Furnace will not stay lit


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Old 11-18-13, 04:52 PM
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Furnace will not stay lit

I am having a problem with a Modine Hot Dawg unit heater. It was just converted from LP to natural gas. The orifices were changed and the gas valve spring was changed and the gas pressure checked to be at required pressure. Now the burner ignites for a few seconds and then goes out, but the exhaust fan does continue to run. I have removed the flame sensor rod and although it was clean I scrubbed it with scotchbrite. The gas valve is a smart valve and it is blinking a green light which I understand means it is functioning correctly. Any ideas?
Thanks,
Boyd
 
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Old 11-18-13, 05:18 PM
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Was the gas piping to the heater changed when the conversion was done? If not, the piping may be too small causing starvation.
 
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Old 11-19-13, 11:39 AM
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No the piping was not changed. It is 3/4" and a run of only 20 feet from the meter. The heater is 45K btu, so the table says it is adequate. The flame comes on full blast for 3-4 seconds and then immediately shuts off with no diminishing of the flame. It appears that the gas valve is closing, presumably due to a sensor. I was reading an old post on this forum and someone suggested removing the sensor from its mounting and heating it with a propane torch while still in the control circuit. I have not tried this, but have to wonder would the torch not have to be grounded for this to be a meaningful test? The flame sensor looks like new with no cracks in the insulator and no residue on the rod. It seems unlikely that this is the culprit. Thank you for your response.
 
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Old 11-19-13, 12:52 PM
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Any chance of a wiring change where polarity was reversed or ground lost?
 
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Old 11-19-13, 07:41 PM
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No wiring changes occurred and there is a continuous path to ground.
 
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Old 11-19-13, 08:13 PM
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Please get the number off the gas valve & post it here. I will try to find a troubleshooting procedure. The number should begin with SV, followed by four numbers, (maybe a letter), a dash, & four more numbers.

See if this helps. https://customer.honeywell.com/resou...0s/69-2014.pdf
 
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Old 11-20-13, 04:05 PM
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The valve is a SV9501M2528. I did a few more tests today. Verified that ground is good, polarity is correct. Before ignition the flame sensor has 2 micro amps, after ignition is shows 3-4, but after a few seconds the valve closes and then then sensor amperage drops back to 2. Both before and after the flame extinguishes, the valve has a pulsing dim and bright light which is normal according to the book. Removing the thermostat from the system has no effect. The problem still exists. The system tried to ignite 4 times before locking out. Thanks again for your assistance.
Boyd
 
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Old 11-20-13, 05:05 PM
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Here's more stuff I found: https://customer.honeywell.com/resou...0s/69-1180.pdf
https://customer.honeywell.com/resou...0s/69-1270.pdf

A couple of things jumped out at me. The first is I guess you have to have a jumper rig to test flame current since they say you can only test it in the pilot mode.

Secondly, you said the gas pressure had been checked & set. If the heater hasn't run, the pressure could not have been set properly. In order to set the gas manifold pressure, the appliance must be running.
 
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Old 11-22-13, 07:15 PM
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I am not sure why, but some of my replies are not making it through. The gas valve pressure was checked at the outlet when the gas valve opened and during the few seconds that it stayed lit. This unit does not have a pilot. There is a heating coil that ignites the gas from one orifice and the flame spreads to the rest. Both the exhaust and circulation motors are operational. I do not have the harness to test the flame sensor circuit. Is it possible that a flame sensor that is not physically damaged and is clean could be the source of the problem? Or is it more likely something on the board that is not getting the signal from the sensor?
 
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Old 11-22-13, 08:52 PM
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With electronics anything is possible.
The heating coil lights a pilot which in turn lights the main burners. The coil doesn't light the main burners directly.
If I remember correctly, the pilot stays on the whole time the main burners are lit. If that is true, maybe once the main burner fires the pilot is not strong enough to stay on the sensor. There is a pilot adjustment on the valve. The last document to which I linked shows the location & explains how to adjust the pilot. Was the pilot orifice changed when the conversion to natural was made? (I'm not sure if the SmartValve uses different pilot orifices for LP & natural or not.)
 
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Old 11-23-13, 04:25 PM
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Here is what the Modine advertising says:
Modine Advanced Burner Design
Modine HD's burner design is more akin to that of a fire-tube boiler than a unit heater. With these in-shot burners, the flame is retained inside the heat exchanger and fires down the inside of the heat exchanger tube. This design accomplishes three things: First, it eliminates the need for separate conventional ribbon-style burners; Second, it enhances flame quality and heat transfer; and Third, it eliminates most burner corrosion problems. Conventional burners fail when moisture from condensation drips on them. This problem is eliminated with the Modine in-shot burner system.

The igniter and the flame sensor are not integrated. The igniter is on one orifice and the flame sensor is on another.

Thanks for bearing with me. I appreciate your assistance.
 
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Old 11-23-13, 04:59 PM
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I'm not disputing what Modine says but everything I find on the SmartValves shows a pilot burner/flame rod combination. Have you verified the system is as described in Modine's literature?
I suspect what you are reading is for the current models. Many manufacturers have gone away from the SmartValve system. Service people hate them.
 
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Old 11-24-13, 10:19 AM
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I just went and looked again. There is no pilot or separate pilot line on the gas valve. You can hear the click as the valve opens and all three orifices light up. The igniter is on the top orifice and the flame sensor is on the middle one. I would imagine that if there were a pilot that the igniter and the sensor might be a combination unit, but at least would have to be in the same location to get a trial for ignition off of a pilot.
 
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Old 11-24-13, 10:30 AM
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This is really odd. Everything I've read & seen on the SmartValves shows a pilot. Yours seems to be set up more as a conventional hot surface gas valve. Is there a separate ignition module or does the valve supply power to the ignitor? Where does the wire from the flame rod connect?
 
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Old 11-25-13, 02:32 PM
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There is a separate ignition control board. The igniter and the flame sensor both connect to this board.
 
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Old 11-25-13, 02:54 PM
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Is there any led lighting up from the board you will be looking for either red or green led blinking
 
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Old 11-25-13, 04:00 PM
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I'm completely baffled. I'm no SmartValve expert by any means but that system having a separate ignition control board is completely foreign to the whole concept of a smartvalve as I understand them. If I have asked this previously, forgive me but did the heater work properly on LP?
 
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Old 11-26-13, 09:14 AM
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The heater worked fine on LP. The contractor that did that did the conversion says he tested it and it worked fine after conversion. It did not work the next day.
 
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Old 11-26-13, 09:49 AM
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There is no led on the control board.
There is a green led on the gas valve that pulsates bright and dim.
 
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Old 11-26-13, 07:55 PM
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The only thing I can suggest is to make sure the flame rod is sticking into the flame far enough. I know, the next question is "what is far enough". That's a good question for which I don't have an answer other than into the heart of the flame. There may be something in the installation manual for the furnace.
 
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Old 12-06-13, 01:03 PM
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Since my last post, I have replaced the flame sensor with no joy. I have grounded the flame sensor and moved it around in the flame with no joy. I even tried grounding a propane torch and holding it on the flame rod with no joy. The unit fires up for a few seconds and then goes out. It retries 4 times and locks out. Since I do not understand the relationship between the gas valve and the ignition control board I do not know which might be the problem. I assume that the pressure switch would prevent ignition if it were bad. Any ideas?
Thanks!
 
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Old 12-06-13, 04:31 PM
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If the pressure switch were to fail to close, yes it would fail to ignite. Since you mentioned the pressure switch, that's something else to check. Presuming there being two wires attached to said switch, you should have voltage across the switch to start then the voltage should go & stay away indicating a closed switch. If that voltage reappears upon ignition, it would indicate a loss of draft.

Sooner or later we'll catch that straw blowing in the wind.
 
 

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