What is the purpose of the "lockout timer" in a home heater/ac system?

[Kat Rabun]

What does the lockout timer do on a 20-year-old Payne home gas-fired heating/ac burner?

The technician came to see why the heat kept shutting off the pilot flame and he took the "lockout timer" with him after testing. This black plastic box had wires attached to it which he wired to the furnace with jumpers.

Before, the pilot flame would light, then the burners, and then after a bunch of clicks, the whole flame would die out before the fan blower kicked in. Now, with the lockout timer removed and its wires taped up with jumpers, the pilot lights, then the burners light, and then after a few minutes, the blower kicks in forcing heat throughout the house.

As he cheerfully left the house, with the heater finally working, he said he'd be back with a new lockout timer in a few days.

My question is: If the lockout timer isn't needed, what does it do?

Also, he drilled some holes in the before and after ducts and inserted a meter which showed 0.39 inches of water of pressure. He said it should be much larger than that and that I needed a whole new system. Any information on what the pressure means? (He didnt' even tape up the holes.)

[The Daring Dufas]

I believe he was measuring 0.39 inches of water column. It has to do with the static pressure in the ductwork when the blower is running. Here's a link that may help you understand it a bit better:

The static pressure in your system may perhaps be raised by the service tech changing the speed jumper to a higher speed for the blower motor.

TDD

[Ralph Mowery]

"Kat Rabun" wrote in message news:hnhmat$c1m$ [e-mail]...

If the circuit does not think the main flame is going after a short time period the lockout timmer will cut off the gas. This is so that if the flame goes out you do not have an open gas line. It could let gas out and if there is a spark the whole house goes up in flames.

[jamesgangnc]

Not likely. He would have measured the temp as well. You can't raise the speed if the outlet temp was already in the correct range. It would cool the heat exchanger too much.

While you system muight be undersized that's info I'd just hang on to until it HAS to be replaced.

[Stormin Mormon]

Sounds like a safety protection. To shut off the gas valve if the burner doesn't light. So that the gas doesn't keep flowing, and fill the furnace, or cellar.

[The Daring Dufas]

After posting, I had a thought that the evaporator fins could be clogged. I've seen so many things affect airflow including the blades of the squirrel cage blower being loaded with dirt. I have also seen the blower motor jumper plugged into the wrong spot, insulation flapping loose and the infamous too many registers shut off. As you know it's impossible to diagnose remotely with limited info. Assuming the tech measured the temperature, why would he not inform the customer of this? From the OP, it appears that the only measurement made was static pressure.

A friend called me to his house because his furnace was making a funny rattling noise while running. The schematic diagram had come loose from the blower compartment door and had been sucked into the blower acting like the playing cards we clipped to our bicycle frames to snap against the spokes as we rode our bikes when we were kids. All sorts of things went through my mind from his description of the noise until I actually heard it in person.

TDD

[DerbyDad03]

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So why would the tech just jumper around the lockout timer and leave?

Is he hoping (probably with the odds in his favor, but still just hoping) that there won't be a flame problem within the next few days?

Doesn't sound prudent...

[Ralph Mowery]

He is thinking the home owner needs heat and is willing to take a chance that the flame will not fail and then leak gas and blow the house up. It could also be a timer on the air system that if the blower fails or the air pressure drops too much the gas is shut off and the fire goes out. This will also be a trade off as to safety vers the need for heat.

Without seeing the exect system I am guessing. That lockout timer could be any of several safety devices or atleast tied to them. Sort of like where I work as an electrician. Every time something stops, I get a call that something has "kicked out". It could have stopped for any reason.

[The Daring Dufas]

The unit may have draft inducer fan. There are two different safety switches that are often used with draft inducers. One that I see mostly on older units is a centrifugal switch on the back of the draft inducer motor, newer units have an air pressure switch to prove air flow through the combustion chamber before allowing the main gas valve to open. Those safeties will often be in series with other safety cut outs like the roll out switch and different over temp switches which are series wired to cut the control voltage to the system.

TDD