HVAC Furnace

Published on June 18th, 2017 | by Henri


Furnace must be properly sized

Q.  I had my old furnace replaced this fall with a new furnace and A/C. The new furnace is a Bryant and is 60,000 BTU whereas the old furnace was 70,000 BTU. Bryant doesn’t make a 70,000 BTU furnace.

I am unhappy with the new unit as it doesn’t seem to heat the house well. There is always a discrepancy of about 2-3 degrees between the thermostat (which has been replaced three times) and the other temperature gauges I have around the house. If it’s mild outdoors the temperature difference is about 2 degrees. If it’s below 15 degrees then the difference is 3 degrees and sometimes more. I haven’t had any below-zero days this past winter but am scared about that in the future. Do you think I should demand the company replace this furnace with the next size they have which is 80,000? I am afraid that might be too big and I’ve been told it may turn off too soon. This furnace is a 95% efficiency. But if I continue to raise the temperature, I will not be saving anything and actually using more gas.

A. Did the HVAC contractor who removed the existing furnace and installed the new one size the system using the manufacturers’ sizing software or follow the guidelines of the Manual J heating and cooling load calculation?

Simply looking at the input of a furnace is not a good guideline; the output is more important, as it is what determines how quickly the warmed air is delivered to the house. The calculations involve the equations of the furnace K-rating and the AFUE (Annual Fuel Utilization Efficiency) of the new furnace, which may be very different from the AFUE of the old furnace.

What is also important is the type of home. If it is an energy efficient and tight house, the difference between the old and the new delivery may not be felt as much, but in older, less efficient and leaky houses, it will be very much felt. In the past, furnaces were oversized to make sure that there would be no complaints regardless of how cold it would get outside, but it represented a considerable overuse of energy when it was abundant and cheap. It was also an inefficient system because it would overheat the space, shut off and come back on when the temperature had dropped uncomfortably. This caused undesirable fluctuations of temperature in the house. It was also not the best for the health of the occupants as the relative humidity would also fluctuate and cause nasal dryness.

But when the energy crisis hit us, we became more conscious of these problems, and furnaces were sized to run more frequently to the point that, when the outdoor temperatures were below 20 F., the furnaces would run constantly. When it got colder, the occupants had to wear sweaters.
Depending on the age of the old furnace, it is possible that it was sized just right for the space to be conditioned, but with a new high AFUE furnace, the dynamics have changed.

You should ask the HVAC contractor to run the calculations mentioned above and make the necessary changes, even if it requires replacing the furnace with the appropriate one. That’s what you paid him or her for.

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