Heatpump on. Fan, not so much
January 5, 2020 10:14 AM   Subscribe

HVAC-oriented Mefites; Why does my air handler not come on with the heatpump?

I have a relatively new heatpump system and, 99.9% of the time, the thing works like a champ. However, on random occasions, the heatpump will cycle on, but the air-handler does not come on with it.

When I realize this has happened, I run to the thermostat and power-cycle the system off, wait a bit, then power everything back on. When the system comes back on, both the heatpump and air-handler come on and all is well.

There is no rhyme-or-reason why this occurs. It happens in cold or heat, day or night, all year-round. Oddly, when it happens, I've tried to just switch the thermostat fan control from Auto to On, but that does not bring the fan on. My only option is to do the power-cycle.

The thermostat seems to be calling both units to run. At least, it makes the normal double-click sound when it makes the calls. And, it makes the same two clicks when I power the system off. I know that's not necessarily a reliable indicator, of course.

I assume this is also happening at times when no one is home to do the power-cycling, and I've not come home to a dead system, so it's apparently handling it somehow, but I'm afraid this might be slowly damaging the compressor or some other part of the system.

Any guess why this might be happening?

(FWIW, the system is not brand-matched. The heatpump is a Goodman and the air-handler is a Lennox. They're both 3-ton units, though.)
posted by Thorzdad to Home & Garden (9 answers total)
I know nothing about HVAC but that hasn't stopped me before! Could it be that the heat exchange has to gather some heat before the fan comes on to distribute that heat?
posted by tmdonahue at 10:48 AM on January 5, 2020 [1 favorite]

Relay for the fan? The relay might be engaging, but not enough to close the contacts. Turning the system off forces the relay open, and it can close on the next cycle.
posted by notsnot at 10:59 AM on January 5, 2020 [1 favorite]

When I realize this has happened, I run to the thermostat and power-cycle the system off, wait a bit, then power everything back on.

The "wait a bit" part might well be the only step that's actually necessary here. How long have you tried waiting before giving up on the fan coming on by itself?

This thought is prompted by the behaviour of my own split-system living room heat pump, which regularly runs its compressor for a minute or so before opening its vents and firing up the room fan. Seems to happen whenever the amount of temperature swing I'm asking it to generate is quite large.
posted by flabdablet at 4:43 AM on January 6, 2020

The coils on a heat will frost up. To defrost the coils the unit will run in reverse, so when in heat mode it will switch to AC to clear the coils of frost. When it does this the air handler fan will not run. You don't want the cold AC air blowing in the house in the cold months.
posted by tman99 at 6:46 AM on January 6, 2020 [1 favorite]

fladablet...Normally, there is never a delay between when the heatpump and the fan come on. It's always simultaneous.

tman99...I'm familiar with the heatpump defrost cycle. The activity I'm seeing isn't related to a defrost cycle.

In my case, when the system goes into defrost, the fan continues to run. According to the operating manual, this is normal. If the inside temp does drop below the set point during defrost, the auxiliary heating coils in the air handler will come on momentarily to keep things level. They've never had to do so during any defrost cycle, even in the bitterest of cold weather.
posted by Thorzdad at 8:52 AM on January 6, 2020

It's always simultaneous

except when it isn't, hence the question. How long have you tried just sitting and waiting for the fan to come on by itself, when it hasn't done so immediately?
posted by flabdablet at 2:20 PM on January 6, 2020

Well, there are the times when I'm in bed when this happens and I have to get up and run downstairs to the thermostat. So...30 seconds? That a pretty long time, relatively speaking.
posted by Thorzdad at 4:16 AM on January 7, 2020

Intermittent faults like this are shits to diagnose. The only way I know to do it is to start as close as possible to the load and work my way back to the controls until the point where control has broken becomes apparent.

So if this were my problem, I'd be working out a way to get a multimeter attached to the air handler's motor wiring, verify that the meter does show mains voltage present when the motor is running and no voltage when it isn't, then leave it set up that way until the fault recurs. When it does, check the meter. If it shows voltage, the issue is with the motor's internal starting switch and/or capacitor; if it doesn't, the issue is upstream and I'd move the meter back to the supply side of the motor relay and check for voltage there. And so on.

That said: if you're not experienced with poking around inside mains powered equipment with a multimeter, unsupervised is not the safest way to gain that experience.
posted by flabdablet at 5:01 AM on January 7, 2020

Sometimes a low wattage desk lamp, preferably with an incandescent bulb because those have fewer failure modes than other types, and possibly with the plug removed so that the bare wire ends can be screwed into the same terminal blocks as wiring to motors and whatnot, makes for a more convenient and reliable go/no-go semi-fixed mains power detector than a multimeter. A similar rig based on a five watt automotive bulb can also be useful if the thermostat side of things happens to be designed to run at 12 or 24 volts, which some are; use a multimeter to find out.
posted by flabdablet at 5:12 AM on January 8, 2020

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