Lennox G23 problem

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Old 10-01-04, 11:29 PM
mcg731
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Question Lennox G23 problem

Hi -

First, I live in SE Michigan, and have owned this house since 1996. A year or two before I bought it, the previous owner replaced his old furnace with a Lennox G23Q3/4-100 w/Central Air. For the most part, the furnace has worked fine. About 2-3 years ago, I did have a no-heat situation, at which time I had the Johnson ignition control module replaced with the Honeywell Ignition Control Replacement Kit (53L90). Since then, I have had no problems. Until yesterday, that is.

Two days ago, we used the furnace for the first time this season. It appeared to work Ok, it produced heat. Yesterday, we woke up to a 65 degree house, and took a look at the furnace. I did some simple stuff, vacuumed the filter screen I have in front of the Trion AirBear filter media, vacuumed the area around the burners, (the furnace is in a work room, and can get a bit dusty). Then, I tried to run the furnace. here is what I found:
1. Ensured the furnace switch was on.
2. Switched the thermostat from off to heat (Honeywell round unit with Cool/Heat setting).
3. Set the heat to above the room temp on the thermostat.
4. Induced Draft Fan started.
5. Induced Draft Fan stayed on (for minutes) until I turned the thermostat down, or turned the furnace switch off.
6. I noticed no other sounds to indicate anything was happening, besides the fan.
What can I look at to diagnose the problem? One thing to note, when I change the Fan switch on the thermostat from Auto to Fan On, the blower fan does go on, but still no heat.

Is there an easy way to test the Combustion Air Sensor to see if it is functioning, or if it is not getting enough air flow? What are some things that could cause no/low air flow, and where can I look?

Where is a good starting point?

Any help would be appreciated!

Thanks,

Mike
 
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Old 10-02-04, 02:05 AM
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I would suggest checking the vent to see if there any blockage. This could cause the pressure switch not to close. Also check the pressure tube for any cracks that could prevent the negative pressure from building up and closing the pressure switch.

Assuming the venting is OK you could also try bypassing the pressure switch. If the pressure switch is faulty the furnace should fire up.

Other possibilities include a faulty gas valve (you can check this by checking for a voltage on the circuit board using a multimeter) or a fault on the control board itself. However, in this case the ignitor should still operate - if it does not the pressure switch is the first thing to look at in my opinion.
 
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Old 10-02-04, 02:47 PM
mcg731
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Lennox G23 problem

Thanks. I have looked at the top of the chimney, and did not see any obstructions. I then went down to the furnace, and put an ohm meter accros the combustion air prove switch, the switch was open, then turned the furnace on. The induced draft fan started, and the the switch closed. I guess that proves the switch is fine, as well as the the air flow.

At this point, what would be the next step? Would the flame sensor come into play here? Could a bad gas valve cause the next step in ignition not occur? Could it be a bum Honeywell module? How can I test it?

Thanks for any ideas.

Mike
 
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Old 10-02-04, 04:35 PM
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Usually the next step in the process is for the ignitor to heat up (for hot surface units) or for the gas valve to open (if you have a spark unit). In any event the flame sensor does not come into play until the furnace has lit. So the things to check would be to see if the pressure switch is actually connected properly to the control board. If the next step in the process does not occur it could be a combination of a gas valve, ignitor or control board problem.

It would be harder to test the control board unless you can get the manufactuer to supply a circuit diagram. You can possibly try to trace out what it is doing by visual inspection if you are good with circuits - typically it will have small micro-controller of some sort connected to a few peripherals like relays. The micro is the "brains" that decides what happens in the ignition sequence. In any event I would suggest looking for loose wires.
 
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