Lennox heat pump issues [Archive] - HVAC-Talk: Heating, Air & Refrigeration Discussion

gave20

01-24-2010, 12:16 PM

I have a Lennox XP19 heat pump paired with a G71 furnace and controlled with a honeywell HZ432 zoning board. Last weekend I started having issues with the HP. when running the DATS temp. would climb quickly, a few degrees a minute, then shut the HP off when the DATS exceeded 120 degrees. The call for heat would still be there, the furnace would move enough air to cool the A coil to about 70 degrees. The HP would then kick back on and run again to 120 degrees and shut off and repeat the cycle. I then called the HVAC company and a day before they showed everything worked fine. The Tech figured either the DATS temperate probe is going bad or the zoning board but he was perplexed as to why the HP would shut off at 120. (DATS high limit is 170 and low is 40). This weekend it was acting up again. I took my multimeter temp. probe and stuck it beside the DATS probe. The multimeter read about 110 pretty much the whole time, the DATS reading continued to climb until reaching 120.

My question is does the outside unit have a high temperature limit like the G71 does? The tech said the outside unit shouldn't be shutting off until the A coil reaches 170 degrees and if it did it would need a manual reset.

I'm all for replacing the DATS probe I'm just concerned there's a problem with the outside unit and not the zoning board.

I'll also note, when in HP mode the furnace moves 671 CFM for the first 7.5 minutes, then 819 CFM for another 7.5 minutes. It with then jump to stage 2 heat and run for 15 minutes moving 1171 CFM. If the DATS doesn't reach 120 before 7.5 minutes enough air is moved across the coil to keep the DATS from tripping the high limit.

brewchief

01-24-2010, 02:21 PM

I'm not sure why the contractor used that honeywell zone control instead of a lennox harmony zone control, while that honeywell control does appear to have some blower control it's pretty limited compared to the harmony board.

The DATS upper setting may be for fossil fuel and there may be a fixed setting for the heat pump.

It sounds to me like it's overheating because of the a/c fan program that decreases blower speed at the beginning of an a/c call to provide better dehumidification, switching this to allow full airflow may cure the problem.