blower won't stop

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Old 05-17-09, 08:21 PM
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blower won't stop

Hi, got a rheem furnace (rgda-075a-cr). Recently, the blower started staying on constantly, regardless of whether the thermostat is calling for heat. We don't know when this started, but...

recently had the floor drain back up. This is right next to the furnace and the capacitor for the blower is sitting on the floor.

I thought the capacitor might have gotten wet. It hasn't dried out and started working normally in the last day (had fans drying the area out), but removing the electrical tape on the wire-to-capacitor terminal contacts revealed a little rust.

I don't know if the flood thing is a red herring or if a capacitor short could cause a constant "on" state. Figured I should mention it though.

The only way to shut off the blower is to cut power to the furnace. Digital thermostat looks okay.

Any ideas? Thanks!
 
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Old 05-17-09, 08:53 PM
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Not likely to be the cap. The capacitor helps start the motor, and gets cut out of the circuit when the motor comes up to speed.
 
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Old 05-17-09, 10:16 PM
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So does this mean that the programmable thermostat is constantly calling?
 
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Old 05-18-09, 03:33 AM
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Is the fan running at the slower heating speed or at the faster cooling speed? Take off the covers and look for the control board. See any flashing codes? See the G and C terminals where the cable from the thermostat is hooked up? Make sure the thermostat is not calling for heat and see if you have 24 volts from G to C. No voltage?- bad fan relay on the board, bad board, or maybe an open limit switch BUT that should give you a code and no heat. Yes, there is voltage? Disconnect the wire at G. Fan goes off now? If yes, problem is external to the furnace- bad stat, wires shorted, etc. Let us know what you find. Thanks.
 
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Old 05-18-09, 06:36 AM
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Hi Daddyjohn,

Thanks for that. I'm heading over at lunch to have a look. I'll bring my multitester and check the wires as per your recommendation. The furnace is the original from the 1987 construction, the subdivision builder model, so I don't think there's a control board (or lights for blinking codes). I'll look, but I haven't seen one fooling around with the thing over the years (not that I looked for one so far)!

If this isn't an easy fix, I think it might be time to shop for a new furnace since it's so old. Definitely have corrosion on the heat exchanger these days. I get it checked each autumn, but why ask for trouble?

Thanks again and I'll post back this afternoon.
 
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Old 05-18-09, 08:42 AM
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Is it hot in your area? Could be the fan switch is just reacting to a hot ambient. Meaning the fan switch thinks the heat is from the furnce and not nature. If you have a/c run it for a few minutes.
 
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Old 05-19-09, 07:55 PM
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Howdy again folks,

Sorry for the delayed reply. I'm leaning towards replacing the furnace after looking into the heat exchanger compartments. I have the thing serviced each year with a leak test. I've never had a leak, but the service guy usually cringes at the rusty state of my exchanger, starts working on me about a replacement. There seems to more rusty flakes than is usual in there, which makes me nervous. Starting to shop around.

As for reporting the facts (just for the curious!) my control board was tucked up in there all right, but no lights. Just a 2" square chunk of pressboard with the terminals. I tried testing the terminals that Daddyjohn recomended (couldn't remember whether I want AC or DC, so did both). One yielded near zero and the other about 36 volts. After disconnecting the G wire from the control board, the fan did indeed still run.

Noticed that the fan doesn't kick on as soon as you put power to the furnace. It comes on with the call for heat and just fails to turn off. My buddy in the house says it performs the same for the AC system too. Maybe a sticky relay?

wrt MBK3's thought about the ambient temperature, I'm not so sure. I believe there is no ambient temp sensor on this furnace. The (upstairs) area where the thermostat is certainly wasn't cold. I did fiddle with the thermostat (call for heat, then reduce temp below actual; call for cooling, the raise temp above actual) and the heating/cooling shut down as it should, but not the fan.

Could only discern one fan speed, so I can't answer that one.

Thanks to everybody. I'm still interested in trying to figure it out, even if just to learn at this point.
 
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Old 05-20-09, 05:01 PM
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Sio with the house cooled down in A/C mode, the blower still does not shut off?

Do you have furnace wiring diagram pasted on say the inside of the blower door in the furance, that you can photo and paste here?

Just so you understand, since MBK is not here right now - the issue he brought up regarding ambient temperature applies if the house is too warm -NOT too cool. If a blower fan's switch is set to high or malfunctions, it may not shut off if the house and furnace is too warm. Specifically that the house is actually warmer than the shut off setting off the blower fan switch. But not all furnaces control the fan by heat. Some use a timer. This would be clear on a wiring schematic as to what you have.
 
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Old 05-21-09, 09:22 AM
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Okay.

I might be able to take a picture later today. Will post something.

Roland
 
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Old 05-22-09, 04:16 PM
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Looks like it's the Honeywell limit control switch. Burner goes on, switch temp increases until fan goes on, burner goes off, fan stays on, switch temp decreases a bit, gets stuck. I tapped on the switch housing and the temp dial jumped and the fan went off.

Thanks for the input!

I'll post a separate question on replacement.
 
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