Weathermaker 8000 ignition problem

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Old 12-18-13, 08:21 AM
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Weathermaker 8000 ignition problem

Hi,
I have a Weathermaker 8000, Model # 58zav096 -16. For the last week or so it has had intermittent ignition problems, sometimes it lights up, sometimes it fails.
When it fails the indicator light shows a 31 error:

PRESSURE SWITCH DID NOT CLOSE OR REOPENED - If open longer than five minutes, inducer shuts off for 15 minutes before retry. Check for: - Excessive wind
- Proper vent sizing
- Defective inducer motor
- Low inducer voltage (115VAC)
- Defective pressure switch
- Inadequate combustion air supply
- Disconnected or obstructed pressure tubing
- Low inlet gas pressure (if LGPS used) - Restricted vent
If it opens during blower on-delay period, blower will come on for the selected blower off-delay.

Every time the furnace fails to light up, I find that the "Blocked Vent Safeguard Switch" has tripped. If I reset the switch, the furnace will fire up, then trip the switch again several hours later. None of the 6 causes above apply. I have taken the pressure switch out and it appears to work just fine. The furnace has not been serviced (cleaned) for 10 years.

Do I need to replace the "Blocked Vent Safeguard Switch" or what
 
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Old 12-18-13, 08:47 AM
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I would look into the vent pipe for an obstruction (like a large wasp nest).
I' d also verify that the vent cap on the roof is not damaged.
 
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Old 12-18-13, 09:31 AM
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Went on the roof and removed the vent cap, it is undamaged. Looked down the vent with a flashlight, there are no visible obstructions. Cannot see past an elbow joint about 6 feet below (inside the roof space)
The vent pipe was renewed when the system was installed 10 years ago. The vent pipe is shared by the gas water heater which runs continuously, so I believe that the vent portion that I cannot view internally would be too hot to allow wasps to nest there.
I will open the vent next to the furnace as soon as I have the required sealant I ordered for the refit.
Is it worthwhile ordering a Blocked Vent Switch at this time. Any other thoughts
 
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Old 12-18-13, 11:30 AM
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We have had to extend the vent pipe on the roof to stop this before but it looks ugly and you have had 10 years of operation the way that it is currently vented.
I haven't installed a gas furnace for a few years but I believe the manual wants at least 30 inches clearance between the bottom of the cap and the nearest part of the roof. Refer to your installation manual for a more accurate clearance specification.
 
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