Furnace Ignition Issues

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  #1  
Old 01-16-18, 12:55 PM
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Furnace Ignition Issues

I have been attempted to diagnose a furnace ignition issue for weeks now and I can't get to the bottom of it. I have included a video of the problem below. In short, the furnace inducer motor turns on, the HSI glows. This continues for sometime until the gas valve opens. When you hear this the gas valve opens (or at least clicks) and the HSI Immediately turns off. The inducer keeps running for about five seconds and then it retries the startup cycle and throws error code 11 (pressure switch stuck closed prior to activation of inducer motor). Eventually the furnace does ignite but the number attempts it takes varies.

This code doesn't seem to make much sense considering it isn't thrown until ignition is about to occur and the inducer motor has been running for quite some time. The fan itself never starts (main blower fan).

Here are things I've done:

1. Checked condensate drains and traps. They seem to drain no problem
2. Replaced pressure switch
3. Replaced Gas valve
4. Replaced control board
5. Cleaned condensate lines internal to the furnace.
6. Cleaned flame sensor

https://youtu.be/Yi6VB2KAlZg
 
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Old 01-16-18, 01:29 PM
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Welcome to the forums.

Indeed a strange problem. It would appear the inducer and pressure switch are working correctly or the igniter would not come on.

The igniter warms up.... you hear the click of what could be the gas valve or it could be the igniter relay shutting off as it goes off at the same instant it should be lighting. Normally the igniter stays on for a few seconds during burner firing.

Can you post the full model number for that furnace for me ?
It will be on an ID tag on the side wall near the burner.
 
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Old 01-16-18, 02:44 PM
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Hi thanks for your reply!

The full part number is EL195UH110XE60C-03
 
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Old 01-22-18, 07:27 AM
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Just wanted to update everyone (and anyone that might read this in the future) on what the fix was.

The problem was the Nest thermostat. I can only hazard a guess that when the gas valve turned on it created noise on the 24V line that caused the Nest to briefly release the W wire causing the ignition process to shutdown. No error was reported by Nest.

Bypassing the thermostat and manually shorting out the wires resulted in proper startup every time. I replaced the stat with an Ecobee and it works great no more issues.
 
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Old 01-22-18, 02:57 PM
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I have seen this many times after everything was replaced by several technicians.
The typical issue is not the pressure switch but the port or orifice on the induced draft housing where the rubber tube from the pressure switch attaches to "sense" and prove the inducer motor is running, if it is 100% blocked by carbon the next sequence of operation never happens that being a trial for ignition which it will attempt 3 times before it locks out. If the orifice or port is partially obstructed which is usually the case the pressure switch will waver open, close open , close, just the slightest opening of the pressure switch will cause a problem as it is proving the induced draft motor is operating and has purged the combustion chamber. ream out the port where the pressure switch's rubber tubing attaches to the induced drafts housing using a tiny drill bit or strand of wire, be sure to actually clear it as it can be tough to clear and can fool you, the induced draft motors squirrel cage resides inside the inducer motor housing and usually if you have cleared the port the drill bit or wire you used will touch the squirrel cage and then you know the ports opened. On some units if the pressure switch remains closed in the off cycle the unit will not operate until the pressure switch opens after the end of the induced draft motors operation, the tubing cal collect condensation which will collect in a low point or loop of the tubing, the tubing itself can be old and not seal well enough to operate normally and the pressure switch itself can fail however you replaced this already making me thing the port or orifice on the induced draft motor housing is clogged or obstructed, this port if not reamed at least every 2 years will create this nuisance lock outs and many times everything was replaced and still the problem persists this is why. you can pretend you are a normal pressure switch by closing and opening the pressure switches circuit by touching the 2 wires together as you hold them and releasing them when the sequence of operation would normally do so, if you act like a good pressure switch and the problem goes away you know this is why.
Another issue which is probably not your problem this time is the flame sensing and the signal reaching the ignition modules "sense" or "ground" terminals on the ignition module.
They way the unit proves a pilot flame exists is bt flame rectification, this is a flames ability to rectify AC power to DC after it has passed through the pilots flame thus proving a pilot is established by a DC signal.
The DC signal is very weak and is easily disrupted if the mating surfaces are not clean such as the terminal where the flame sensor attaches to the sense terminal on the ignition module. The ground sterminal of the ignition module is common to the sense terminal and often the ground wire is patine'd and simply removing the wire and replacing it will clean up the surfaces to allow the very wek DC signal that proves the flame exists, the pilot assembly is screwed onto the metal cabinet of the unit and the ignition modules ground wire is attached to the metal cabinet as well often using a screw that mounts the ignition module, paint and moisture or patina will interrupt the weak DC signal from reaching the ignition module through the units metal cabinet, I often use 105 celsius rated wire and attach one end to the pilot assembly nout screw and run it to the ignition modules ground terminal, this creates a much better path to each other rather than expecting millivolts of DC current to make it from the burner area to the ignition models ground terminal using metal cabinetry as the conductor a wire is a direct path to another.
You can buy a flame simulator that clamps on that simulates a DC signal too and if it operates normally with a flame simulator then you know the DC signals the problem in that its not reaching the ignition module or has even the tiniest interruption of the weak DC signal proving the flame exists which signals it is safe to transition to main burners
 
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Old 01-22-18, 10:04 PM
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Thanks for letting us know how you made out.
Sometimes the nest doesn't have a full charge and can cause a hiccup like that too.
 
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