Furnace doesn't start

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Old 12-10-18, 11:33 AM
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Furnace doesn't start

I've read several threads here with similar titles, but did not find one matching. (I'm sure it's here somewhere!) I am at work, so don't have all the details of the unit with me, but really might only need a more detailed sequence of events as they should happen to help me diagnose my unit.

Central unit is 35+ years old and on its last legs according to servicemen, who have all recommended replacing. Have DIY'd it to keep it alive for last 4 years. I believe it is a Carrier, or possibly a brand that Carrier bought out. "Generic" Carrier-type control board, but LED is not programmed to display any error codes. It is apparently just for looks.

No heat is the problem. Opened it up to see what could be seen. When thermostat asks for heat, it clicks the ignitor, there is a blue flame for a second or so, then pilot flame goes out, clicks and relights, repeat, repeat, repeat. Never gives up and was probably clicking away for a few weeks before I opened it up last night (house has 2 other units, just this side of the house is chilly). So the ignitor is working in that it gets a pilot flame. Gas is there as the pilot flame burns for a second or so (sometimes a whole 2 seconds).

I've tried looking for the flame sensor, but don't see anything that looks like the images I've seen of one online. Not sure if that would even be the problem.

Question for the experts: What is the normal sequence of events for a working system? 1) Thermostat sends heat signal; 2) ignitor/auto spark lights pilot light; 3) gas turned on to burners; 4) pilot light lights gas = heat. What goes in between 2 and 3 that gets the gas turned on to the burners? Seems like it must be a sensor of some sort?
 
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Old 12-10-18, 11:19 PM
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Welcome to the forums.

With that type of system there is no "flame sensor" per se. The flame is detected by the pilot being lit. If the flame isn't large enough.... it will go out and relight. Some of those systems use the spark electrode as a type of flame sensor. If you look up the part number on your generic control box..... you should be able to find a manual on it.

With a 35 year old unit...... you may have to replace the pilot and igniter assembly if you plan on keeping that running. Also...... with a unit of that age..... the heat exchanger needs to be checked for integrity. A crack will allow CO to fill the house.
 
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Old 12-11-18, 08:16 AM
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This is the control board:

ICM275 | Blower Control | Delay Timer | ICM Controls

The unit has been serviced semi-annually, so heat exchanger is okay, and this particular one is in an exterior mechanical room so less of a danger of CO, although we do have combo smoke/CO detectors throughout home.

The flame looks to be a decent size when it lights. Since there is no flame sensor, can I manually light it to get this side of the house warmed up, at least temporarily? Since I don't smell that much gas, it just doesn't seem like the gas for the burners is being turned on. There's just the little whiff you can smell associated with the pilot light's gas. If there isn't a flame sensor, how does it know to turn on the burner gas?
 
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Old 12-11-18, 09:24 AM
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Thermocouple? Is that the step between pilot flame and turning on gas that isn't working?
 
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Old 12-11-18, 09:38 AM
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That is the fan control module..... not the ignition system.
 
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Old 12-11-18, 12:41 PM
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Below is an example of a thermocouple.

https://www.supplyhouse.com/Honeywel...2dfafdc29a6a90
 
 

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