Boiler burners shut off early

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Old 02-25-11, 11:06 AM
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Boiler burners shut off early

I have a Weil McLain boiler that is shutting off early during cold weather.
Model HE3, Honeywell S890H1010 controller.
With low demand, the system runs properly. Gas shuts off when the boiler reaches the upper limit setpoint (190F) and the boiler cycles until the thermostat is satisfied.

When all 3 zones call at once, the burners fire up and run for about 10 minutes before shutting off. It hasn't reached the high limit and the thermostats are still calling for heat. The fan keeps running (30 sec purge) and the burners re-light. The next 2 runs are shorter (again shutting off before upper limit or thermostats are satisfied) and the control goes into lockout (fans and circs running continuously, but no attempt at lighting). I had the same problem last February during the coldest days, but the boiler went back to 'normal' once peak demand was gone and we didn't follow up on a diagnosis.

Any suggestions on what the problem might be? Voltage appears steady at 25.6-27V. I've verified the pressure switch (replaced it last year). Grounding appears solid. Flame looks good, but could the flame sensor be intermittent with long burn times? (I don't have a meter sensitive enough to test it.) Should I try replacing the sensor? Or should I look at replacing the controller? Anything else?

If its the controller, would the S8910U (available locally) be a good replacement (4 or 7 seconds instead of spec'd 6 sec) or should I search for the original equipment S89C (6 sec single failure lockout)?

Thanks.
Baffled
 
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  #2  
Old 02-25-11, 01:16 PM
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Could be lots of things. Does not make sense if one or three zones are going and you get a burner off situation. There is a series of events that take place before the burner will fire, and you need to find hich one.

Heres your manual. No help but at least you have it.

http://www.weil-mclain.com/en/multim.../he3manual.pdf

Igniting procedure page 7 for Honeywell. All the trouble shooting procedures. Let us know if you need help following the procedures listed. Get a multi meter if you do not have one.

http://www.weil-mclain.com/en/multim.../hevhe3hsi.pdf

Mike NJ
 
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Old 02-25-11, 08:44 PM
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Have you taken the flame rod out and cleaned it? I would try that before i start buying parts. Just clean it with some real fine sand paper or steel wool. they should be cleaned every year or so. Paul
 

Last edited by NJT; 02-26-11 at 08:37 AM.
  #4  
Old 03-14-11, 05:19 PM
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Smile Resolved! Hope the long write up may help someone else

[lawrosa] Thanks, I had already used the troubleshooting guides but was stumped because the boiler did light, it just didn't stay lit through a full cycle.
[paul52446m] You were right to suspect the flame rod, but I did clean the flame rod and it made no difference. I also verified that the wiring from the controller to the rod was continuous (0 Ohms). The boiler would still light normally, but run through 3 short cycles before locking out.

Fortunately, after spending the weekend resetting the thermostat to release the controller lockout so the boiler would run and keep the house warm, I confirmed the problem in the flame sensor. There was an intermittent break in the connection from the flame sensor lead wire to the sensor rod where they join inside the sensor insulating sleeve. At room temperature, the connection from the controller to the rod was continuous. However, it appears that when the flame sensor assembly heated up during a long burn cycle, the assembly expanded just enough to break the wire's contact to the sensor rod.

A replacement sensor turned out to be a hard to find. (Weil-McLain part # 511-330-182 made by Sapco - originally included with the Norton 201 to 201n ignitor conversion kit.) Without specs, I didn't trust replacing it with a different model. As a temporary fix, I improvised a way to bind the sensor lead wire tightly into the insulator so that it maintained contact with the rod even when hot. The boiler ran normally. I have since found a replacement sensor/wire assembly and the boiler has continued working properly.

A few other notes:
The controller was providing about 8 volts AC to the sensor. I still don't have specifications to show if this is the correct voltage. I discovered that if the sensor is grounded, then the controller will not attempt to ignite the burners. Once the burners do ignite, a very small DC current (~1 micro amp) is conducted through the flame to the grounded burners. An intermittent ground fault could cause similar problems and shut off the burners prematurely.

Burn cycle sequence of events
  • exhaust fan starts
  • exhaust pressure switch detects sufficient air flow and allows current (through the thermal fuse element)
  • after the purge time (about 30 seconds) the controller checks for no current through flame sensor if no current then
  • ignitor heats up (about 15 seconds)
  • gas valve opens and burners ignite
  • controller checks for small current through flame sensor
  • burners continue firing until one of the following occurs:
  1. thermostat has been satisfied
  2. boiler reaches the aquastat's maximum temperature setting
  3. flame goes out (the flame sensor is grounded or no current flows through it)
  4. exhaust pressure switch detects insufficient airflow
  5. thermal fuse element reaches it's limit and cuts off power
In all of these cases, the gas shuts off.

If 3, 4, or 5, the controller will go through the purge sequence and attempt to relight the burners up to 3 times. If it fails 3 times the controller locks out with the exhaust fan on, the circulators on, and the gas valve off.

The following is a dangerous test! If the gas were on and the flame went out, the controller would not know it and gas would continue to flow! Do not bypass safety systems with the gas on!

The controller would not attempt to light the burners if I placed a meter between the flame sensor and the controller (as suggested for testing), but if, within 6 seconds after the controller opens the gas valve, I connected a volt meter between the controller's sensor connection or the flame sensor and ground, the meter will draw off enough DC current to simulate a working flame sensor and the controller will keep the gas valve open. Disconnect the meter or ground the sensor connection and the valve will close. (In my case, the cycle ran correctly whenever I tested without gas and the sensor did not get hot.)

All of this would have been much simpler if I'd had a replacement flame sensor to test with or if the sensor had failed completely....
 
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Old 03-15-11, 09:40 AM
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Originally Posted by sdrmnw View Post
Ö I confirmed the problem in the flame sensor. There was an intermittent break in the connection from the flame sensor lead wire to the sensor rod where they join inside the sensor insulating sleeve. At room temperature, the connection from the controller to the rod was continuous. However, it appears that when the flame sensor assembly heated up during a long burn cycle, the assembly expanded just enough to break the wire's contact to the sensor rod.
....
Donít have anything significant to say. But that is amazing. Iím just a newbie but that looks like really impressive troubleshooting. Are you Dr. House by any chance? LOL
 
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