to good sealing, the 27 KBtuh figure seems good to me. I've done a lot of calcs on houses over the years and if I had to pick one w/out knowing much more about your house, I'd say the 27KBtuh is the closest of the 3 you provided. First house I built was 1,800 sqft and I calc'd it's load at around 35 KBtuh and it had relatively a lot of windows. 157 sqft for windows AND doors ain't much window area, but it's a small space.
I've always used an old book reference for calcing heat loss and in a pinch sizing ducts. It's like the beginners guide to HVAC sizing. "Concepts in Thermal Comfort" by M. David Egan (I think I spelled it right). Got it back in the 70's but it's still a good simplified reference that uses an old ASHRAE methodology.
IMO many contractors will tend to oversize the systems just to ensure that their customers aren't cold, ever. They often use very broad rules of thumb to size their systems or even WAGs. But that is really a diservice to the client as an oversized system doesn't run efficiently. Besides, you calc the size and then you are most usually forced to pick a piece of equipment that is somewhat larger.
Oversize furnace and ductwork is a waste of money both construction and long term costs. We have to start demanding better calcs.