Bryant Thermostat Troubleshooting


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Old 01-01-16, 02:52 PM
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Bryant Thermostat Troubleshooting

Hello Forum,

I'd like to get some information on my circa 1988 Bryant thermostat. It's a basic model. From the label under the cover:

Bryant (BDP Company)

Type: 271-3495

8803 E6 2

Heating: 0.15 to 1.2 Amps
Cooling: 1.5 Amps Max

Max. 30 Volts AC

I'm experiencing problems with my gas furnace. The blower motor runs continuously with both thermostats off, Fans set to AUTO and temperature control set to low.

It was working fine until I set the fans to ON to circulate the system when the gas furnace wasn't running. Then I moved the temperature control to high, but the furnace didn't heat up. Then I tried to shut the entire system down using the thermostat controls and the furnace blower motor continues to run and the system is cold.

I'm looking for any assistance to troubleshoot this system.

It's a gas upflow furnace installed in 1988. The AC cooling has been removed from the system so it only a forced air gas heat system at the moment.

Regards
 
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Old 01-01-16, 02:57 PM
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Welcome to the forums.

I don't recognize that as a Bryant part number.

I doubt the stat is at fault.

If the furnace has a draft inducer blower...... is it running too ?
 
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Old 01-01-16, 03:47 PM
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No, this furnace only has one blower with two speed settings.

At this point in time, I think it's a thermostat issue or a control board issue. On the control board, I suspect the HFR relay. The relay part number is KU-5399-3.

I want to try to isolate the thermostats from the system to determine if it's a thermostat problem or a control board problem.

Could the gas valve cause the fan to run continuously? One of the numbers on the gas valve is V800 A1070.

Regards

The control board on the link below is a direct replacement.

http://www.americanhvacparts.com/p-1...ier-payne.aspx

The link below appears to have a replacement relay.

https://acpartsdistributors.com/inde...ducts_id=16967
 

Last edited by tjg79; 01-01-16 at 04:20 PM.
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Old 01-01-16, 04:16 PM
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Attached are pics of the thermostat.

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Old 01-01-16, 04:42 PM
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That's a Honeywell stat re-branded as a Bryant.

Disconnect the wire from the R terminal on the control board..... that will isolate the stat.
 
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Old 01-01-16, 04:53 PM
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I've completely removed the thermostat from the control board and the fan still runs. Does that indicate the board is the problem? I would think the system wouldn't run anything with the thermostats completely disconnected.
 
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Old 01-01-16, 04:57 PM
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It could be the limit switch circuit. What are the correct readings for testing this circuit?

Is the circuit open or closed when hot?
 
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Old 01-01-16, 05:10 PM
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It sounds like a limit trip.

If it was a stuck fan signal you would still be able to get heat.

The limits are normally closed.

Post a picture of the furnace nomenclature (tag). It should be located in the burner section of the furnace.

I picture of the wiring diagram might also help.


I suspect that it looks very much like this...

 
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Old 01-01-16, 05:39 PM
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The data plate info reads:

Bryant

Model: 394GAD036975
Series: 5087A04544
Input: 75,000 BTU/Hr
Natural Gas

3C6234-307

The board is getting 120VAC and 30VAC
 
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Old 01-01-16, 05:53 PM
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Do you have a voltmeter ? Can you check from R to C for 24vac ?

On your furnace....an open limit will keep the blower running. Your blower timer board is "power required to turn blower off".
 
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Old 01-01-16, 05:59 PM
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Yes, I've got a couple multimeters.

When I checked the limit switch earlier, the blower was running, the system was cold, and there was continuity at the limit switch.

I'll reconnect the control board and check the voltage between R and C.

What should the thermostat be set to to check the voltage between R and C? Does the thermostat influence the voltage?
 
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Old 01-01-16, 06:01 PM
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You don't need the stat connected. Just power up the board.
 
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Old 01-01-16, 06:02 PM
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Are you measuring at the control board Lim1 and Lim2 terminals or one of the safeties?

Looks like you have a manual reset button on the vent pipe spill switch.
 
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Old 01-01-16, 06:46 PM
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R to C voltage measures 1.2VAC with blower running and no thermostat connected.
 
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Old 01-01-16, 06:56 PM
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Ok.... in the diagram below that was taken from the diagram you posted...... you'll see three limit switches in series. I used pink to show the loop. One of them is open keeping the 24vac from getting to the board. You'll have to trace the loop from LIM 1 terminal to LIM 2 terminal.

Houston mentioned a limit with a reset button. Look for that one. It should be above the burner or near the exhaust flue.

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Old 01-01-16, 06:56 PM
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Are you measuring at the control board Lim1 and Lim2 terminals or one of the safeties?

Looks like you have a manual reset button on the vent pipe spill switch.
I was measuring the limit switch circuit across the thermocouples.

I don't see a manual reset button on this system as installed.
 
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Old 01-01-16, 06:59 PM
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Could I just short the connectors across the circuit components to identify the open device?
 
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Old 01-01-16, 07:02 PM
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VSSS is on your diagram.

If a bird or wasps builds a nest in the vent pipe this safety would detect the restriction and lock out heating.

You should be able to trace the wiring across the LIM circuit to see it.
You also have a fuse link. No reset on that.

Place one meter lead on LIM 2 and place the other lead on LIM 1. Move the meter lead from LIM 1 to the next point and the next on that circuit until you get 24 volts.


Bypassing these safeties for anything other than testing can be fatal.
 
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Old 01-01-16, 07:04 PM
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You can check any way you want. I highlighted the diagram. It's a simple loop. It's open. You need to find out what is open. I can't do that for you.

Put your meter on R and C. Short across one limit at a time.... when you find it.... the meter will read 24vac.
 
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Old 01-01-16, 07:17 PM
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The AC voltage between the R and terminal L1 on the board is about 18VAC.

The AC voltage between R and before the device pictured below is about 18VAC

The AC voltage between R and after the device pictured below is 0VAC.

I'm not sure what that device is. I've posted two pictures, one connected in circuit and one disconnected for better image.

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Old 01-01-16, 07:20 PM
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The first picture has a limit and a fuse link.

The second is the fuse link.

http://www.americanhvacparts.com/p-1...t-carrier.aspx

Is the old evaporator coil still sitting on top of the furnace?
Is it dirty?
 
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Old 01-01-16, 07:22 PM
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That is shown in the pink loop as FL. It is a fusible link.... a heat activated fuse. Once open... it's defective. If yours is open..... you have a problem with flame rollout or high heat in that area.

You can check that with an ohmmeter. It should show continuity.... 0 ohms..... shorted.
 
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Old 01-01-16, 07:24 PM
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You can see the two terminals for the limit switch circuit on the control board pic below. The limit switch circuit are the red and brown wires in the upper right quadrant of the board to the left of relay and in line with the bottom edge of the relay to the right.

From visual inspection, there appears to be three devices in this circuit. The first is depicted in my previous post and the other two are thermocouples. The voltage in this circuit goes to zero after the first device pictured previously.
 
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Old 01-01-16, 07:30 PM
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The first picture has a limit and a fuse link.

The second is the fuse link.

Is the old evaporator coil still sitting on top of the furnace?
The old evaporator coil is not installed on the system. It's completely removed. It was done several years ago.
 
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Old 01-01-16, 07:35 PM
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That is shown in the pink loop as FL. It is a fusible link.... a heat activated fuse. Once open... it's defective. If yours is open..... you have a problem with flame rollout or high heat in that area.

You can check that with an ohmmeter. It should show continuity.... 0 ohms..... shorted.
My FL is open. If I bypass it by connecting the bare wire before the FL to the limit switch thermocouple, the blower motor shuts down.

I don't know what could have caused it to blow.

I could just replace it and see what happens.

It may be prudent to replace both thermocouples in the circuit as well. Then if it blows again, the problem is likely on the control board.
 
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Old 01-01-16, 07:41 PM
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$39.00 for a fusible link kit. You must be kidding me.
 
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Old 01-01-16, 07:45 PM
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I'll check on a FL at my local Aireco Supply. I'll see what they cost there.


$18.00
https://arnoldservice.com/product/br...it-307566-751/

$5.50, shipping included on eBay.
http://www.ebay.com/itm/Thermal-Cuto...kAAOSweW5U9kF3

I need to get the specs for my FL.
 
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Old 01-01-16, 07:54 PM
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Can you read the numbers on that FL. Should be like 102c.
 
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Old 01-01-16, 07:56 PM
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I could have caused this by initiating heat with the blower motor running. Normally, when the furnace starts, the heat exchanger is started, then the blower starts. But, by initiating a heat cycle with the blower running, it could have cause a heat event that blew the fusible link.
 
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Old 01-01-16, 08:14 PM
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it could have cause a heat event that blew the fusible link.
Not likely. You'll need to monitor the furnace during operation for a flame roll out.
 
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Old 01-01-16, 08:25 PM
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Thank you for all the help gentlemen. You save me a lot of time troubleshooting this gas furnace issue.

I realize that I didn't check the limit switch circuit the proper way and I missed the FL issue. It caused me to focus on the control board.

Happy New Year to you all.
 
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Old 01-04-16, 10:28 AM
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FYI

I called a local Bryant HVAC dealer and they identified the Bryant part number for my model gas furnace as 307566-751. This is a 144 degree Celsius/287 degree Fahrenheit fusible link or thermal fuse. For up-flow gas furnaces only.

Generally, these are sold as a "kit" with the connectors attached. The best price I found with Google, Amazon, eBay was about $17.00 for one each plus anywhere from $5.00 to $10.00 for shipping.

I did locate several sources for the thermal fuse rated @144C/291F and bought 10 for $7.99 plus $4.50 shipping.

Incidentally, the temperature rating of the gas furnace at the outlet is 200F max.

Thermal Fuse 144 Degrees Celsius Thermal Fuses - Thermal Fuses - By Univen - 744539034043 at Goodman's Small Appliances Housewares and Parts
 

Last edited by tjg79; 01-04-16 at 12:55 PM.
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Old 01-04-16, 10:30 AM
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Just keep in mind... those are thermal fuses.
You cannot solder to the leads and they should not be cut either.

A crimp connection is recommended.
 
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Old 01-04-16, 12:08 PM
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Thanks for the info.

The original appears to have been soldered. I've got a Hakko solder station where I can control the temperature. I think the connectors can be soldered, but it's got to be done with good temperature control, good solder practice, and heat sinks to protect the device.

Because these devices are temperature sensitive and I intend to solder the connections, I bought 10 each, because things don't always happen correctly the first time. Also, I don't know what caused the thermal link to blow in the first place. If it happens again, I'll have to do a more thorough investigation to determine the cause.

Regards
 
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Old 01-04-16, 05:01 PM
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Correction - The original is installed in the limit switch circuit with crimps. So, I won't attempt to solder in the replacement.
 
 

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