I am planning to replace my Oil fired furnace with a gas fired furnance. How do I determine the right size, type and effeciency of furnance I should purchase? I live in the DC area.

 

the HVAC people do this

 

Thanks. But how do I know if the system they are proposing is too large or too small?

 

For heating only you do a heat loss calculation. For cooling only you do a heat gain calculation. For both heating and cooling you do both calculations.

These calculations take into account the desired indoors temperature, the expected low or high outside temperatures, the style and construction of your home including the type of windows and doors and amount and type of insulation.

Many dealers when installing a replacement furnace or A/C do not bother to run the heat loss / gain calculations but instead ask if you have been comfortable with the existing equipment. This is a quick and dirty method and you should insist upon a full calculation because most furnaces are way oversized.

As for the efficiency rating...in the D.C. area I would not have anything less than a 90+% AFUE furnace.

 

There is an excellent program available online from http://hvaccomputer.com. The cost for said program, for a homeowner, is about $50. It could very well be the best fifty bucks you spend on the job.