Gas mail burners won't fire

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  #1  
Old 02-06-13, 10:35 AM
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Gas mail burners won't fire

Hi, I have an older Duomatic Olsen HCS 90/80 s#01521HCDN Honeywell S860C intermittent pilot ignition. 100% shutoff, 30 sec purge.
There is also an air switch sencing the induced exhaust thru 2" dia pvc to exterior.
The unit was cycling ( shutting down and then relighting quite often.) This has been a problem for years but continued to provide heat. I have in past years adjusted the air switch but was never able to correct the prolem. Now the main burners wont fire. I have cleaned the pilot orfice and checked connections.
(Previously) When calling for heat, the pilot lights, after a 30 sec purge, (seems weak) but cotinues and the main gas valve wont open. I verified 24V on the PV terminal yesterday but
(Today) While checking there was 0v and no pilot. I found a small fuse open on the ignition box and replaced it.
Now i measure only .6 V after purge and the pilot wont light. I assume the ignition box is the problem. I verified 24V through the air switch. I think I may still have a problem with the main burner as I had 24 v previously, a working pilot but no main fire.
How is the pilot flame verified at the control? There is no wire connected to the sensor. Shall I again clean the pilot orifice and what I think is the sensor?Before the pilot flame didnt look as high as it should have. It probably should be surrounding the sensor to sence properly.
Any suggestions, warnings will be greatly appreciated. It is cold!
Thanks in advance.
 
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  #2  
Old 02-06-13, 01:50 PM
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Fist verify 24VAC between terminals GND and 24V TH-W and if your control has it 24V and GND. Likely your spark rod acts as the flame sensor. Normal yearly maintenance is to remove the pilot burner and orifice and clean them out and clean the spark rod with #000 steel wool. Once you get a good pilot check for 24VAC between MV/PV and MV terminals. If you have 24V there and no main burner ignition then you likely have a failed Gas valve.

How old is your furnace exactly? Is it worth putting time and money into Vs. replacing is a question you should ask yourself.
 
  #3  
Old 02-07-13, 03:00 PM
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Update today

GAS MAIN BURNER WONT LIGHT
What I did so far:
Replaced the intermittant pilot control.
Removed and thourally cleaned the spark rod, pilot orifice and clened the green ground wire and connected it directly to the pilot orifice bracket.
I cycled the furnace and after purge the pilot lit the first time but the main did not .There was a good strong pilot flame this first time. Upon further tries the pilot will not light? I have verified gas pressure under the main valve.
The main valve coil and rod bench test with 24V. It snaps open.
The pilot side of the gas valve however does not. There is no movement when 24V is connected to TH-TR and ground to TR.
Also after the purge, the ignitor continues to spark and 24 v readings to ground on all three gas valve terminals are bouncing, fluctuating rapidly from 13 to 18v as good as I can read them.

According to my wiring diagram, all connections to the gas valve come from the ignition control as follows
PV ------------ TH / TR
MV/PV --------- TR
MV --------------TH
The main coil wires connect to TH ------- TH / TR
The controller has
---Ground connection to burner
---Ground
---25 V ground
---25V
The thermostat circut is series through the air switch, high temp safty switch and a 1 A fuse before connection to a relay and the pilot controller.

I think I need to replace the gas valve as the above check of the pilot is proving falty. It is a Honeywell VR8440M2008

RE replacement, I plan to replace and install a boiler, hot water system in the near future as part of my house is already being heated with two zones of a radient floor pex loop. I am using a simple water heater now for this system. I would like to nurse this air furnace along just one more year.
Thanks for your suggestions. Please advise if yo think I approching this correctly.
 
  #4  
Old 02-07-13, 03:28 PM
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DID you check voltage at the valve with a call for heat? The main valve CAN NOT open unless the pilot valve has voltage.
 
  #5  
Old 02-07-13, 04:09 PM
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Yes I did. The voltage on the valve while is sparking but not lighting is fluctuating 12--18 V. This is on all three valve terminals while calling for heat. I think I should check all the wire from the stat as I could have something wrong.May be causing the flucuating. I originally burnt the transformer and replaced it with a new one. Maybe I have something incorect. My diagram lacks for thermostat connections. I am only using heat.
 
  #6  
Old 02-07-13, 05:26 PM
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remove the wires from the GV and then check voltages.
 
  #7  
Old 02-13-13, 10:59 AM
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I checked ove the furnace wireing and did have voltage at the gas valve. I removed and installed a known exact replacement valve and now the furnace is working good. It lights very quick and fires the main valve. One small concern is that it will cut the voltage after running awhile and a short time later refire. In checking voltages I discovered the flame rollout sensor and switch is opening and then closeing. Maybe the sensor tube is geting too hot or maybe it needs replacing.
Thank you hvastecfw for your replys.
 
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