Hi! I’ve got a large home (approx. 6000 sq. ft.) with two furnaces that I don’t think are sized correctly. I’m looking for help to determine how they should be properly sized next time around. I have run the HVAC-Calc program and I believe I did it correctly but my calculations don’t match up with what actually seems to be happening in our home.
I live in the St. Paul, MN area which according to the HVAC-Calc program has a winter temperature design point of -12 degrees. Here is a breakdown of what I have calculated and what is currently happening:
The current heater is a Trane XE90 at 100K BTU. At -5 degrees today with the thermostat on hold at 69 degrees, the furnace would run for 3.5 minutes and then stay off for 11 minutes (on 24% of the time). The HVAC calculation is 70.8K BTU/hr loss for a tight home and 79.6K BTU/hr for an average home. There is in-floor (transite?) heating and the exhaust side of the air handler goes to 21” x 25” ductwork.
The current heater is a Trane XE90 at 120K BTU. At -5 degress today with the upstairs thermostat on hold at 73 degrees, the furnace would run for 3.67 minutes and stay off for 9 minutes (on 29% of the time). The HVAC calculation is 70.2K BTU/hr loss for a tight house and 85.5K BTU/hr loss for an average house. The air handler goes to 20” x 23” ductwork.
The HVAC-Calc manual states that the calculations are for actual heat loss and that usually you would want to oversize the furnace by 25%. The calculations would suggest that our lower level furnace is the right size but given the 24% run time, it seems that the furnace is substantially oversize. Similarly, based on run time and the HVAC-Calc, our upstairs furnace is also way oversize.
Ideally, when it comes to replacement time, we will replace with something that is a two (or more) stage and hopefully with a variable speed motor. Is there a way to “back into” the correct size given the run times on the existing furnaces? Is it possible the furnaces where chosen not so much for their BTU ratings but rather to be of the physical size or fan horsepower (blower capacity) to match up with the existing ductwork? Let me know if I need to add cooling information as well. We've never turned on the AC in the lower level and the AC on the main level seems to be sized properly at 4 tons (the total calculated heat gain for the entire house is 51K BTU/h with 15.8K BTU of that being for the lower level.).