Defrost Coil Temps are High

Curt312February 8, 2012

The Specifics:

- York 2 stage 3 ton heat pump

- York 80% Modulating furnace

- VisionPro IAQ tstat

I tend to be a bit anal in my data collection. I monitor most aspects of my heat pump (hp); hp off/fossil fuel on, defrost cycles and temps, etc. That said, I've had 4 new controller boards put in my hp since purchased 2 years ago; the latest one was 10 days ago. I've notice a change in the temp of the outside coils during defrost. Typically the coils reached 80 or 90 degrees during defrost. Now they are reaching 110 to 125 degrees.

Is this normal? What triggers the end to a defrost cycle.

A little more detail about my heat pump:

The four controller boards I spoke of have been replaced for various reasons. The most recent was because I detected the hp was shutting off three times an hour and the fossil fuel was kicking in. According to York, once the fossil fuel is energized, it should not shut off until the demand by the thermostat has been satisfied. The tech was able to duplicate my findings and he replaced the controller board in hopes of rectifying the problem.

My observations of the outside coil temps is a new issue. I have data going back two years and I have never recorded coil temp this high however, I don't know if this is in the normal range of operation.

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It is possible (probable)for the coil to reach those temps during defrost. Once the fossil fuel kicks in, the evaporator coil is right there at the heat exchanger and it gets pretty hot right there. Your HP is now in the cooling mode and removing some of that heat from inside the furnace to the outside, to melt any ice accumulation.

Check the outside coil this summer and according to temperatures inside and outside the house, you may see the coil read 110-115 degrees on hot days. It'll change according to surrounding temperature.

    Bookmark   February 8, 2012 at 10:36AM
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I believe I have a pretty good understanding of heat pumps (born out of necessity). The returning fluid from the "evaporator coils" in my house is about 45 degrees during a defrost cycle (according to my sensors). So I don't believe my additional heat is coming from that.
Anything I find in my research only mentions the outside coils get to 65 or 75 degrees during defrost however, this doesn't mean York has not designed something different. I can't find any engineering spec which actually states the temp range of the coils during defrost.

    Bookmark   February 8, 2012 at 3:01PM
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"The returning fluid from the "evaporator coils" in my house is about 45 degrees during a defrost cycle..."

There better not be any "fluid" being returned from the evaporator during defrost.

The system is in cooling mode, so the gas from the inside coil (acting as an evaporator) is going to be cool.
It is the compression of this gas (and the heating during compression) that forces the outside condenser to heat up for defrosting.
The heat from the hot high pressure gas is then lost to the condenser as the gas cools and condenses to liquid (and melts any frost present on the now heated condenser coil).
If the defrost cycle is driven by a simpler timer, the condenser may get rather hot, especially if there is not much frost present on the coil.

'Demand' defrost systems should help limit unnecessary defrost cycles that simpler timer systems can initiate.
The more complicated system (of course) costs more, requires additional components in the system, and thus decreases reliability.

    Bookmark   February 8, 2012 at 3:28PM
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when you are saying controller board I assume you are talking about the defrost control board. Where are you getting the temp values from? the sensor on the control board or your own homeowner installed sensor? The defrost is terminated by the sensor on the board when a certain temp is attained for a period of time.

    Bookmark   February 8, 2012 at 3:48PM
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I use an Oregon Scientific weather station with 10 temp sensors. I have one in the burn chamber, on the gas/fluid return pipe, on the outside coils, on a floor vent, e2tc. This data is recorded and graphed on my computer. It is easy to tell when the system is in a defrost cycle.
The controller board I reference is the main board in the outside heat pump (York has their heat pump do all the "thinking", not the Honeywell thermostat).
I can only measure results, I can't measure performance against a standard. I don't know what the standard Is! I do know that immediately after the main board to my heat pump was replaced, I see higher defrost temps. To put this in perspective, last year my usual vent temps were 90. After a new controller board was installed , the temps werewere 110. It turned out that the board was bad. When measurements change, I assume the worst.

    Bookmark   February 8, 2012 at 8:42PM
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