MDT at Paragon Home Inspections, LLC wrote:
This one may be pretty tricky. My guess, offhand, is that the
thermostat might not have been "defective" in the sense of a
manufacturing defect, but rather that this might have been a design
defect: that it was possible to program the heat and cooling
set-points such that the thermostat could call for both simultaneously.

If so, a system might operate fine as long as 'stat was in ether the
"heat" or "cooling" mode (which is how it would likely be
tested during installation) but if it was switched it to the "auto"
mode afterwards, both might be active

I can see how an HVAC contractor or Home Inspector could easily "miss"
this unless they had a prior reason to test for it - both would likely
force the system directly into the 'stats "heating" and "cooling" modes
as required for routine testing - and I'd have to agree that it's
unreasonable to "expect" a typical homeowner to detect the problem
other than in the form of a high electric bill.

Michael Thomas
Paragon Home Inspection, LLC
Chicago, IL
mdtATparagoninspectsDOTcom
847-475-568

Hi,
In design aspect that can't happen. In auto mode it's either cool or
heat, not both. ASIC can't do both at the same time.