Bryant Plus 80 not calling for gas, intermitant

[magnumip]

The unit is a 12 year old Bryant Plus 80 376cav downdraft installation. Within the last 18 months the mainboard (would not call for blower), draft inducer fan assembly (squealing), and hot surface ignitor (not needed but replaced anyways) were replaced by a technician. Filter replaced as it should be. The wired thermostat is a digital programmable Honeywell (about 5-7 years old).

Typically, I leave the thermostat on, but below a household temperature that would call for the furnace. When I need to elevate the temperature, the unit goes thru its cycle properly on the first attempt and runs on, off and back on as it should to maintain a setting. Everything works as it should for days.

Now here is the issue. If I turn off the thermostat (or if the thermostat setting has been below a heat call for over say, 24 hours) then come back a day later and turn it back on the unit will go through the start up - draft inducer motor running fine, hot surface ignitor glows and looks as it should, however the gas is not "called" for ignition. Cycle stops. No code. A minute or two later, it attempts the start up cycle. No go again, which may happen at least four times. No code. Fifth time, the cycle - gas, blower on and the furnace will work as it should, cycling on and off to maintain thermostat setting.

In summary, this happens typically after thermostat has been turned off (or not call because of low setting for extended period). Gas is not called for. Cycle repeats multiple times until successful cycle. No led status codes. Light continuous on.

No code baffles me. I also inspected the area around the draft inducer motor for any air breaches. When the inducer assembly was replaced, the technician replaced the motor, as well as the fan (squirrel cage) and a new gasket set. No shortcuts there. No reason to suspect the hot surface ignitor which looks good. All wires to mainboard look secure. Have not tried component test yet.

What else might be checked here before I call my guy back in? Afraid it will work as it should and incur a service call.

[magnumip]

Yes, I replaced the two AA batteries when I started noticing this.

Maybe the thermostat is faulty? Thanks for your reply...

[how]

An obvious fault with out a code is almost always the thermostat (because it lies outside of the board diagnostics) or a faulty motherboard $$$$$.
Intermitant furnace problems are not easy but you also have a tech who should have known the truth of this opening sentence.

Let your unit get cold again , turn off the furnace power, remove the thermostat furnace board connections from R & W, jumper the board connections R to W, turn the power back on.
If the short cycling occurs.. then the board has a problem. If it runs perfectly OK then your problem probably lies with the thermostat wires or the thermostat.
The thermostat change is easy.
Most thermostat wire problems are dealt with by ohming the lines but if you don't have a meter, redo the last inch of thermostat wiring at the board and at thermostat (if you have enough excess wire available) because thats where 95 % of the poor internal wire continuity occurs.

[magnumip]

How,

very interesting reply. thanks for taking the time to explain the procedure. I will try this when I get a chance and report back.

[heatseeker]

I wonder if he lived thru it.

[Chillerman]

He's just troubleshooting the individual components, what could happen? The furnace fires?