Lennox XP14 heat pump showing abnormal behaviour

[frodier]

I have a 3.5 ton XP14 Lennox heat pump that has not been behaving normally this heating season. I say this because I have been keeping track of its "heating" profile for the past 2 years. Basically I have a temperature sensor in the output plenum right after the coil which sends my PC temperature information during the heating cycle.

Last year at 32F outside the sensor showed 94F during heating, at 26F outside the sensor showed 92F at the coil.

This year at 32F outside the sensor reads 89.5F and at 26F the sensor shows 88F, at 40F outside the sensor sits at 91.8F.

No auxiliary heating was on during all these measurements.

This year the heat pump regularly goes into defrost, I noticed that sometimes it would do so even without frost visible. At 41F it was still going into defrost every so often. The XP14 has a demand defrost system.

I suspected a low charge may be causing these output temp variations, however since the pump behaved normally otherwise and gave me enough heating I hesitated to call for service.

Last night the temperature dropped to the lowest this year to 23F, and I found the pump this morning all frosted up and running continuosly, the sensor showed only 83F at the coil, when outside it was 29F. It was obvious that the demand defrost stopped working at some point during the night. I shut off the system for 10 minutes and on restart the pump immediately went into defrost, melted all the accumulated ice and starting working within the coil temperature profiles recorded this year ( which are on average 4F lower than last year's)

My question: are these symptoms indicative of a low charge?

I'll be calling for service next week and I'd like get a forecast of what could be wrong with the pump.

Thanks in advance

[martyinlincoln]

Could be low on refrigerant or something is effecting the air flow at the heat pump. It's past due for service anyway so the tech will let you know when he comes out.

[TwincamDave]

Sounds like a good guess, only a good tech can look at it and tell you for sure. Has there been any service done to it in the past year?

[wahoo]

Since you monitor this system, you surely know the importance of at least an annual tune up. Have your system put on a pre-paid annual check with a reputable contractor.

[hkempf]

Ha your my kind of geek, I mean guy, I'm a total computer geek and love that you have your system linked to your PCM, now that's just awsome

[hkempf]

I mean pc, damn auto correct

[frodier]

The tech came over to check the heat pump a few days back. Checked system pressures and said all looked normal, but slightly on the low side. He added some R410 . I kept monitoring the plenum temp inside with my iphone as he was charging. There was no noticable change in heat output. He then changed the defrost board cause he said it seemd to be not working properly. As it was under warranty I told him to go ahead.
I've been monitoring temp profiles since then, there is no change since before the service call. However I looked down the top of the outdoor unit and noticed that the TXV valve is strapped on the suction line, but there is no insulation around it. Is this normal for a Lennox XP14 heat pump? The inside coil TXV has insulation around it, but the TXV outside has no insulation. Was just wondering if this could be causing a loss of heating efficiency, since in heating mode it's the TXV outside that is active.

[frodier]

Above I meant that the sensing bulb is strapped to the suction line...not the valve itself

[joey83]

Did he give the outdoor coil a good wash? And drybulb temp does tell a lot but the wetbulb temp is just as important.