Bryant Gas Furnace will not stay lit


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Old 11-10-09, 04:41 AM
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Bryant Gas Furnace will not stay lit

I am helping a lady in the neighborhood fix her old Bryant gas furnace.

The main burners will stay lit for 3 seconds and then go out. I've read many of the posts on this site and I figured that either the pilot switch or the main gas valve had failed, so I replaced them both.

The behavior is exactly the same with the new equipment as it was prior to the installation. So clearly, this was not the root cause of the problem.

Here is a link to the circuit diagram
http://www.xpedio.carrier.com/idc/gr...t/58ss-1si.pdf

Here is the order of operations as I've observed:
1 - Call for Heat
2 - Inducer Fan starts simultaneous to the spark Igniter
3 - Spark Igniter clicks and lights a healthy blue pilot flame
4 - About 5 to 10 seconds after the pilot lights, the main burners come on. They look good and with strong blue flame.
5 - 3 Seconds after the main burners light, they extinguish. Please note that the length of time until the burners go out is very consistent which makes me suspect that it could be commanded by the control board.
6 - This step I'll have to confirm, but I think the furnace immediately tries to re-light at step 3 above.

I've checked the voltage across terminals 1 and 2 on the gas valve and I'm getting 22VAC after the pilot heats up. When the pilot is cold, there is no voltage. The voltage stays at ~22Vac when the main burner extinguishes per step 5 above.

I've also checked the voltage across the hold coil (between 4 and 2) and I see it also is at 22VAC. However, I see the voltage move/change slightly when the the main burners cut off. I still have ~22VAC, but it changed/blinked. I have a digital multimeter and I'm thinking that the voltage may drop for a very short amount of time which may be too fast to be picked up on my meter. This drop could let the pick/hold valve close and thus the flames go out.

Here is one more piece of information that I think is significant. If I place a jumper on the pilot switch between yellow and green, the furnace seems to operate perfectly with no issues.

Can anyone help me understand whats going on? BGE home wants this lady to purchase a new furnace for ~$3,000 because they can't solve the problem. She does not have the money to replace this furnace.

Thanks for your help.

Mike
 
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Old 11-10-09, 06:38 AM
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Measuring low voltage from terminal 2 on the gas valve ought to be OK, but better would be measuring it from the furnace cabinet and best from the "c" terminal on the circuit board.

You do an excellent job of describing your tests, which are reasonable, but you don't understand some of the details of the Carrier 3 wire pilot ignition system. That's not surprising, since there are a variety of obscure twists and turns in how it works.

You get a "Very Good" though, if you are a DIYer.

The fact that the ignition system recycles and relights the pilot suggests that this isn't the more common dirty pilot type problem.

I'd look carefully to see if the pressure switch is opening, which would cause the ignition to recycle. When the thermostat is calling for heat, you should get a constant 25 VAC on the "C" terminal of the pressure switch.

When the inducer motor comes up to speed, you should see the "NO" terminal energized with 24 VAC, and that should stay energized as long as the thermostat is calling for heat. Any interrption in the 24 VAC would cause the pilot to shut off and recycle, which could be the cause of the problem you describe.

I'd check that carefully. A low voltage test light available for $3-4 from a hardware store would allow you to observe what is happening if you don't trust your meter.

Also---- test the voltage at the "W" contact on the circuit board for continuous voltage once the thermostat calls for heat. If you have a heat anticipator on the thermostat that's set improperly it can cause the furnace to cycle rapidly as you describe, which would turn the 24 VAC on and off to the "W" terminal.

Very likely the cause of the problem is that something is interrupting the 24 VAC to the hold coil (terminal 4 on the gas valve). Find that cause and you've probably solved your problem. The pressure switch or the thermostat are the most likely causes of that problem.

Unfortunately, the color of the wires to the pilot switch aren't indicated on the circuit diagram, and I can't remember which color goes to which contact on the switch.

But what you are probably doing is applying voltage to contact 5 on the gas valve (you can check that). That energizes the pick coil, but the pick coil is supposed to be shut off when the pressure switch makes, switch the 24 VAC from the pilot switch to energizing the hold coil.

The bottom line is that you probably need to identify what is interrupting voltage to terminal 4 on the gas valve.
 
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Old 11-10-09, 11:17 AM
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Dear SeattlePioneer,

Thank you for your reply.

Regarding the pilot jumper...Sorry, I forgot that there was no color on the schematic. You were correct that placing a jumper between Yellow (10B4-1) and Green (10B4-3) forces power from (10B2-6) to the MGV's PICK coil (Terminal 5) regardless of the state of the pilot switch.

I've also confirmed the operation of the pressure switch (item #7V on the schematic) and cleaned the contacts. I also bypassed this switch with a jumper and it did not fix the problem.

I haven't looked at the thermostat yet. I just pegged it at 90° so that the furnace comes on each time I push the door safety switch.

I will try the low voltage test light to see if I see a flicker in the power to Terminal 4.

My brother also suggested trying a capacitor (for test purposes only) in the terminal 4 line to prove if intermittent power is the reason for the failure.

Do you think that there is any way the the control board could be commanding the hold coil to drop?

Thanks again,
Mike
 
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Old 11-10-09, 02:26 PM
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I wouldn't use the capacitor.


Does the inducer motor stay on when the pilot/burners shut off?

Does the main fan motor that circulates air around the house ever turn on, stay on or cycle on and off while you are experiencing these symptoms?


If the inducer motor is shutting off, the thermostat is probably turning things off.

It's unlikely that the circuit board is shutting things off. It's very likely that some control connected to the circuit board is doing so.

What connections did you jumper to eliminate the pressure switch?
 
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Old 11-10-09, 03:57 PM
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The inducer motor stays on after the pilot/main burners shut down.

The main blower to circulate the house air never comes on in the current failure mode.

However, as I said before if I jumper between Yellow (10B4-1) and Green (10B4-3) which holds the PICK open, the furnace seems to operate normally and the house circulator comes on normally after 30 of 40 seconds.

Off hand I cannot remember how I jumpered around the pressure switch, but I think it was pretty straight forward.

Since I seem to have good power from the Yellow (10B4-1) wire going to the pilot switch, I'm thinking that I should jumper from it to terminal 4 on the MGV. This should force steady power to the Hold coil which I suspect has been dropping due to some inexplicable power fluctuation.

If this works, I think it will prove the intermittent power to the Hold valve theory.
 
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Old 11-10-09, 04:55 PM
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Does the pilot flame extinguish with the main burners flame? It sure seems to me like a flame rectification problem with that same exact 3 second timing. If no other flame sensor involved except the 3-wire system, maybe it is dirty, or the pilot flame is pulled off course by the main burners flame, or loose flame rectification connection. ???
 
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Old 11-10-09, 06:44 PM
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Yes, the pilot goes out at the same time that the main burners shut off.

I've done two tests to eliminate the pilot safety switch as a source of the problem.

First, I replaced the old switch with a brand new one and the problem did not change. I also confirmed the operation of the new switch using a multimeter and matches prior to installation.

Second, I substituted a 3-way wall switch in place of the pilot switch. Switch position one connected the yellow wire (10B4-1) with the PICK coil (Terminal 5) of the MGV. Switch position two connected the yellow wire (10B4-1) to the white wire (Terminal 1) on the MGV.

With the switch on position one and a command for heat, the sparker immediately lit the pilot flame. The pilot flame remained lit until I flipped the switch to position two, upon which the main burner immediately ignited. After 3 seconds, the main burner extinguished.

So I’m pretty sure that the problem in not in the pilot safety switch.
 
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Old 11-10-09, 07:39 PM
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Unfortunately, you don't have the right test equipment to troubleshoot your equipment and wind up using a lot of second rate compromises because of it.

You should really call a good repairman to do the testing and repairs that need to be done. The reason for the burners to shut off is now pretty obvious, but it requires still requires significant test equipment and skills you don't have to do the job properly.

I'm concerned that the compromises and guesses you make could cause safety problems, so I'm checking out at this point.
 
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Old 11-10-09, 09:01 PM
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Dear SeattlePioneer,

I appreciate your help. Thanks again for your advice.

Best regards,
Mike
 
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Old 11-11-09, 05:45 PM
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I looked at the schematic and also the sequence of operation. Your pilot should be lit and rectified for about 60-70 seconds before the main gas valve comes on. Is that happening? (By 'is', I mean is the pilot staying lit that long, first.)

If so, it seems odd after all that time that coincidently it stop recognizing that rectification, right when the main burners came on. Unless - the burner flame is pulling the pilot flame slightly away from where it senses it, perhaps.

Other than this theory, nothing more makes sense to me at this time, without further study and contemplation. I read nothing else in the sequence, that at this late stage in the operation, that could affect the flame going out, other than what I just said.
 
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Old 11-12-09, 04:59 AM
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After the call for heat, the pilot flame ignites within a few seconds. It seems to take another 10 to 20 seconds for the pilot switch to change states and the main burners ignite. It defiantly does not take 60 to 70 seconds.

Then, like clockwork, the main burner shuts down after 3 seconds.
 
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Old 11-12-09, 07:43 AM
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Take the wire off terminal 4 of the gas valve then check for continuity across terminal 4 to terminal 3 --- the hold coil.

A burned out hold coil can cause these symptoms.
 
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Old 11-12-09, 07:49 AM
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Take the wire off terminal 4 of the gas valve then check for continuity across terminal 4 to terminal 3 --- the hold coil.

A burned out hold coil can cause these symptoms.
 
 

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