Anyone in the HVAC industry?

[Rick Parker]

I've got an intermitant problem with my home furnace that I'm attempting to fix. The fan motor is attached directly to the squirrel cage without any belts or pulleys. It is a Rheem Highboy unit, '79 vintage combination heater/AC unit. When the burner lights, the heat/cool fan relay clicks signaling the motor to start. The capacitor start motor just "hums" without begining to spin. I can manually give the squirrel cage a nudge and it starts spinning without any unusual noises. Question: assuming the motor itself is OK, do capacitors fail or loose their ability to store energy to provide the bump? In removing it from the unit, what is the correct way to discharge it.

Thanks

[Ron61]

Rick,

I am not sure about your case but I had a capacitor go bad in my heat pump unit a couple of years ago. The symptoms weren't quite the same but it was one of the can capacitors and I could see the swelling of the can so I got it fixed before it blew. And yes, they do fail. And as big as these normally are, turn the power off and get a good cable to discharge it to ground. Or if you are in a hurry just put your fingers across the terminals. I will be watching for the first human satellite. Didn't you ever take the old capacitors out of the distributors on a points system and charge them and then just lay them down and wait until someone picked them up to watch the fun? Except the capacitors in my unit carry such a charge that no way do you want to discharge them through your body.
If you have an induction current meter and know what the motor should draw at start that would be an easy way to see if it is getting the initial high bump charge. Also make sure that even though the relay seems ok the contacts haven't burned and it isn't putting enough juice through to charge the capacitor fully. I don't know what yours would cost but my unit has two and they cost enough to buy a small car.

Ron

[Excaliber]

Capacitor is a likely problem for these symptoms all right.

[392cobra]

had it happen many times,it is the capacitor.always goes out at the worst possible time.

Finally learned to replace it when it loses 10% of it's rating.That stopped the surprises.

[427 S/O]

My outside unit wouldn't start, the guy said your capacitor is bad, replaced and cooooool again....

[Seagull81]

I would try to give you an answer, but I can't get past Fred's avitar!
Steve

I agree with the other guys, Capacitor.

[Ron61]

Fred's avatar is completely destroying concentration on here. I keep trying to type to the bouncing rhythm of the breasts. He needs a new one. Maybe Frenchy.

Ron

[Rick Parker]

If those were "fannin" me I'd sell the heater and A/C.

[Ronbo]

Yep sounds like starter cap. They are oil filled and over time start to dry out, especially your outdoor ones. You should be able to pick one up at a industrial supply house like grainger or an appliance parts place as well as a AC parts place.

Don't be suprised if you can't find that exact capacitor, so you'll need a different bracket as well.

[Wes Tausend]

Originally Posted by Rick Parker

I've got an intermitant problem with my home furnace that I'm attempting to fix. The fan motor is attached directly to the squirrel cage without any belts or pulleys. It is a Rheem Highboy unit, '79 vintage combination heater/AC unit. When the burner lights, the heat/cool fan relay clicks signaling the motor to start. The capacitor start motor just "hums" without begining to spin. I can manually give the squirrel cage a nudge and it starts spinning without any unusual noises. Question: assuming the motor itself is OK, do capacitors fail or loose their ability to store energy to provide the bump? In removing it from the unit, what is the correct way to discharge it.

Thanks

I think you've gotten good advice. One point I question is that if this an intermittent problem, it doesn't sound like it is the capacitor to me. I've always thought that once a cap fails, it's done permanently.

On the other hand, I think some motors have a centrifugal switch that always starts the motor with either an out-of-phase separate winding or temporary insertion of a capacitor in parallel(series?) with the main winding which causes the main winding to start with a phase shift that gives it intended direction.

If you hear a click from the motor after the hand-started motor comes up to speed, that may indicate that a centrifugal switch has kicked off. This could mean that the intermittent problem is due to bad internal switch contacts. I think that contacts are more likely to be intermittent than a capacitor. Just a thought.


...

[trularin]

Caps do not just hum...unless they are doing background for a barber shop quartet.

Seriously, pull the cap and take it to a shop and ask someone to put it on a Hunter, it will tell if the cap is any good.

If you know the size, PM me and I might just have a replacement. I have a few left and may have the size you need.

Last time I had this problem, and it was about the same as yours, it turned out to be the motor.

Hope this helps.

[trularin]

BTW, the cap will discharge through the coil winding inside the motor.

I could explain in detail the physics of all this, but; the cap is used to delay the current, by 90 degrees, to one winding before the other winding forcing the motor to turn in one direction ( laymans terms ).

They do dry out, but it has been many years since the industry has used oil filled caps on small applications like this.

[392cobra]

Rick,
I hope you got it fixed by now.

What did it take to get it going again ?