oil burner flame sensor control problem 1

[ruggb]

I am having a problem with a Honeywell oil furnace flame sensor control. It is, IMHO, a design issue as it has been a problem for years since it was new. Maybe someone can tell me how to solve it. I am a retired EE so I can't just replace it (u know how that goes) and everything else has the same ckt so they probably will all act the same. The attachment shows the ckt. Everything works fine as long as there are no power interruptions. Unfortunately, this house is in the mts and when there is a storm of any kind the power does strange things. The other issue is, I don't live there. It is a vacation home and things freeze when it trips out.
I thought that it might be because the burner was shut down and restarted too fast so I put a TD relay in the thermostat ckt. One blew out (permanently open) and the ckt tripped at the same time.
I am now thinking that a rapid off/on of the power is generating an inductive spike on the secondary of the transformer which is getting to the triac and turning it on, and may have fried the TD relay.

ckt operation: When the thermo goes on, 24V is applied across R1 and the CAD cell (high resistance) and the triac is turned on. This pulls in K1, which starts the burner and holds the relay on. When the CAD cell sees a flame, its resistance drops and turns off the triac, and everything runs fine. If it does not see a flame in about 20 sec. the heater trips the safety sw. and everything shuts down.
The 1st question is, if there is a power transition is it reasonable to assume an inductive spike will be generated on the secondary?
The 2nd question is, if the TD relay trips in 10ms after power failure, the safety sw must trip out before K1 drops out. This I cannot explain - IE, what sequence of events will do this?
The 3rd question, how to I stifle it?
One thought is to place the TD relay in the primary of the transformer. I have not done this because it involves surgery on the control unit. But I am about to as soon as someone can explain #2. Maybe sooner.
The TD relay will not come back on for 5 min. after power is restored.
Thanks in advance for ANY ideas.

 

[waross]

I had issues with an industrial burner that had a leaking pilot valve. With the valve leaking, the pilot flame would not go out. As long as the "Eye" saw flame, it would not restart.
What is the connection?
Possibly on very short power interruptions the flame has not extinguished by the time the power is restored.
Arrange your time delay to interrupt ALL power to the control board, not just the thermostat circuit.
PS The the fractional distillation tower was down for 2 1/2 days plus another day to bring it back online before the issue was solved. Fortunately for me I was only involved during the last day.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ruggb]

thx Bill - I am leaning that way - But since it involves some surgery I would like to understand the mechanism for the failure. The fried TD may have been a coincidence, but a large transient may have caused both failures. I just can't rationalize that. The only way to trip the safety is for the triac to be on for about 20 sec (actually closer to 15). If the flame is there, the sensor wants the triac to be off. I have tried to trip it manipulating the thermo but cannot so it does not seem to be a function of the flame going on/off/on/off/on, which is why I am looking at how a transient could cause it. But with the heater in there it becomes more of a mystery.
One possibility is if 1k2 is not pulling back in, then the ckt is on w/o the motor running. That would trip it. But why would it do it only when there is a power interruption, and not every interruption will cause it.

 

[waross]

Some additional thoughts. Strange things can happen on rural distribution lines.
A tree branch on one phase can cause a transient over-voltage on the other phases.
As the re-closers try to clear a fault, there may be several transients on the un-faulted phases.
I was watching TV one night when the TV flared brilliant white and then the power failed.
Our distribution line was under-built on a 60 kV transmission line on wood poles.
A car had hit a power pole 5 or 6 miles away. A 60 kV conductor slapped a distribution conductor.
We were really lucky. Homes a few miles closer to the accident lost TVs and refrigerators.
The point is:
Power failures are not the only issues on rural distribution lines.

Another possible factor is the motor. Re-energising a running motor before the residual EMF has decayed can cause severe torque and current transients.
There are protection devices available that will trip out a circuit on either under-voltage or over-voltage. They then allow enough time for a refrigerator to bleed off the high side pressure so the compressor may be safely re-started.
If one of those is not available, consider an on-delay relay controlling all the power to the furnace controller. This would drop out all power to the furnace controller on loss of power and when power returns, wait a minute or so before closing to put power back on the furnace control.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ruggb]

Thank you.

 

[VEBill]

In my part of Canada, cottages are typically closed out for the winter. Pipes are drained, toilet tanks emptied, plumbing antifreeze dumped into the water traps.

If any heat is left on, it's one electric baseboard set to something like +7°C. Just enough to keep the frost out of the interior furnishings. People have to search around to find a thermostat that works that low.

Oil heat wouldn't be worth the energy-cost savings. The cost of such minimal electric heat is so low, the power company doesn't even bother reading the meter until Spring.

I'd never trust an oil furnace in such a situation. Too much can go wrong.

At the very least, you could wire in an electric baseboard heater, set low, as backup to your oil furnace. Minimise the impact of inevitable furnace failures. But you'd still want to drain the plumbing in case of power failures.

Good luck.

 

[waross]

A good suggestion, VE1BLL.

Bill
--------------------
"Why not the best?"
Jimmy Carter