I'm finishing my basement and I'm trying to identify if my existing furnace is large enough to heat the basement. Here are the details. I have a Trane XE 80 LP furnace with a 100,000 BTU input rating and it's an 80% furnace. My house is 2250 square feet EXCLUDING the basement. I'm finishing 700 square ft of the basement which would take the living space to nearly 3000 square feet. I live in Ohio so the winters do get cold and the summers get hot.

I'd like to tap into the plenum to add two 4 x 12 registers with 6 inch duct work, but I want to make sure I do not adversly affect the heat in the rest of my house. Any suggestions on how to calculate?

An accurate calculation is very difficult and varies with many factors. A better way to start would be to check local codes. If you ever sell the house, you could be forced to replace an furnace that worked fine for many years because the code is too conservative. If code is going to demand replacing the furnace, may as well do it now. Having passed that hurdle, think back to the worst weather you can remember. Did the furnace run for long periods only shutting off short periods? If so, that would suggest a bigger furnace.

If the furnace is marginal, it might be smarter to put money into tightening up the house instead of a new furnace. Maybe a good idea anyhow, while you are remodeling. LP prices can only be expected to go up. Depending when your house was built and any retrofits, you may be losing vast amounts of heat out the basement.

My house was built in 1970. In 1995, I replaced the original 60% furnace in the garage with an 80% one in the crawl space. You could see daylight in places between the plate, foundation and band joist. Your basement may not be near as bad, but likely the combustion air for the furnace is leaking in there, or warm air drawn down from the rest of the house. It cools the floor as it is drawn to the furnace. You would do better to carefully caulk all the joints, and then insulate the walls. The furnace must have air, 52.5 sq. in. free area or a 9'' diameter duct for a 105,000 BTU furnace. I have a vent in the band joist, and use 2 joists as the sides of the vent. My furnace was not set up for a combustion air duct, so I just ended the duct near the furnace. I also insulated it. With no heat outlets in the crawl space, it stays about 60 degrees in the coldest weather. With a good tight, insulated basement, you may be able to keep it cozy with less load than on the furnace before.

to do a heat lost calculation these are the information you need: area of walls, window, door, materials wall made of plus thickness, window R-value(u factor), door R value ( u factor)... it much easier to ask the sale person to help you to select the size of furnace,, if she/he can't help... go for the 120-125 k if the existing furnace was able to give "you enough heat"