Gas and Oil Home Heating Furnaces - How does Honeywell S8600M sense pilot and main burner heat

This is to Grady if he is still a moderator or any one else who can answer this rather technical question.

This is in reference to the thread
http://forum.doityourself.com/gas-oil-home-heating-furnaces/251717-lighting-up.html

The above thread helped me get my furnace working much better. It cycled 1, 2, 3, 4, and sometimes 5 times before it got happy even though both the pilot and main burner lit as far as I know ... based on sounds heard at the other end of the basement or upstairs ... whenever you were close enough to see what was happening, it would not fail:-)

I disconnected the pilot burner/igniter/sensor from the manifold and cleaned up the metal there BUT I also ran a copper wire from there to a frame screw. The manifolds sit on/over a gas nozzle at the front and over a bar at the end ... hardly good electrical contacts ... amazing it works at all.

BUT my question is about how the S8600M knows that the pilot lit and the main burner started. I have checked rather thoroughly and I cannot find any other wire going anywhere into the "fire box" from the S8600M other than the spark wire.

Does this spark wire manage to serve a dual purpose?

Furnace is now working but I am trying to better understand how/why this works so I can debug it when something else goes wrong.

Thanks in advance for any info you can post.

Dave

Here is the Honeywell instruction manual. Notice on the first page under application that some units have separate spark and sense probes while others have them combined.

http://customer.honeywell.com/techlit/pdf/PackedLit/69-0463.pdf

Start reading on page 12 to see the typical operating sequence.

14Dave, as you can see from the manual, the ignitor on your model does indeed act in dual roles as an ignitor and as a sensor.

Thanks to Goldstar and Furd!

I already had the manual and had read most of the relevant pages BUT I didn't appreciate the significance of that little dash:-)

It must make it difficult to measure the current if the meter and wires have to also cope with 10,000 volts of spark ignition ... glad I just assumed that a bad ground must be the problem and acted accordingly based on the threads in this forum.

Anyway, thanks a bunch!

Dave

While the instruction manual does distinguish between different ignitor-sensor types, I frankly don't know what that difference is supposed to be nor do I see the difference illustrated in any of the curcuit diagrams provided.

If anyone can point out difference to me, I'd be glad to have them do it.

But in my experience, all the S8600 modules have a separate spark ignitor electrode connected to the "Spark" connection on the module.

The flame sensor is any part of the steel pilot burner that the pilot burner flame strikes.

The circuit for the flame rectification flame detection is through the spark electrode, then through the pilot burner flame to ground on the steel part of the pilot burner, and from there back to the "Gnd" terminal on the ignition module.

If you want to test the flame rectification current, you do that by connecting a microammeter between the "GND" terminal on the module and the chassis ground or the ground wire from the pilot burner.

The most common reason for unreliable operation of the S8600 ignition system is a dirty pilot orifice. Cleaning or better yet, replacing the pilot orifice is the best place to start.

After that, disconnect the MV connection to the gas valve so you are just testing the pilot ignition and operation. If the pilot lights and the sparks shuts off, the module is sensing that the pilot is lit and should turn power on to the MV terminal.

To SeattlePioneer,

Thanks for the additional information ... this level of detail was what I was really looking for.

It sounds like you have done this microamp measurement once or twice:-) Do you have to have a meter and probes that are rated for the 10,000 volts of ignition voltage to do this? This is not quite as high as the voltage used in your car ignition or on a CRT in analog TVs or computer monitors BUT I would think it was high enough that the average VOM would not like it not to mention that the voltage could jump out of thinly insulated wires and through you on its way to ground ... not good.

You did not comment on what I found to be a poor electric contact ... though things might be a little more forgiving at 10,000 volts ... but what is the voltage when the microamp current flows for sensing ... I suppose it could still be 10,000 but that seems excessive ... anyway, I thought that depending on gravity to make a good electrical contact between the burner manifold and the frame after cleaning up the surfaces where the screw connects the pilot light assemble to the manifold to be too much wishful thinking so I added the copper wire. I never bothered to try it without the copper wire addition.

The furnace is 3 or 4 years old and uses natural gas. I did not do anything about cleaning the pilot orifice.

As far as I can tell, the furnace fires and continues operation from the 1st opening of the pilot and main valves.

But thanks to you guys I think I am well prepared to fix/debug it if it acts up in the future.

Thanks.

Dave

The microamps are measured between chassis ground and the Gnd terminal on the module, so that is at ground potential and not affected by the spark voltage.

A good ground is important, but usually isn't an issue, since the pilot burner tubing back to the gas valve usually forms a good ground. Still, a ground wire from the pilot burner back to the Gnd terminal is often provided by Honeywell.

As previously stated, the micro-amp read is taken between the ground terminal and the ground wire when there is no dedicated sense terminal. Meter must be installed in series. AND you must know what values you are looking for, most Honeywell modules will work with a minimum .5 Ua read, although more is better. Look for a sustained read of around 3 or 4 Ua and you will be good.