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Old 12-22-2018, 03:57 PM   #1
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Circulating fan stops unexpectedly during heat cycle


I have a Tempstar model NUG9100FHA1. During a regular heating cycle with the Fan set to Auto, everything works perfectly for about 7 minutes, and then the air circulating fan will suddenly stop for no apparent reason. During this time the burners and draft inducer motor are still running, causing the heat exchanger to overheat. Once the temperature exceeds 160F, everything shuts off for about a minute and then the whole cycle starts all over again since the thermostat is still calling for heat.

So far, I've changed the circulating fan motor and capacitor, furnace control board and the 24V transformer. None of these new parts have had any affect on this problem.

As a temporary fix, I am able to keep the fan from stopping by switching from Auto to On at the thermostat. The heat cycle works fine like this, but now the fan is running all the time.

Also, when I changed the IFC, I replaced the original Honeywell ST9120G4012 with the Universal ST9120U1011. The diagnostic light on the new IFC never indicates that there is a problem.
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Old 12-22-2018, 04:41 PM   #2
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Re: Circulating fan stops unexpectedly during heat cycle


check the heating speed terminal and neutral for line voltage, seeing if it drops out when the blower shuts off. you can pull off the blower motor leads by 1mm so you can get in there with your meter leads.

you'll have to tape or clamp the blower safety switch when checking.

If it drops out , you need a new board. otherwise, maybe a motor issue which only shows up when the heating speed tap is in use.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 12-22-2018 at 04:49 PM.
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Old 12-22-2018, 05:03 PM   #3
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Re: Circulating fan stops unexpectedly during heat cycle


Before purchasing the new motor, I did try connecting the different speed taps to the Heat terminal on the board. It made no difference. Also I checked the resistance of the 4 speeds on both the new and old motors. They all had resistances between 2 and 20 ohms so I'm fairly sure both motors are good.

Rather than try to measure the live voltages when the motor stops, I just bought a new board, and it produced the same problem. I find it hard to believe that the original board and a brand new board could have the exact same problem, but it is starting to look like there's no other possibility.

Tomorrow I will try to measure the voltage when it craps out and let you know what I find...
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Old 12-22-2018, 05:12 PM   #4
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Re: Circulating fan stops unexpectedly during heat cycle


Can you post a clear schematic?

Should be on the blower access panel, part facing in.
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Old 12-22-2018, 06:17 PM   #5
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Re: Circulating fan stops unexpectedly during heat cycle


This was on the back of the main cover. There was no schematic on the back of the blower access cover.
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Old 12-22-2018, 06:21 PM   #6
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Re: Circulating fan stops unexpectedly during heat cycle


a smart valve system.

can you give me the model number off the valve so i can see which terminal does what?

I suspect the board gets a signal indicating the flame is on to run the blower timer circuit and it's something external causing this issue.

Edit: yes, pdfs some if not all of the smart valves communicate with the control board to govern heating fan operation. If this communication is messed up, for example the valve is dropping signal after a few minutes, it may cause this problem.

I'm unsure if it's a straight 24v signal or it does something more complex.
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Last edited by user_12345a; 12-22-2018 at 06:43 PM.
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Old 12-22-2018, 06:55 PM   #7
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Re: Circulating fan stops unexpectedly during heat cycle


" In those systems using an EFT the Smart Valve sends a 16 VAC signal to the EFT to start the time delay which dictates the amount of time that must pass prior to energizing the air handling blower." https://www.hvactrainingsolutions.ne...-valve-part-1/

-maybe this is dropping out mid cycle and messing with the fan control circuit?
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Old 12-22-2018, 09:19 PM   #8
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Re: Circulating fan stops unexpectedly during heat cycle


The SmartValve is a Honeywell SV9500M-2674, Part #HQ1009089HW.

It seems to be working fine, but it is the only thing that I haven't checked yet. I will follow the links you provided. Your theory sounds very plausible. I never would have suspected the SmartValve, but now that you mention it, it seems to make complete sense.
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Old 12-22-2018, 09:36 PM   #9
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Re: Circulating fan stops unexpectedly during heat cycle


See the attached diagram.

Find and very carefully stick your meter leads into the appropriate parts of the back of the molex connector (with it still plugged in) to check "ETF output" and "24V common"



Set your meter to AC volts, put the furnace through a heating cycle and see if you get voltage there when the main burners ignite.

(if you don't, try with one lead in contact with ETF output and the other in contact with the chassis)

If you do and the voltage drops out or becomes erratic just as the blower shuts off mid cycle, it's a pretty good indication that you have a bad smart valve.

The ignition control is integrated into the top of the valve so the entire thing must be replaced if the control has failed. It's not a diy job.

Quote:
I never would have suspected the SmartValve, but now that you mention it, it seems to make complete sense.
I'm unsure of if the ETF output from the valve only gets the board to start the fan timer or if it can have an impact during the cycle.

But you've already ruled out the board and motor.

Electronics can fail in odd ways and just from this board I know the smart valve system is notoriously unreliable.
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Last edited by user_12345a; 12-22-2018 at 09:42 PM.
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Old 12-23-2018, 04:51 PM   #10
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Re: Circulating fan stops unexpectedly during heat cycle


user_12345a,

You are a genius! You correctly identified the source of the problem without even seeing my furnace. I'm certain that most HVAC guys would just do what I did -- change a bunch of parts and then say I need a new furnace.

In the end, I could not get my multimeter probes to check the voltages at the SmartValve. Honeywell appears to have deliberately used an extra long connector so that it cannot be backprobed without specialized equipment. According to the SmartValve manual, they sell a special wiring harness for taking voltage measurements.

Even though I couldn't get the voltage measurements, I realized that the connector wiggled excessively. After you explained that this SmartValve is notoriously prone to failure, I began reading more about it. I remember reading somewhere that when the connectors are loose, you should use cable ties around the whole valve to secure the connector from moving. I did this, and now it has been working perfectly for over 1 hour.

I'm not sure how long this fix will last, but at least now I'm certain that all I'll need is a new valve.

I cannot thank you enough. I've struggled with this problem for weeks, and you solved it in a few hours.

Thank you so much for all your help.

Happy Holidays!
Fred
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Old 12-23-2018, 05:01 PM   #11
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Re: Circulating fan stops unexpectedly during heat cycle


You're welcome.

I wonder if you can probe the connections at the circuit board instead if you can't get them into the valve's molex connector.

Hopefully it'll hold up because replacement could be pricey, figure 100% markup of online prices plus at least an hour of labour plus a service call.
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Last edited by user_12345a; 12-23-2018 at 05:04 PM.
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Old 12-24-2018, 12:36 PM   #12
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Re: Circulating fan stops unexpectedly during heat cycle


That's a good suggestion, to probe the board instead. If it stalls again, I'll give it a try. Right now it is running like new, and doesn't seem to have a problem in the world.

Unfortunately, you are right, the SmartValve is pricey, and on a 24 year old furnace it is cost prohibitive, so it looks like I'll have to get a new furnace anyway at some point. At least now, I can wait until Spring when the gas companies offer big incentives to upgrade old equipment. For now, I'm looking forward to a nice, warm Christmas at home with my family.

Thanks again for all your help!
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Old 12-24-2018, 12:40 PM   #13
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Re: Circulating fan stops unexpectedly during heat cycle


yah, the heat exchanger could be cracked by now. didn't realize it was that age.

there are some rebates available in ontario - https://www.enbridgegas.com/Using-Natural-Gas/Rebates

https://saveonenergy.ca/en/For-Your-...ng-and-Cooling
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Last edited by user_12345a; 12-24-2018 at 12:45 PM.
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