ok its been 20 years since iv looked at this stuff i'm going to have to do some more reading to come up with the answer. I'm leaning toward the fact that it can react with the presence of less voltage?
After some investigation of a similar issue (warm weather, room temp at 78, STAT temp set to 74, cooling comes on and display temperature rises to 80, system runs continuously, forms ice on evap actual temp drops to 73), I have found that there is an incompatibility between this power stealing thermostat and the control board HK42FZ011 on a Carrier 9200 series system rated 42,000 BTU. The temperature display and the internal controller on the STAT are both wrong, causing it call for cooling all the time. The light on the STAT works fine telling me that there is sufficient power in the wiring, however, something inside this STAT is not getting the right power or is overheating internally in the STAT throwing off the temp measurement when cooling is on. 24V is fine and no other symptoms. Also, the temp display is accurate until the cooling is turned on, so the internal thermistor seems to be OK.
My guess is that the system was running 20 hours or more in this house over the past many summers (6 years from this STAT replacement), consuming lots of energy and building lots of ice in the evap A-coil.
Cleaning pitted contacts on OLD mechanical STAT which was lying around in the basement for the 6 years and the system cycles like a charm, holding the room temp to within a half a degree. Lesson learned: avoid the power stealing STAT devices unless you are sure they will be compatible and test thoroughly through heating and cooling cycles or go with a battery powered device.
Hope this helps anyone facing a similar situation.
Company still buys these and other non-powered stats, but I refuse to use them just because of this type of issue.
We charge the same for the basic digital Pro5000 as the digital round or non-digital round, so install a battery operated(and connect to "C" when possible) and not worry.
Also, the non-digital rounds seem to be off in temp more often than correct. Just another reason not to use them.
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from an excerpt by Paul Jacob in Sun City, AZ
Honeywell claims that it is a bad STAT and not a compatibility issue, but how would the installer know without waiting out an hour or two and testing it through multiple heat and cold cycles allowing everything to stabilize between changes etc. Nobody is going to pay for that time.
Instead of paying $80 for the service call including cleaning and replacing the old STAT 6 years back, these people paid $240 to install the new one and then perhaps $300 to $500 per year in extra cooling costs to generate ice in the evap coil. Then again today to go back to how things should have been all along!
I agree with pacnw - will never install a power stealing STAT.