Oil furnace problem

The yearly oil furnace inspection my wife ordered included a switch to a smaller fuel nozzle. Performance improved (it was short-cycling) but the next day the furnace cut out and would't come on again. She called the people back and this time a crew of 3, after four hours of effort, get it working and chalk it up to "a bad transformer."
But yesterday it wasn't working again, so I looked at it. Nothing nada dead - not even a click from the control relay. I check the thermostat and there's 24 volts AC that switches to 0V when it calls for heat.
It is a 5 year old in-floor furnace that appears to be in good shape. I pulled the air circulation blower and beneath it found what I suspect they replaced and called a transformer - a Honeywell control module that has an input for the flame sensor and seems to control the whole shebang - and appears to be brand new. All connections are good but the red button on the top is up and resetting it returns the furnace to operation only to have it stop again after a few hours.
There's a small spring loaded trap door in the cast iron firebox/heat exchanger that allows me to see the flame.
I suspect an air/fuel mixture problem either causing a flame out (can't run it for hours with the inspection port open) or failure to reignite. flame is blue/yellow
My questions are how is the flame mixture adjusted - by flame color? There appears to be a sleeve around the burner assembly with a series of rectangular holes so that moving the sleeve would open or close the air intake. How does the flame sensor work? Is this something that requires a probe to be placed where it senses heat like a thermopile or light like a photocell? I couldn't see the actual sensor just the wires to it.
What should I be checking and can I tinker with the fuel mixture in safety? (I'm a retired electrical engineer - and good at repairing things)
And do these guys seem competent based on what I've written?
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wrote:

Floor furnaces useally have high heat sensors as a safety feature. They will turn the furnace off if the floor grate is blocked preventing a fire or would turn off if someone fell across the floor grate. They are wired in series with the thermostat an if there is a loose conection to one of the sensors it will cause the furnace not to ignite. Also one of the sensors may be defective. This is just one of many things that may be the problem. Good luck.
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default wrote:

Smaller nozzle change may be the cause of trouble if it was running fine before. Heat chamber burning temperature might have changed.
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wrote:

That's what I would think too, but for three guys to spend 4 hours and then replace a "transformer?"
I know damn well that if something is working before I mess with it and isn't afterwards, I should start troubleshooting at the place I came in.
But then there's always that one time I replaced the fuel filter on my truck and the oxygen sensor, failed the day after, and I'm figuring I introduced some dirt into the fuel injectors.
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wrote:
Problem solved. Idiot that replaced the fuel filter didn't bother to tighten the bowl, or connections. Some $1000 worth of oil is now polluting the ground water.
Damn thing was out of fuel because the tank, which was topped up before the service, ran out on to the ground.
Cost for initial service $70 Cost for second service and "transformer" $200 Cost for third service (they actually tried to charge for fixing their mistake!) $50
I'm considering small claims court. I also want a record of who's fault it was, in case the environmental people get wind of it. God knows what the clean up costs would be.
I sure wish I'd been in town to see these jokers...
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default wrote:

Your main problem is that you still heat with oil.
Get out of the stone age and connect your home to a natural gas pipe.
Heat your house, your water, your clothes, and maybe even cook with natural gas.
I don't know why people deal with oil any more.
Having oil delivered to your home is practically like having coal delivered through a chute in your basement. It's for cavemen.
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I grew up during that time. The coal (and ice) man still used a horse drawn wagon in the 50's.
Gas isn't an option, unless it is propane in tanks.
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