I live in the DFW area, it was raining last night, so it was steamy, high humidity. I have one unit for both heat and a/c. Our home is 2720 s.f. The unit is a two-zone unit, which means we have two thermostats, one upstairs, and one downstairs.

Here's what happened last night:

1. I noticed the upstairs zone was slow to come on, the thermostat was set at about 78 and it took at least two to three minutes for the unit to come on. Finally it came on.
2. I noticed the filters needed changing. I changed all three filters. I have a two month old filter grill in the down stairs ceiling, and two filters upstairs. Boom, now the unit was not coming on slowly, now it was coming on right away, good news.
3. I noticed the upstairs zone was not turning off. For example, the inside temp was 80, so I set the thermostat to 78. The unit was cooling, and within a "normal" amount of time, the inside temp registered 78 also. But the unit wouldn't stop running. I may have waited 15-20 minutes before I gave up and bumped the thermostat up to 79, and this made the unit turn off. I also checked the outside unit to make sure it wasn't still running. The outside unit appeared to be doing what it is supposed to do.

4. I decided to isolate the problem, by trying the same experiment with the downstairs zone. First I made sure the upstairs zone wouldn't come on, by setting the thermostat up higher, about 83, then I set the downstairs thermostat at 78, inside temp was 80, the temp came on down, and the zone did shut off in a "normal" timeframe I thought.

I looked in the unit, the inside unit is upstairs. I have a Honeywell "controller", that has a bunch of bulbs in a horizontal line. I shined my flashlight in there, and the purge light, which is yellow is flashing during this time when the temp is not turning off. There is actually an explanation on the board. It says "flashing purge - ZMS sensor failure or no ZMS sensor installed". In fact the purge light is flashing right now. The unit is off now.

At some point last night, I noticed the purge light on, steady, but I can't remember what was happening when the light was on steady.

Now- to this morning.

I performed the same experiment with both zones.

Now they are both doing the same thing, the temperature is coming down at a good pace, but both upstairs and downstairs zones are taking 9-10 minutes for the unit to shut off, AFTER the inside temp is the same as the thermostat setting. To me, this is way beyond normal. I would expect some lag time, maybe the unit is getting rid of the humidity? but 9-10 minutes?

Thanks for reading, and I appreciate any ideas.