I have a 2700 sq ft rambler home with cathedral ceilings over a large portion of it - over the hallway, 2 living rooms and kitchen. We replaced the furnace that was installed when the home was built in 1982. This original furnace was a 115000 btu furnace. Unfortunately, we changed the furnce before we moved into this house a few months ago.

When the HVAC guys did a heat loss calculation they determined that the house needs about 80000 btu and installed a 80 % efficient 90000 btu Lennox (2 stage, variable speed).

The furnace runs constantly even when the temperature outside is 50 degrees and the thermostat is set to 62 degrees. Our utility bills are over 270 dollars for this setting.

The HVAC people have suggested adding more vents in the areas with the cathedral ceilings, changing the duct work to have a 2 zone system. In short they have asked us to modify the house so the furnace operated more efficiently.

I am wondering if the decrease in BTUs is causing the furnace to run constantly, even for low thermostat settings.

  • 90K btu furnace should easily maintain 62F when the outside temperature is 50! Something does not sound right.... We have approximately the same size house and a 93K btu furnace and it maintains 69F even when it is -22F outside and windy (we live in MI). If it is always running when it is 50F outside, something must be not working right! Unless you are keeping your windows open or your house is incredibly leaky and poorly insulated 90K should be plenty for maintaining 62F
    – Yuriy
    Feb 26, 2014 at 3:13
  • What climate is this in? What year of construction or type/R-rating of insulation is present?
    – wallyk
    Feb 26, 2014 at 5:29
  • I wonder if the constant running is a setup issue - ie, the air handler is running full time (usually a very inefficient/power hungry form of whole-house fan) and the actual heat is coming on intermittently? As opposed to only running the air handler when heat is called for, and for a few minutes after the flame shuts off.
    – Ecnerwal
    Mar 1, 2014 at 21:34

2 Answers 2


I hope the 90K is the output or it's truly undersized! Even so, it's quite common to oversize furnaces beyond the calculated heat loss by 20-30%. It's also possible the heat loss calculation is low, it's really just an estimate, and part of why the furnaces are oversized. Even if it is undersized, it would not significantly alter your energy costs. Costs are dependent mainly on the home's overall insulation and the furnace efficiency, the capacity and running time is not a big factor.

Changing ductwork will better distribute the heat, but if more heat is leaking out than the furnace can generate, it will not help. While a larger furnace will better heat your home, it will not help your energy costs. If your costs are out of line for your type and size of house in your area, then more heat is being lost than is typical. In the long run, you are better off preventing heat loss than generating more heat.

There are energy consultants in most areas that have the proper tools and equipment to advise you where most of the heat is being lost and what could be done to most effectively reduce heat loss.


Well it's a 2 stage so most of the time it's running at about 36k btu (80% 90k btu=72k btu max output) I'm no HVAC guy but to me it's a little undersized, a 90k btu 96% AFUE would have been better or 100k btu 80% AFUE.

I think 2 stage furnaces are designed to run constantly, giving you a more comfortable environment, but they cost more and will need service sooner because they run all the time.

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