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Old 11-10-2008, 07:42 PM   #1
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Blower fan runs constantly


I've got a Carrier downdraft htg/fan coil unit, 1990 Model No. 58DRC080-JB.

Unit was working fine. Last weekend main power to the house went on and off about 5 times in 1 hour. Next morning the blower kept running even after the heat setpoint was satisfied (and burner properly cut off). I made the blower stop by opening the access panel to trip the safety switch.

To test for Tstat damage I completely disconnected the thermostat wires - The blower still runs constantly when the panel door closes, so I think Tstat isn't the problem. Heating and cooling both function properly from the Tstat, just that the blower stays on constant; I have the access panel ajar until I want to run it.

Looking at the unit and the wiring diag inside the panel, I wonder if there's an airflow safety switch or temp safety that got shorted out.

Anyone heard of power outage causing a fault like this before?

Thanks in advance for any ideas on troubleshooting, or other advice.
Made the mistake of asking on hvac-talk forums, just for any known DIY sites for homeowners, and within mins, a handful of unfriendly posts and the thread was closed.
-Mark

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Last edited by epowerfan; 11-11-2008 at 05:37 PM.
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Old 11-10-2008, 08:58 PM   #2
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Blower fan runs constantly


Sounds like a problem with the fan relay circuit. maybe it got welded in the closed position?

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Old 11-10-2008, 09:22 PM   #3
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Blower fan runs constantly


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Sounds like a problem with the fan relay circuit. maybe it got welded in the closed position?

VERY possible. Carrier and all down flow furnaces have auxiliary limits mounted on the blower. It's a manual reset. That many voltage surges could have welded a relay like Chris said and it could also have caused the limit to trip.
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Old 11-10-2008, 10:12 PM   #4
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Blower fan runs constantly


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VERY possible. Carrier and all down flow furnaces have auxiliary limits mounted on the blower. It's a manual reset. That many voltage surges could have welded a relay like Chris said and it could also have caused the limit to trip.

Or someone left the fan mode on the tstat in the ON position again.
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Old 11-10-2008, 10:14 PM   #5
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Blower fan runs constantly


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Or someone left the fan mode on the tstat in the ON position again.
Had one today
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Old 11-10-2008, 10:20 PM   #6
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Blower fan runs constantly


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Had one today
My favorite is always the emergency switch in the "Off" position.
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Old 11-11-2008, 02:18 PM   #7
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Blower fan runs constantly


Thanks guys-- that's at least 2 possibles to investigate.

The fan control button on the programmable T500 electronic Tstat is def. not on. I don't know what an emergency switch is. Wish I had an electronic version of the wiring diag to share-- any chance I could find one on the web?

There are 2 relay/contactors in clear plastic cases inside the unit's main control box-- labeled HFR & CFR on diag. The htg fan relay says N.C. spst, the CFR just says dpdt.

Below the fan and above the burner are a gray box Ignition Lockout (flowswitch?) and a black-back thing that looks like it might be a temp sensor.

Now this is odd... last night I ran the htg and when the setpoint was satisfied, the fan went off normally on it's own. I thought it had worked itself out and I was home free. But this morning, after htg came on auto by Tstat program, the fan stayed on constant again after the setpoint was satisfied.
So I'm back to access-door ajar to keep the fan from running.
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Old 11-11-2008, 04:44 PM   #8
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Blower fan runs constantly


I have the same problem as epowerfan - fan doesn't turn off - the only way to turn off the fan is to switch the furnance master switch on the wall.

I jumped the high limit switch as well as disconnected it - no effect on the fan. Thermostat fan switch is on "AUTO".

Is it worth replacing the control board (about $40 online) or is this something that a service tech needs to diagnose?
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Old 11-11-2008, 05:33 PM   #9
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Blower fan runs constantly


x47-- high limit switch? Temp hi lim? airflow switch? and where is it? Could mine be one of those in the picture?

I watched the HFR & CFR while i closed the access panel interlock switch. As the blower started I didn't see either relay close.

What gives?? Does that mean my Htg Fan Relay is normally closed like the wiring diag says.. and a N.O. temp limit switch is in series with it, so that the blower is ON unless a temp safety switch is below setpoint temp?

-Mark
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Old 11-11-2008, 05:43 PM   #10
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Blower fan runs constantly


if you still have heat (which means hi limits are closed and not the prob) and you arent getting any lo voltage sig at the low volt term on the board (G) than id say its the board pull the low speed fan wire see if it shuts off you might also try gently smacking the relay while the fans running
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Old 11-11-2008, 06:02 PM   #11
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Blower fan runs constantly


the high limit switch is the one on the left in your photo - has 2 wires coming from it. I believe it is normally closed.

I am going to try what kennzz05 recommends - that should confirm whether or not the fan controller board is working right.
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Old 11-11-2008, 08:01 PM   #12
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Blower fan runs constantly


One more thing to check, is yout t-stat programmed to re-circ the air? this would basically turn the fan on for a set amont of time per hour.
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Old 11-11-2008, 08:42 PM   #13
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Blower fan runs constantly


results?
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Old 11-12-2008, 12:12 PM   #14
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Blower fan runs constantly


tapped on the relay - fan has been working fine for half a day. something is probably bad in the relay. I will see how long it goes without reverting to the fan being on all the time.
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Old 11-12-2008, 12:29 PM   #15
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Blower fan runs constantly


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tapped on the relay - fan has been working fine for half a day. something is probably bad in the relay. I will see how long it goes without reverting to the fan being on all the time.

the points on the relay are pitted and will stick every now and than if it weren't a sealed relay you could sand the points but its pretty much a throwaway item unless you want to try and solder a new relay in but this isn't Russia so we just toss em and replace since its not coming out of our pocket lol

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