Fixing furnace lockout condition

wdn

Messages
1,640
Location
NH
This weekend the 8 year old York furnace for the second floor was not heating, I noticed the thermostat was set for 65 and was calling for heat but temperature was 61. I want to the basement and the LED was blinking red a 7 flashes. That means lockout condition. I fixed it. I am not a licensed HVAC technician just a homeowner so if you blow yourself up don’t come looking for me. It’s a gas furnace. A normal York blinks a slow green LED.

Lockout occurs when the furnace tries to start up but does not detect a flame. Normally when call for heat happens the ignitor will glow red hot then the gas valve opens and fires, and the flame spreads across the gas tubes. Most furnaces have three or four gas jets mine has four. At the gas tube furthest from the ignitor there is a flame sensor used to detect the furnace is lit. At that point the furnace shuts off the ignitor, which is only used to start the fire. If flame is not detected the furnace shuts off and turns of the gas valve. The York and it is a common pattern, will retry three times and if the furnace does not stay lit it goes into lockout mode. The whole furnace shuts off for an hour. Likewise if your furnace was running and then the flame sensor fails to detect flame it shuts off. This is a necessary safety feature. Otherwise gas would fill your house and could have a gas explosion.
A flame sensor is just a heat sensitive variable resistor. As it heats resistance drops some and conducts more current to ground. The control board of the furnace measures this flow of current. We are talking microamperes. A flame sensor in spec is 1 uA to 7 uA. If the metal rod becomes tarnished the resistance is increased so it passes too little current. The control board interprets this as a no flame condition, even if there is a flame.

A lockout can occur for several reasons but most common are dirty/clogged furnace filter, or a dirty or defective flame sensor. I changed the filter. It was kind of dirty but not that bad. Then I removed and cleaned the flame sensor. First you turn off the furnace power switch next to the furnace. Then open the top panel where the burners are. Follow the gas pipe and locate the burner furthest from the ignitor. On my York furnace the ignitor on the left, flame sensor on the rightmost burner. There is a single wire with a spade connector going to the flame sensor. On a York I don’t know if it is by heating code or by convention, it is a purple wire. Then remove the flame sensor which in mine is a 1/4” hex head self tapping screw that attaches the flame sensor to the burner. In my case there was not even enough clearance for a stubby screwdriver. I used a 1/4 nut driver bit I stuck onto a 1/4” open and wrench and taped. This pYork is a boneheaded design has a brittle plastic condensate tank directly under the flame sensoe that makes it hard to access.

Once the screw was removed the sensor dropped free. It has a metal rod that was plainly discolored with white deposits and black carbon deposits. I polished if with fine steel wool until it was shiny. Then I polished it with a dollar bill. I out it back in the furnace, reconnected the wire, closed it up and turned the furnace switch back on.

It works now.
 
Messages
2,528
Location
WY
I use scotch brite and don't even remove mine. Seems you have one second from the time the gas comes on to the burners until it senses flame. I found this out when my meter which had filled with condensate froze up and restricted flow into my house. A pitfall of being the only house on a gas line in a new neighborhood of 60 future homes. The gas company came out and changed the meter, it sounded like a maraca when the tech shook it. There is also the evacuation fan that comes on before any flame can happen.
 
Messages
3,670
Location
Chicagoland
Been there, had to do it at our old house. Also had a piece of.... something... get stuck in one of the pressure sensing tubes which locked out my furnace. Took it off, blew through it with my mouth and it rocketed out like a BB!

I also had to replace the board because the relays were starting to fail on it. Super easy. I stop at easy to do stuff like that, I won’t touch the gas valve or lines.
 

wdn

Thread starter
Messages
1,640
Location
NH
Scotch Brite works too. I used steel wool because the manufacturers recommend it.
 
Messages
903
Location
New Hampshire USA
After reading the OP post a couple thoughts. Don`t think a dirty air filter has an effect on the gas controls. The flame sensor in most cases uses flame rectification. that is A/C current is rectified to D/C passing thru the flame. There are several methods of flame control and all are sensitive to dirty flame rods. One more thing, natural gas usually gets 3 tries for ignition and propane only one.




















after
 
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