Equipment: New 3 ton Trane XL19i 2 compressor heat pump with matched variable speed air handler and 9.6kw 1410 single-circuit one-phase 240v backup strip heat. Trane residential zoning (6 zones) and electronic air filter. Ducting per Manual D based on HVAC-Calc load, oversized somewhat for zoning. 8” bypass duct (per recommendation of JR Benny on this forum) can be either routed to the return plenum or dumped to basement hallways.

Test modes for stage 1 and 2 work normally and charge pressures were checked at both. Approximately one lb. additional R22 added to Trane factory charge as recommended for difference in length of 37 actual feet vs. 15 feet for factory charge.

Except for problem (1) below, the system acted normally for awhile. Both stages performed defrosts as expected with defrost lasting 1 ½ minutes at about 40 degrees outside temp and just under 3 minutes at 20. Stage 1 produced 80-90 degree air at 30-40 outside temp. Trane says coefficient of performance is above 1 for 1st stage down to 7 degrees and for 2nd stage down to 2 degrees (reduced 5-10% for 7500 ft. elevation).

Problems:

(1) When the heat pump switches from 1st to 2nd stage, it always turns on the strip heat at the same time. As set points are met or nearly met, it turns off the strip heat first and leaves the 2nd compressor stage on for awhile before dropping back to first stage.

Various people at Trane have said to include or not include outdoor thermostat TAYSTAT 250A between R and F at the heat pump to keep the strip heat from turning on until a set outside temp is reached. Currently this outdoor thermostat is disconnected, because the latest Trane recommendation was that it’s needed only for dual-fuel heat pumps. But when the ODT was connected the system still turned on the strip heat whenever it switched from 1st to 2nd stage.

Same story on W1, W2 and W3 connections at the air handler. Some Trane personnel have said to jumper W2 to W3 with the outdoor thermostat, and others have said to jumper W1 to W2 without the ODT (current setting).

(2) Alarm condition A: One cold morning (-5 to +10), after operating for 30 – 60 minutes on second compressor stage and strip heat, “Service” blinked on the master zone controller display, and the fault light on the system controller board alternately flashed once per second for 10 seconds, followed by continuously on for 10 seconds. Both warnings disappeared after cycling power.

Contractor said to “clean filters” and we did. Trane regional office said probably windy conditions did not allow defrost to terminate normally. Later that day we noticed that a defrost cycle lasted around 20 minutes with no wind. Following the failed defrost cycle we found no heat from the heat pump, and

(3) Alarm condition B: Status light on the heat pump defrost board normally blinks once per second but was instead doing two quick blinks and a one second pause. Once again this warning cleared after cycling power.

(4) Later when we noticed heat ramping down again, we found that the outdoor fan was running and the stage 1 LED was on, but the compressor was not running. No warning lights this time.

Because of the risk of compressor damage, we have switched to emergency heat and are not running the heat pump at all. Getting our contractor to our house is always a 2 to 4 week wait. We have asked Trane for the technical documentation listing the warning flash sequences and their meanings, but were told this information is proprietary and not available except to dealers. As far as we know, no one has looked up the flash sequences anywhere, but instead has simply told us the first thing that popped into their head to explain unusual conditions.

Any theories, recommended tests or suggestions would be greatly appreciated.