Poor air flow -- undersized system?

Yesterday I inspected a house built in in 2005 that had 2900 sq ft, but was much larger in volume than a typical 2900 sq ft house, because about 1/3 the 1st floor area was taken up by a great room.

The furnace was a Carrier ultra-high efficiency model with an output of 112,000 BTU and 3/4 hp motor, and the AC condenser was a Carrier 4 ton unit, which was not run due to cold weather. The latter seemed a bit under optimum size to me.

The furnace did appear to heat the house well enough throughout, and did not have to run for too long in my judgment to do so for an outside temperature about 32 F / 0 C. But the flow at many registers was barely detectable, especially in the 2nd floor and basement, though all were fully open.

The rating / data plates for the units are attached. The AC rating plate was already just about worn off, so I had to really push the contrast to make it readable.

Thoughts, HVAC people?


It appears to be adequately sized, but it’s not our job to determine the adeqacy of heating and A/C systems.

I’ve got a calculation guide you can give to your clients…HERE

Thanks, David. I will forward that along to my client.

It often appears that higher efficiency systems tend not to crank out hot air as noticeably as the older, low efficiency units. Is this just “in my mind” or is there some logical explanation for such a perception, such as slower air movement in the heat exchanger being necessary to transfer the radiated heat more effectively?

There too much to consider when sizing a heating system…

Window sizes and quality
Insulation R-Values in walls and ceilings
Seals in/around all wall penetrations
Distance from furnace to registers
Insulated supply ducts
Sizing of supply ducts.
Return air
ETC, etc, etc…