Defrost sensor is located in the correct location? to me if you already replaced those three parts, and the sensor is mounted in the correct position possible your refrigerant charge is off or your OD metering device is not throttling 100% correctly, or a very slight restriction in the drier ( do not know if you have one or two driers ) changing the temperature of your tubing enough to initiate a defrost.
You mention 85 degree discharge air temperature, but you do not mention the room temperature , @ 40+ degree outdoor temperature to me you should easily be over 20 + degree temperature difference between supply and room. Hopefully your return duct is sealed tightly. If your positive your 15 KW is only coming on only during very cold weather, then to me it's coming on during defrost , your electric rate seems high for only running 1st stage Heat Pump only,
What is the history of your HP compared to last year? Where you paying the same monthly charge ,give or take last year?
The only thing I would recommend is when you get into the warmer months I would have a competent Tech. Fimiliar with how a Trane XL 15 works and have them check the refrigerant charge according to subcooling or superheat or whatever Trane recommends to adjust and check the charge. You look to be a detailed oriented person, and you can download the install guide to get a heads up as to what Trane recommends for sub cooling before you call .The base line sub cooling temperature that Trane recommends may be located on the outdoor unit electrical data plate.
Bazooka, firstly I'd like to thank you for your professional response. The defrost sensor (yellow) is mounted low on the coil set on a pipe that has a copper 'bell' sort of brazed onto it. The defrost sensor is nice and snug in that housing, same as the previous sensor it replaced. As for as the drier is concerned, I only see a single large Grey Drier unit outside on the line set midway between the egress of the house and ingress into the outside unit housing.
Sorry, I did forget to mention the inside house temperature. The house is set between 68/70* (permanent hold) I work from home so , I set and forget the thermostat. The main supply and return trunk(s) are encased in drywall, however first thing I did when i moved in the house was find all the return feeds and seal them up with aluminum HVAC tape. They used the cavities in between the floor joists in some cases. They did foam seal the insides of those runs but not the sheet metal they used to box the runs, that is what I sealed.
I verified on multiple times with a clamp meter that heat package is only coming on with call for aux heat when it is very cold (~15* F or below). I agree that the elevated utility would be tied to the frequent calls for defrost when not needed (no visible ice AND temperate).
I've checked my rates they are the same for this and last year.
February, Most current bill: 409.00$
Used: 3,005 KW
Daily Usage: 103.6 KW
avg Daily temp: 35*
Last year same month/cycle:
Used: 2823 KW
Daily Usage: 97.3 kw
Avg Daily temp: 29*
January 2016 bill: 420$
Total Usage: 3142 kw
Avg Daily Usage: 89.8Kw
Avg temp: 42* F
Last year Same Time:
Total Usage: 2674 kw
Avg Daily: 76.4 kw
Avg Daily Temp: 36* F
So, you can see it was colder last year and I used LESS energy and rate are the same. February had 6 shorter billing days so that basically accounts for the small difference in that bill.
I can hear when the unit fires up as it has a distinct sound when in heat mode vs cool. When i'm on the first floor I can tell when it sin defrost because I don't hear the fan on the unit spool up, as it come sup to speed gradually and whirs. When I don't hear it I go see if its in defrost, and most typically it is as the fan isn't engaged as it shouldn't be in beginning of defrost.
I thank you for your honest feedback. I'm not a HVAC pro, but i'm not a dummy either. I was reasonably confident the defrost cycle was initiated and controlled by the defrost sensor (yellow) the ambient sensor (black) and the Defrost control board. When the tech pointed out that additional sensor I needed to investigate. At first it seemed plausible because the sensor he pointed out is spring clamped to the refrigerant line inside the coil next to the compressor. I looked at the service papers for the unit and did not see that part listed as defrost, but as a high temp thermostat base don the wiring and schematics. But again not being a pro, i'd thought i'd investigate.
Nick
