Originally Posted by 6T6Cuda
Go with the one that did a Manual J calculation (which I assume none of them did or they would explain it to you).

Here is a presentation I found real quick that appears to explain it: https://www.energy.gov/sites/prod/files/2013/12/f5/webinar_hvac_calculatingloads_20110428.pdf

basically, you calculate the the loss through the walls (based upon the insulation, windows, size of the external walls), roof, etc... the area the house is in (the need for heating and cooling based upon outside temperatures), and it will calculate the number of BTU of heating and cooling you need; then you can size up a "hair" from there to account for the non-seasonal days, or when the load changes quickly.

Rules of thumb work, but it is very easy to get it wrong because they do not take into account many important factors, such as the condition of the house (insulation, etc...) or area in the country (which obviously makes a difference).



Being an engineer, this is what I did. I worked with contractor while we were remodeling the house. We had the house extremely well insulated, R60 in attic, R-15 (limited to existing walls) to R-23 in walls. House is 2600 sqft. Ended up with a 90,000 btu high efficiency unit with variable speed blowers. The furnace automatically slows the blower or speeds it up depending on thermostat set pint and temperature. I did a similar calculation for the garage when i added heating system (radiant floor heat). I ended up with 60,000 BTU water heater which is a tad undersize for cold winter months based on the calculations. But for a garage, it works well. Garage is 2x6 wall so R-21 in walls and R-60 in ceiling rafters and about 1600 sqft room space.

For A/C we have a 2.5 ton unit. And I think thermally its fine. The distribution system for the furnace/ a/C isn't optimal so the two front upstairs rooms don't get adequate cooling in the summer. I may install some booster fans. Its not an issue with heat, because heat rises and the upstairs room get a lot of residual heat up from the stairway.

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