Unit works fine a couple times then fires and wont kick in fan to complete heat cycle. this happens a couple times but does not go into lockout. unit was cleaned and had co tested and gas valve and output checked last week.the air flapper was also replaced. the fan limit switch was replaced 2 years ago because of a no heat situation,the furnace works erratically but doesnt fail ?
Most of them 90+% units I've worked on ended up being a secondary heat exchanger drain issue, drip leg issue in gas line, or venting/combustion air issue. Had any roofers on the roof lately? Or mud dobbers(bees)/bird nest in the flue? Now if it's an electrical issue inside of the furnace, there no easy way to fix it, without being there, and catching it acting up. If it's intermittent, have the tech remove each wire connection in the unit and plug it back in, tighten all voltage lugs. And if there are limits inline with the blower only, check/replace them for certain.
Have seen a rash of faulty blower time delays in lennox equipment. Lennox has a recall on one currently, not sure if your system is effected but it can be bypassed if in fact the problem.
If you need further help, reach me via phone at https://www.6ya.com/expert/thomas_092728000e6acb79
SOURCE: Furnace gas valve wont open (roll out switch failure)
Sequence of operation, Gas Furnace.
On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the ignitor. The ignitor will glow and you will be able to see it if viewed thru the small inspection port. Once the ignitor gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.
What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the ignitor does not energize (bad control, bad ignitor), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not lite (dirty pilot), the burner does not lite (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).
Most newer furnaces will have a diagnostic center or control built into the control module. You may be able to view it as it is running thru a small viewing port. Some models will require the removal of an access panel prior to finding it. On the reverse of the access panel, there will be a diagnostic chart that will aide you in understanding any error code recorded (usually a blinking light).
This is why we technicians earn the big bucks!
SOURCE: Lennox Gas Frunace fires goes out, fires goes out, fires goes out
furnace was working fine until the electricity went out for 5 hours now furnace is just humming and then nothing...HELP...
SOURCE: Furnace unit to be louder now than last season
Check your air filter, make sure all registers are open full and there is no restriction in all ducts. Could be an unbalanced fan, clean the squirll cage fan with a brush and vacuum.
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