How to Size an oil furnace and AC unit?

[emailcab]

An HVAC installer I know is planning to install an oil furnace and AC unit. I'm trying to figure out the appropiate size units for my home. It's a two story house with a basement. The 2nd floor is heated and cooled by a heat pump which in in the attic. The 1st Floor and Basement are about 2400 square feet. This is what the new units will be servicing. The windows on the first floor are about 10 years old. The basement has small windows and I plan to install new windows there. The exteriors walls on half of the first floor are insulated with r-13 and the rest has an inch thick layer of plaster. The facade of the house is brick. Hope this helps.

[Hube]

You need to have a "heat loss/gain" calculation done. This calculation will give you the amount of btu's that are needed for heat/cooling. Any hvac person in the know should be able to give you the particulars. it will involve taking sizes of widows, doors, R values of insulation ,locality, area temperatures,, ,etc,etc, This will take about 2-3 hours to do, but it will be well worth it in the long run.

[emailcab]

Thanks Hube. All the HVAC companies that came out to give me a price never did that calculation. I understand what you mean, though. But can't I get a range of the size furnace and AC, a pretty good guess on what I should have based on the square footage of the house, the insulation, the number and size of the windows and the area where we live? Can't someone based on experience of doing these calculations take a pretty good guess on what size range of furnace and AC I might need? For example, they could guess that its definitely bigger than an 85000 btu but no more than a 120,000 btu. (based on their experience) They might be able to get a good feel from the size and type of windows , the amount of insulation, climate, etc. Also is there a simplified version of the calculation that I could use to put in these figures? Thanks.

[HayZee518]

I found this site which addresses your issue. It's written in html script and is a pdf file. There's formulas all over the place so you can figure out a heat load.
http://www.engineer.gvsu.edu/house/h...use%20Heat.doc

[Hube]

educated Guesses" and "rules of thumb" for calculating heat loss/gain sometimes work out ok, but what happens if they don't? The answer is YOU, the homeowner suffers not only in comfort but also in the wallet($$) too.
For best results do a complete calculation (manual J, Hrai, etc.)

[wingardclocks]

I agree with everyone, you have to do the math! You will be sorry if you don't. Don't take an opinion of a service person unless they do the work. Don't be afraid to let them know this has to be done.

After all they want to sell you the unit. Make them do the work for the heat loss/gain calculation, but ask for a copy of heat loss/gain work sheet.
T