Blower cuts out

Problems related to residential installations.

Blower cuts out

Postby AdamMikalauskas » Fri Jun 17, 2011 1:58 pm

I have a SnyderGeneral GUH075A012IN high-efficiency furnace with a direct drive blower along with a central air unit (unknown brand/model) all being controlled by a Honeywell CT1501P thermostat. I am having a problem with the blower and am not sure what exactly the cause is. Problem is that the blower will intermittently (without any pattern that I can figure out) shut off for approximately 90 seconds and turn back on. This happens when in Auto Heat, Auto Cool, and Manual Fan On modes.

Due to the intermittent aspect of the problem I have been unable to diagnose whether the blower is still getting power at this point. Currently I believe that it can be; the blower, the blower relay, or the thermostat. I intend to replace the thermostat this year anyways. Using a clamp on ammeter I have found the blower pulls 18.3A peak at startup and when up to speed a constant 8.5A.
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- Blower cuts out

Postby nomadpeo » Sat Jun 18, 2011 6:33 am

you need to know the amp rating of the motor. it will be shown as fla (full load amps) or simply 'a' for amps (- 12.5 a). the actual amp draw needs to be less than this rating. if the amp draw exceeds the rating, the motor will open internally on a thermal switch. the cause could be the motor developing a winding to winding short or a bearing starting to wear, or it could be the capacitor going out. the other possibility for an overload could be air restriction. i'm sure you would notice cooling performance problems if your evaporator coil or filter was restricted to the point that the motor was suffering.

you said the unit was high efficiency, so i'm assuming a modern unit with a circuit board. the relay is not a servicable part in that case. if it is failing, the board would have to be replaced. if the board or the thermostat was the culprit, it would probably be failing solid. it is more likely an overload. verify that first.
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- Blower cuts out

Postby Freon » Sat Jun 18, 2011 12:26 pm

Has anything changed with you system... new duct work or new filters? Have you had an electrical storm before this problem started? Has the system always operated as expected until this problem?

If possible, the next time the motor stops, go to the furnace, open the blower compartment and CAREFULLY put you hand on the motor. Is it very hot? Do you smell anything that might be from excessive heat? Spin the blower wheel. Does it spin freely and quietly? If the motor is excessively hot then the thermal safety may be shutting it off, as previously suggested. However if that's the case, then you'd expect to find an excessive load on the motor (depending on the type of motor) for some reason... closed supply vents, blocked returns, clogged filters or a blower wheel that does not spin freely.

It would be nice to measure the voltage at the furnace control board where the thermostat wires connect between the G (fan) terminal and the C (common) when the blower stops. If the voltage is 0 then you know it's the thermostat. If you are reasonably handy with wire, you might try the following. This may be risky so think the logic through before attempting. I do not know your specific system or how it has been wired.

Turn OFF the power to the furnace. At the control board, disconnect the wire from the thermostst going to the G terminal. Label that wire G and put tape over the bare end. Now disconnect the wire from the thermostat going to the R terminal of the control board. Again label and tape it. Be sure you get the thermostat wire in case there is more than 1 wire at the R terminal. You have now removed power going to the thermostat and isolated the fan circut from the thermostat.

Now make a jumper wire and connect it at the control board from the R terminal to the G terminal. You are effectively putting 25V AC directly to the fan circuit thereby having continuous call for the fan and no thermostat intervention. See if the blower motor cuts out as it had before. If it does then the problem is with the control board/relay or the motor.

Testing the motor can be done but is more involved. You'll need to know about the kind of motor it is (PSC or other) and possibly make a direct connect to the motor bypassing the control circuit completely. This is not something most people would want to attempt.
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- Blower cuts out

Postby AdamMikalauskas » Sun Jun 19, 2011 1:16 pm

Thanks so much for the replies. I rewired the blower to an extension cord that I had laying around and found that the cutout still occurred. I found that the motor was running extremely hot. I then removed the entire blower assembly to remove air restriction from the equation, and again had the cutout occur. When removing the motor I found that my current reading was much higher than the motor plate. Removed the motor and during dis assembly found that the windings were burnt (both visibly and smell wise). Replacement fixed everything, including a large decrease in amperage draw.

Thanks for the help,
Adam
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