Lennox heat pump defrost circuit

[Citation]

Is anyone on the forum familiar with Lennox defrost circuits on heat pump?
My house has a HPXA12 heatpump. Recently it seemed like after a defrost cycle the outdoor fan no longer works when I request heat or AC. This started on a below freezing day when defrosting would have been needed. I noticed the HP was supposed to be running but no heat was coming and the outside fan was off. The unit was well defrosted. As an expedient I setup the system to just use backup heat.

Today I tried to look into the thing and found the following.
1. The outside fan works if I manually press the compressor contactor. I've also verified the capacitors are good. So I do not believe my issue is with the fan itself.
2. Today was warm enough to try both heat and AC modes. In both cases the pump turned on but the fan didn't.
3. Looking at the defrost control board there are status LEDs. The whole time they indicated either ready or short cycle delay (just after I turned the system off).
4. I don't see obvious damage on the defrost control board. Replacements appear to be $200 so affordable but I don't want to guess and check just yet.

Is there anything else I should check?
Are these boards known to fail? I see that Lennox has a new board that supersedes mine.
Any suggestions?

 

[bonneyman]

I've no direct experience with the model of Lennox unit you have, but most defrost boards employ a built-in fan relay that controls the outdoor fan. Usually a NC (normally closed) switch. During cooling it brings the fan on when the unit's on. However, during heating - when the unit goes into defrost - it opens to shut the fan off while the compressor stays on. This allows the heat being produced to build up in the outdoor coil in order to melt the ice. When the defrost thermostat ends the defrost cycle, the fan relay closes to bring the fan back on.
Sounds like the relay on your board has failed open and is bad. They get exposed to inrush current every time the fan starts, so, over time the tiny contacts inside can overheat and go bad. You probably wouldn't see any external damage, and it's integral to the defrost board. But I hesitate to tell you to buy a new board being a "arm chair" technician in this situation.

You could try testing the relay on the board if you feel you are technically up to it. The relays are usually fairly big (comparatively speaking) components on a defrost board, with two wires coming out of it. You could try pulling those leads off (follow the schematic to find the right ones), and check for continuity across the two terminals. Since they are usually normally closed, there should be continuity through that switch whenever there's power to the unit. (Your unit may vary) No continuity could indicate an open relay when it should be closed.

Hope this helps!

 

[fitter30]

Page19
www.manualslib.com/manual/835779/Lennox-Hpxa12-Series.html?controller=view#manual
Fan is controlled by A108 defrost board marked fan two black wires where t stat wires are landed

 

[Citation]

I've no direct experience with the model of Lennox unit you have, but most defrost boards employ a built-in fan relay that controls the outdoor fan. Usually a NC (normally closed) switch. During cooling it brings the fan on when the unit's on. However, during heating - when the unit goes into defrost - it opens to shut the fan off while the compressor stays on. This allows the heat being produced to build up in the outdoor coil in order to melt the ice. When the defrost thermostat ends the defrost cycle, the fan relay closes to bring the fan back on.
Sounds like the relay on your board has failed open and is bad. They get exposed to inrush current every time the fan starts, so, over time the tiny contacts inside can overheat and go bad. You probably wouldn't see any external damage, and it's integral to the defrost board. But I hesitate to tell you to buy a new board being a "arm chair" technician in this situation.

You could try testing the relay on the board if you feel you are technically up to it. The relays are usually fairly big (comparatively speaking) components on a defrost board, with two wires coming out of it. You could try pulling those leads off (follow the schematic to find the right ones), and check for continuity across the two terminals. Since they are usually normally closed, there should be continuity through that switch whenever there's power to the unit. (Your unit may vary) No continuity could indicate an open relay when it should be closed.

Hope this helps!

Thanks for the suggestions. I have no issues identifying the electrical components and have already thought about doing a continuity test on the thing. What I'm confused by in this case is if the relay is NC and I've verified the fan works when I manually close the compressor contactor, why would it open when the thermostat sends the go signal?

 

[Citation]

OK, just did a bit more checking. The fan relay is NC per my continuity check. Also, if I manually press the compressor contactor the fan turns on. If I unplug one of the fan wires from the relay (ie make it always open) the fan doesn't turn on. Finally, I checked and when heat was requested the relay went open. So the relay seems to work but why the system thinks it needs defrost all the time is still something I don't understand. We are almost 50 degrees and no sign of frost so I don't think the system should think it needs to defrost.

As for the temperature probes that decide when defrost is needed, is it possible one of those is the issue? I would assume when the ambient temp is a good bit above freezing and the unit is just started it will default to normal run, not defrost. Thus even if the temp sensors were out of position they would both read the same temp and defrost wouldn't be requested.

We've been using aux heat for about 2 weeks now. If I can't think of anything else I'm going to swap the board. At worst I'm out $200. Fortunately(ish) I don't *need* the system to work until AC season. I can limp by with expensive aux heat for the time being.

 

[bonneyman]

Thanks for the suggestions. I have no issues identifying the electrical components and have already thought about doing a continuity test on the thing. What I'm confused by in this case is if the relay is NC and I've verified the fan works when I manually close the compressor contactor, why would it open when the thermostat sends the go signal?

Good point! Could be something else in the board. You could turn the system on to heat -with everything powered up as it would in heating mode - and recheck that relay. Outside of defrost it should pass power. Perhaps some circuit logic is screwing up and opening the relay whenever it gets a 24 volt "on" signal?

Check the defrost sensor, too. If it's failed closed (rare but not unheard of) it's telling the board to stay in defrost even if the coil is clear. Certain units would also have this happen during the summer in cool mode!

 

[rooster59]

If you haven’t yet, run a test cycle by using the jumper across the test pins and watch the leds.

 

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Thanks rooster. Tomorrow I think I'm going to try the reset option. If that doesn't fix it, well it's $180 for a new, replacement board. I will probably just do that. Doesn't look like a hard install and getting a few more years out of this 20+ year old system seems like a good plan <- saying that means it's going to eat it's pump during the height of summer.

 

[bonneyman]

I'd be interested to find out how things go with this.
The system is old enough (but not so old) that it was made before the time frame of when I saw quality drop in new units (circa 2012 or so). So as long as no major component fails it's worth keeping going IMHO.

 

[Citation]

I gave the reset jumper a try and it worked! Rooster, thanks for suggesting the rest pins. I had been looking at the LEDs and it was always doing the read or delay flashing sequences. It didn't seem like the jumper did anything but when I turned the heat request on from the thermostat, well the fan started spinning.

The up side is this was a zero dollar repair (thanks for all the help!). Well zero dollar other than the extra electricity over the last two weeks using only aux heat. The down side is this feels like one of those "fixes" where you take something apart, find nothing obviously wrong, put it back together and it works.... is it fixed or just going to break again. Anyway, if it does I'll likely get the replacement board.

Thanks all!

(and Merry Christmas)