HVAC fan intermittent

[herbertjd13]

My blower fan is intermittent, and this occurs during both heating and cooling demand.

For example, I am currently running air conditioning and the blower fan will kick out for about 1 to 2 minutes, even though the thermostat continues to call for cooling and the condenser unit continues to run (fan on the condenser unit is spinning). Then the blower fan will come back on and run normally for anywhere from a few minutes to a few hours, and then the problem will occur again.

When the blower fan kicks out, it will not turn on even when I switch the fan switch on the thermostat to the "continuous run" position.

Is this most likely a fan relay problem, a motor problem, or a control module problem? Any suggestion on how to troubleshoot?

The unit is a Rheem propane forced air furnace with a Lennox air conditioner added about 5 year ago.

Thanks!

[HayZee518]

probably is a fan relay because this seems to be super imposed between the thermostat and the motor. if you could post a schematic of your unit, this would help us troubleshoot your problem, otherwise its a shot in the dark what the problem might be.

[herbertjd13]

Many thanks for your input.

A schematic is attached.

[herbertjd13]

Noticed the schematic is not readable; will see what I can do to make it larger.

[HayZee518]

yeah, I just looked at it. my zoom makes it worse.

[herbertjd13]

Thanks again. Here's a .pdf that's not great, but much better.

[HayZee518]

ok print shows a door interlock switch, on the left part of the ladder diagram, next is a fan control [looks like a thermal device] [normally open] then a system relay normally open that goes to the exhaust blower.
if you can open the chat in here - look up top on the forum page for CHAT go in there.

[herbertjd13]

Sorry I missed you. Unfortunately, will be out now until late afternoon/early evening (I'm in Colorado).

It also said I had a private message but could not open that as it was blocked initially and then I coudn't find it again.

[HayZee518]

Furnace

Re-drawn diagram
Print shows 5 & 6 reversed on fan relay. 5 is normally closed and 6 is normally open. drawn 6 is normally open and 5 is normally closed.

[herbertjd13]

Not sure what to do next.

[herbertjd13]

Studied things a bit more, and still not sure what the re-draw means.

But am I close here:

The thermal device is part of the fan and limit control. When the heat exchanger temp exceeds the hight limit, then the limit control opens to kill the flame, and the fan control closes to assure that the blower is running to remove heat. So the fan control is not the source of my problem.

Does this mean that the only two potential problem sources are the thermostat and fan relay? If so, then if I can verify that the green thermostat wire connected to the fan relay measures 24volts to ground when the fan fails (meanining that the thermostat is calling for the fan to run), then the only other possible source of failure is the fan relay? (or an intermittent motor failure, which I assume would be unusual).

I'm mostly guessing here!

[HayZee518]

ok the thermostat is just a switch. the thing in the middle is the fan relay. the fan relay has 6 wires on it. 1-2-3 is one set, 4-5-6 is the other set. the fan relay operates when power is applied to the green wire. contacts 4-6 close, contacts 1-3 close, power path on the 120 supply line is from the hot to relay 6 - through the contacts to 4. from 4 & 1 through its contacts to 3 and to the high speed of the fan motor. if the fan relay fails to pull in, next to look at is the thermostat

[herbertjd13]

Naturally, I can't get the intermittent failure to occur whenever I'm available to do more diagnostics.

But fairly convinced it's the fan relay and have one on order. Will post back after install to advise if that solves the problem.

Many thanks for all your inputs!