Blower Motor Stops Intermittently - then not

[silverago]

Hello, HVAC problem: I have a 13 yr old Lennox gas furnace and outside AC compressor. I'm having a problem with the indoor furnace blower motor cutting in and out inconsistently when the AC is on. It does not appear to be the temp of the motor - when the motor is cold/cool (with the thermostat AC/heat both off), I can cause the problem by throwing the main power switch off for 30 seconds then back on, leaving it on for one minute - doing this multiple times will cause the problem to occur. So the AC does not seem to be the cause. When the motor begins to start, it gets partially or fully up to normal speed before it cuts out again. When it cuts out, it dies for one second to 10 seconds before it attempts to start itself again, and can sputter like this numerous times while trying to restart. If the AC temp is set low so the unit must run for a couple of hours, the fan eventually settles down and stays on continuously until the t'stat turns off the AC at the set temperature. The fan motor feels VERY warm at this point (can't keep the back of your hand on it for more than 5 seconds). When the AC comes on again a short time later, the problem starts again.
An HVAC guy just changed the blower motor circuit board ($236), and I tried a new Thermostat but no luck with either. HVAC guy said he will call Lennox tech support tomorrow morning.
Also: Filter is clean, nothing blocking squirrel cage fan input on either side, fan blows hard when it's running, fan rotates extremely easy when moved by hand, motor shaft has no play when moved up and down - but slides in/out of motor slightly, switching thermostat to "fan on" does not help,
Any help would be highly appreciated - thank You.

[HayZee518]

check for an open aux contactor in the compressor circuit. a motor in the compressor is trying to start against a non equalized pressure between the high and low side and is always going to pop an internal thermal overload.

[silverago]

thank you for the reply HayZee. Note if the system has been turned off for over one hour, and I also set the tstat off during that time, I can reproduce the problem just by turning the furnace power switch off and on as previously described (tstat still set to off). I would think the outdoor compressor would not be a factor after this hour - wouldn't the pressure have equalized by then ? - and with the tstat set to off, the AC would not (and does not try to) come on.
A little more data and observation from this evening -the system also has a TrolATemp TotalZone 3 zone control system. Only 2 zones are wired and the one zone always has the tstat turned off (it has never been on in 13 years - the loft room). It may have been coincidence but when the AC was on and the temp was reached, the TrolATemp AC light (green) switched to the yellow purge light (normal functioning as previously observed), but the fan cut off at the same point.
Does this spark other possibilities ? - thank you

[paul52446m]

Hello, HVAC problem: I have a 13 yr old Lennox gas furnace and outside AC compressor. I'm having a problem with the indoor furnace blower motor cutting in and out inconsistently when the AC is on. It does not appear to be the temp of the motor - when the motor is cold/cool (with the thermostat AC/heat both off), I can cause the problem by throwing the main power switch off for 30 seconds then back on, leaving it on for one minute - doing this multiple times will cause the problem to occur. So the AC does not seem to be the cause. When the motor begins to start, it gets partially or fully up to normal speed before it cuts out again. When it cuts out, it dies for one second to 10 seconds before it attempts to start itself again, and can sputter like this numerous times while trying to restart. If the AC temp is set low so the unit must run for a couple of hours, the fan eventually settles down and stays on continuously until the t'stat turns off the AC at the set temperature. The fan motor feels VERY warm at this point (can't keep the back of your hand on it for more than 5 seconds). When the AC comes on again a short time later, the problem starts again.
An HVAC guy just changed the blower motor circuit board ($236), and I tried a new Thermostat but no luck with either. HVAC guy said he will call Lennox tech support tomorrow morning.
Also: Filter is clean, nothing blocking squirrel cage fan input on either side, fan blows hard when it's running, fan rotates extremely easy when moved by hand, motor shaft has no play when moved up and down - but slides in/out of motor slightly, switching thermostat to "fan on" does not help,
Any help would be highly appreciated - thank You.

Sound like either the motor is getting too tight, Does it roll free. Start capacitor is going bad or end switch in the motor is sticking in or out. The out
side unit has nothing to do with this problem. Later Paul

[silverago]

So the AC guy installed a new fan motor this morning - I'll need to monitor what happens over the next few days. It will be near 100 with high humidity Saturday here in PA so that should be a good test to determine if the problem is finaly fixed.
My only potential issue with this new fan is a short 3 whump-whump-whump sound when the fan is starting up from a dead stop. It comes up to speed as fast as the old motor then runs quiet after the three whumps.
When the AC guy first turned it on and off (twice) after installing it, it did not whump....that started right after he left (typical huh).
Any ideas about the whumping? - normal break-in? - or sign of a defective fan? ...still whumping on startup tonight.
Thanks for any input.

[HayZee518]

belt drive or direct connection? whump would be a belt slipping.

[paul52446m]

So the AC guy installed a new fan motor this morning - I'll need to monitor what happens over the next few days. It will be near 100 with high humidity Saturday here in PA so that should be a good test to determine if the problem is finaly fixed.
My only potential issue with this new fan is a short 3 whump-whump-whump sound when the fan is starting up from a dead stop. It comes up to speed as fast as the old motor then runs quiet after the three whumps.
When the AC guy first turned it on and off (twice) after installing it, it did not whump....that started right after he left (typical huh).
Any ideas about the whumping? - normal break-in? - or sign of a defective fan? ...still whumping on startup tonight.
Thanks for any input.

Are you sure that the new motor is running the same speed as the old motor was? If you are moving more air than before you could have sheet metal popping in and out, due to the increase of air movement??? later paul

[silverago]

The fan is a direct drive, no belt. The fan slides over the motor shaft and the motor is bolted to the fan housing with 3 bolts.
The fan is definitely pushing the same amount of air as the old motor did and gets up to full speed just as fast as the old motor did - it just has those 3 whumps that the old motor did NOT have. Other than the 3 whumps it seems perfect.