Weil Mclain CGi boiler system with intermittent flame sensor issues

Reply

  #1  
Old 03-11-12, 10:48 PM
Member
Thread Starter
Join Date: Mar 2012
Location: USA
Posts: 8
Received 0 Votes on 0 Posts
Weil Mclain CGi boiler system with intermittent flame sensor issues

I've got a 10 year old Weil Mclain CGi gas fired boiler system, series 5. It uses a UT Electrical Controls system that looks like a pretty common system, as I can actively find replacement parts. Service manual here for anyone that wants the specifics.

This system uses a flame sensor that sits in the path of the pilot light, which lights with a spark system. A Honeywell gas valve flows gas to the pilot light and the control system sparks to light the pilot. A flame sensor, also attached to the control system senses the pilot flame once lit. When that happens, the control system turns off the spark and tells the Honeywell to open the main gas valve which fires the rest of the burners.

A few weeks ago, my system was refusing to light. Observing the startup failure, I noticed that once the thermostats triggered, the pilot would light just fine, but the spark would continue to spark for several seconds even though the flame was present. The main gas valve would never open and eventually the system would give up.

So I figured it had to either be the flame sensor or the control computer. I went through the troubleshooting procedure listed in my link above, which had me do a basic inspection of the sensor for shorts, etc. After finding nothing, it prompts the tester to connect a uA current meter in line with the sensor and check for at least 1 uA of current when the pilot flame is burning.

I did this, and saw absolutely no current flow during the pilot flame burn, which was several seconds. So you'd think flame sensor right?

However, the troubleshooting manual says this means I must replace the control computer.

I don't understand how the flame sensor works. When looking at it, there is a single wire from the computer to the sensor, which is housed in ceramic and not grounded at all. In fact, I don't see a complete circuit on this flame sensor at all, and don't understand how any current can ever flow. I figured the computer would supply 24V and look for an open or closed position, or perhaps some current flow, but that would only work in a 2 wire system, or one with a ground to the chassis. If no current flowed, why would the computer be at fault, rather than the sensor?

Can someone give me more information about how flame sensors work and how they interact with the control computers?

To make matters more confusing, once I put everything back together and reconnected, the boiler started working again just fine. This has worked several other times as well, and will work for a few days before starting to fail once again.

I can order a control computer and flame sensor, but they're both pricey. I really want to understand this problem before throwing parts at it.

Opinions?
 
Sponsored Links
  #2  
Old 03-12-12, 12:24 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,489
Received 32 Votes on 24 Posts
Have you tried cleaning the flame sensor? A Scotchbrite pad is preferred but fine steel wool will work. The circuit is closed through the flame which rectifies an AC voltage to DC and in turn signals the presence of flame.
 
  #3  
Old 03-12-12, 08:29 AM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
Dr. J, I think you are reading the chart improperly?

It says:

e. Is flame current signal less than
1.0 microamp?
NO

Replace ignition control
YES

Check for correct gas pressure, clean pilot assembly, and
tight mechanical and electrical connections. Also check for
proper system grounding. See box on page 46.
You are saying that you measure ZERO current when the pilot lights, so the answer to the question is YES, it is less than 1uA of current, and they then instruct to check connections, etc.

The 'return' for the flame rod signal is through the ground connections that are mentioned. These connections need to be clean and tight. Since it's such a small signal all connections need to be very well made.
 
  #4  
Old 03-12-12, 09:11 AM
Member
Thread Starter
Join Date: Mar 2012
Location: USA
Posts: 8
Received 0 Votes on 0 Posts
Thanks for the responses so far.

To Furd: During the last inspection I did actually remove the sensor and wire wheel it to a clean finish, it had a moderate coating of rust and filth. The system has functioned since that cleaning despite my 0 uA readings during my test after the cleaning.

To NJ: You're hitting on what I thought might be a typo in the manual (or my misunderstanding in hindsight). In part D with flame signal notes, it says there should be at least 1 uA current when the pilot flame is lit. I took that to mean I should be seeing a DC current whenever flame is covering the sensor (which I did not see). I couldn't understand why they would say the control module needing replacement if I actually did see 1 uA. I realize now they must be assuming the module wasn't functioning despite getting the correct sense reading, although I would think it could still be a gas valve issue, which I would need to check the MV 24V source for.

After re-testing the flame sensor, I noticed a very fast initial spike in current when the pilot flame first lit, then it would fall to 0 and stay. My multimeter is not fast enough to give a good idea of what is really going on, but if the rectification is only momentary, perhaps that's the real signal. It seams reasonable given that the boiler worked all night after my cleaning, but despite my test results.

If a momentary current of that magnitude is what is required to trigger the main gas valve, I would think all circuit continuity and grounding would be absolutely critical. Based on Furd's response, the current must flow from the control module to the flame sensor tip through the flame and then through the pilot assembly into the center burner on which it is mounted, and finally through the chassis ground. I did note the burner on which the pilot assembly is screwed is not securely anchored like the others, and might be presenting an incomplete ground path.

The only confusion in my understanding is that I couldn't measure any continuity between the flame tip and the actual wire terminal on the other end. I would have expected it.

So it seems like the name of the game here is verifying the integrity of the flame sense circuit through its rather roundabout return path. Crazy!
 
  #5  
Old 03-12-12, 11:36 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,489
Received 32 Votes on 24 Posts
Most consumer grade multimeters are not sufficiently sensitive to show a one micro-ampere current flow. For explanation on how the flame rectification system works, Google flame rectification and read some of the articles.
 
  #6  
Old 03-12-12, 02:10 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
Good point Furd... 1 MICROAMP is 0.000001 A or 0.001 MILLIAMP

So, if your lowest meter scale is 200 MILLIamps, and the display has three digits to the right of the decimal point (most will probably only have TWO), you can see that it will be next to impossible to resolve a current reading as low as 1 MICROamp.

I'm not sure that wire brushing the sensor is the best idea... a scotchbrite pad is usually recommended... but if it works now, give it some time and see if it 'holds'.
 
  #7  
Old 03-12-12, 02:16 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
I couldn't understand why they would say the control module needing replacement if I actually did see 1 uA.
That's where the misunderstanding lies. Not what they are saying. They mean that if you are seeing MORE than 1 microamp, head toward the module. If LESS than 1 microamp, check the flame rod and wiring.

I realize now they must be assuming the module wasn't functioning despite getting the correct sense reading, although I would think it could still be a gas valve issue
Not an assumption, pure logic.

Don't forget how you got to that point in the troubleshooting process!

You were there because the SPARK kept sparking AFTER the flame was lit, and then the pilot shut down.

IF there IS a current produced by the flame sensor, then the controller would know that and would shut down the ignition circuit once the pilot flame was proven.

Since the spark was NOT shut down in the presence of a flame, and IF there IS a current reading on the sensor, then logically the module is the next link of the chain.

The gas valve defective has nada to do with it at this point... the symptoms lead down a different path in the t'shooting tree.
 

Last edited by NJT; 03-12-12 at 03:33 PM.
  #8  
Old 03-12-12, 03:28 PM
Member
Thread Starter
Join Date: Mar 2012
Location: USA
Posts: 8
Received 0 Votes on 0 Posts
Excellent info once again!

To start, my multimeter does indeed have microamp resolution, 0.1 microamp in fact, which from my reading was the minimum resolution required to really understand what was going on with the DC offset current.

That said, I cannot explain my lack of current when the system pilot light was burning. It means I still don't REALLY know if it was the sensor or controller, because my test failed.

However, given what I did the last time in, which was to clean the sensor and essentially disconnect and reconnect all ground path components, I'm pretty certain my issue all along was an intermittent ground to the pilot assembly.

"Flame rectification" search yielded some great articles, most of which clarified the specifics of the sensor functionality for me. I can only image how many of these sensors or control modules are replaced by less than diligent repair technicians.

Thanks again for the very well informed advice. Rarely found on the internet...
 
  #9  
Old 03-12-12, 03:37 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
You've got a pretty nice meter then!

If you repeat the current test now that it's running, can you get a current reading?
 
  #10  
Old 03-14-12, 05:15 PM
Member
Thread Starter
Join Date: Mar 2012
Location: USA
Posts: 8
Received 0 Votes on 0 Posts
I still cannot get a constant DC read of current when repeating the test. Have confirmed my multimeter has a xxx.x uA scale, so I'm not sure what the real issue is. Boiler has continued to work despite this.
 
  #11  
Old 03-14-12, 07:36 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
My Fluke 89IV will report if the fuse in the current measuring circuit is open. Many do not have this feature and it is pretty easy to blow that fuse... I wonder if the fuse in yours is blown? It would read 0.00 all the time.
 
  #12  
Old 03-14-12, 11:11 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,489
Received 32 Votes on 24 Posts
Having a "constant" current flow would be unusual. The system has a built-in time delay to allow for periodic "drop outs" of current flow.

Trooper, if the fuse in the meter is blown then there would be no current flow and the system would sense this as no flame and close the gas valve. Remember that the microammeter is in series with the flame rod and the circuit board.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: