Carrier Weathermaker 8000 short cycling as ignition problem


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Old 02-16-12, 09:33 AM
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Carrier Weathermaker 8000 short cycling as ignition problem

I have a Carrier Weathermaker 8000, 58WAV090-LC, giving a code #34 which results in a code #14 after a few tries. This indicates an ignition problem.

Here's the sequence: Blower runs for 90 seconds after reset and shuts off, Vent inducer motor runs for roughly a minute, HSI relay clicks and HSI glows, 10 or so seconds later another click and gas valve opens, gas ignites and HSI turns off after 10 or 15 seconds. All 5 burners are lit and flame sense rod begins to glow. After 30-60 seconds the blower motor comes on. Flame looks good, things running well for 5-10 minutes when the gas valve relay suddenly clicks and the gas valve shuts off. The Vent inducer fan stops and the blower runs for 90 seconds. Then the process starts over 3 or 4 times until the lockout occurs. It display the 34 code.

The thermostat is still calling for heat as the house is still under the set temp.

Here's what I've done so far. Checked all screw terminals for good connections. Reseated the spade connectors on all sensors. Cleaned the flame sensor rod with light sandpaper. Cleaned the flame sensor connector. Cleaned the ground screw mating surface on furnace and retightened on the control board. Checked limit sensors for continuity.

Ignitor replaced in '04, but glows bright and immediate and lights every time as soon as gas valve opens. Control board replaced 10 months ago. New motor in blower 1.5 years ago.

I haven't measured flame sensor with a meter yet to check 5 microamps flow. I haven't sanded mating metal surfaces to ensure good ground from flame sensor or gas valve. The note talks of a green wire must be connected to furnace sheet metal. What green wire is this referring to, 120 VAC ground?

Any other ideas would be greatly appreciated.

Thanks
 
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Old 02-16-12, 10:53 AM
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I'd start by cleaning the flame sensor. If it's dirty, marginal performance might shut off the furnace at odd times.

It would also explain why you are getting the flame failure diagnostic code.

The flame sensor is on the side of the furnace opposite the hot surface ignitor, and is a rod about 1/8" in diameter and 3" long or so, sticking up into the burner flames with a wire coming out the bottom that goes back to the circuit board.

Remove the flame sensor and clean any invisible oxides from it using a wire brush or fine sand paper. You only need to clean the surface, not cut into the metal.

Reinstall and see what happens. This is ordinary maintenance that should be done once/year in any case.
 
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Old 02-16-12, 12:04 PM
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Yeah, I already tried that. I was hopeful that was the problem, but it still did it after I cleaned it.
 
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Old 02-16-12, 12:44 PM
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I see you mentioned that you did clean the flame sensor on the earlier post.


I'd measure the AC voltage between the flame sensor and chassis ground or the C terminal.


The circuit board typically applies at least 24VAC to the flame sensor whenever the inducer motor is operating. If that voltage goes away it would cause the burners to shut off.

See if an inconsistent voltage could be causing the problem.

That might well be caused by a bad circuit board.


Checking for the flame sensor current would be an alternative --- do you know how to do that?

A good flame sensor system typically produces 4-5 microamps of DC current. If the current flow is 1.5 or less, it may start causing the kind of intermittent failure you have.
 
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Old 02-16-12, 01:22 PM
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Thanks, I'll try that. I have a Fluke meter that might measure that low of current. I'll have to check.
 
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Old 02-17-12, 11:43 AM
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So I brought my meter home and took a few measurements. I measure 2.5 to 3 microAmps on the flame sense lead. However I measure 113 VAC between the flame sense lead and chassis ground or the lead and the common screw terminal on the control board.

The voltage does not fluctuate or change when the furnace goes through a heating cycle. The gas valve does however shut off after a few seconds (as it should when not sensing flame) when I measure the voltage as the flame sensor is not connected while measuring the voltage.

Any ideas?
 
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Old 02-17-12, 01:47 PM
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2.5 microamps should be plenty to satisfy the flame sensing circuit.


That implies that the circuit board is defective and needs to be replaced.


This is a "black box" diagnosis. This suggests that if you have all the proper inputs to a black box, you should get the expected outputs. If you don't you need a new black box. You don't need to know what's going on inside the black box using this method.
 
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Old 02-17-12, 07:01 PM
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I may have found the problem. On a suggestion from a guy at a local HVAC store, I opened up the A/C coil unit cabinet that sits above the furnace. Not as easy as it sounds as I had to remove the water heater to get to it and the unit sits front to back not side to side. It was a royal pain, but I'm glad I did it. Once I could see inside the unit, it was completely black. I carefully reached inside and touched the black stuff. It was a good 1/4" thick of dog hair and dust. We don't have a dog. Previous owners and renters did though, in fact the renters often had two dogs. So I carefully vacuumed out the blanket of nastiness.

Upon putting the unit back together, it pumps out so much more air. And it never shut off until it reached the desired temperature, which it did in about 15 minutes as it was 4 degrees below what the thermostat called for. The fan seems to run crazy fast now, but maybe that's because there is no longer a pressure build-up inside the furnace.

So that brings me to another question. The air is no longer very hot at the register, just warm. Is that how it should be? The blower motor is a 3 speed generic that replaced the OEM 4 speed. The Low lead is connected to setting 1 and the High lead is at setting 3, out of a possible 4. The #2 position was removed. Is that the correct positions?

When does the furnace call for Low versus High?

@SeattlePioneer - Again thanks for the help while working on this thing.
 
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Old 02-17-12, 07:59 PM
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Well, the 3-4 and 1-4 diagnostic codes you reported are for a flame failure. The diagnostic codes for an open limit switch are 1-3 and 3-3.


But making a magical leap to a limit switch problem is fine if it works I suppose.
 
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Old 02-21-12, 07:43 PM
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Nah, that wasn't it. The furnace only ran well for one cycle, then short cycled the next time it called for heat after about 3 minutes of operating. And I had a hard time believing a pressure switch would mask as a flame sensor problem and not a limit problem.

On a whim and a guess, I went and bought a new flame sensor, as that is cheaper than a new board (which I had replaced 8 months ago). It has ran flawlessly since I installed the new sensor. I'm guessing it had a bad connection somewhere in the sensor or wire that would eventually give a bad reading after it heated up.

On the plus side, after doing all that work to clean the inside of the heat exchanger, I do love all the extra air flow I'm getting.
 
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Old 02-21-12, 08:18 PM
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Accurately checking the current and voltage on the flame sensor should have revealed any problems with it if it was done properly.

If you were getting 2.5 ua DC current, the flame sense circuit should have kept the burners lit. Some circuit boards use 24 VAC and some use 120 VAC as the flame sensor voltage --- the higher voltage minimizes outages caused by dirty flame sensors.

If you had an intermittent problem with a bad flame sensor, you should have been able to observe the flame sensor current dropping to 1.5 uamps or less, followed by the burners shutting off.

You might want to check what kind of current you are getting on the flame sensor circuit with the new flame sensor. Usually with everything working correctly you will get a current in the vicinity of 5-6 uamps.

If you are getting the 5-6 uamps now, perhaps the 2.5 you reported earlier wasn't enough to operate the flame sense circuit reliably, even though it usually is on most furnaces.


Checking on the air flow through the furnace and cleaning up any problems is part of the things that should be done during annual maintenance on the furnace. I'd be looking for defects in the filter system that allowed the AC evaporator coil to get plugged up with debris. If you don;t find that defect, it will probably get plugged up again.


I encourage DIYers to follow a rational process of diagnosing problems with their equipment. A series of guesses that aren't confirmed by careful testing is a typical pattern for DIYers, but something I try to improve upon for people.

But I suppose magical thinking, guessing and parts changing can pay off sometimes!
 
 

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