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  1. #1

    Exclamation Munchkin M199 Boiler Fails to Light (no faults generated)

    I have an M199 Munchkin boiler that fails to light. When powered up, it will repeatedly purge, fire the igniter but gas valve does not open. Gas pressure is normal, and hot water heater on same gas manifold with standing pilot working fine. Control board and gas valve have been replaced along with low voltage wiring harness. It should be throwing faults after 4 tries but keeps on trying. Yesterday, tech thought he found bad ground connection and the system operated completely normally for about 12 hrs. CO2 when running on spec. This has the 926 controller with Vision option that resets HW set point with outside air. It is correctly reading OA temp (d5 code) and HW setpoint (d11) is per the reset schedule (e.g. 40 deg OA --> 149 deg HW set point). System when running modulates combustor to hold that schedule set point just fine as zones cycle to satisfy T-stats.

    When it fails to light, ionization probe looks clean and has 110 AC on it, spark looks normal on igniter, but something is inhibiting the control board from opening the control valve. What is very strange is not seeing any fault codes even after many tries at ignition. HTP factory has so far not suggested anything that is helpful as suggested replacements do not fix problem. The fact that it is intermittent is troubling, but with new wires and all connections double checked, not sure where else to look.

    Anyone with ideas?

  2. #2
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281
    Thread relocated to "Tech to Tech"

  3. #3
    Thanks Chris, new to the forum. Slight addendum for what it's worth. Service tech here yesterday thinks induction fan (more specifically its control board since the fan runs fine) may have defect. Here's why: normal ignition sequence would issue command to open gas valve followed or concurrent with ~6 sec spark, then valve latches open when flame detected with ionization probe. When this itermitent problem goes away, that is exactly what we see. When problem is present, ignition is very short along with short pulse to gas valve, insufficient to open it. Keeps repeating this sequence. It is as if something is inhibiting the sequence from completing early, as if a start permissive initially OK goes false due to transient error from somewhere, most likely the induction sub system (my tech thinks). Still a mystery why this never generates a fault code. All connections look good, control board and valve new along with low voltage harness so this only place left to look. Cant find any tech data on logic tree or sequence diagram to understand what is supposed to happen. We think all the other sensors OK or ignition sequence would never start (temp and H20 pressure...there is NO duct pressure sensor on this Rev 1 unit built 5/2004).

  4. #4

    M199 Problem Resolved

    Quote Originally Posted by paulhoupt View Post
    Thanks Chris, new to the forum. Slight addendum for what it's worth. Service tech here yesterday thinks induction fan (more specifically its control board since the fan runs fine) may have defect. Here's why: normal ignition sequence would issue command to open gas valve followed or concurrent with ~6 sec spark, then valve latches open when flame detected with ionization probe. When this itermitent problem goes away, that is exactly what we see. When problem is present, ignition is very short along with short pulse to gas valve, insufficient to open it. Keeps repeating this sequence. It is as if something is inhibiting the sequence from completing early, as if a start permissive initially OK goes false due to transient error from somewhere, most likely the induction sub system (my tech thinks). Still a mystery why this never generates a fault code. All connections look good, control board and valve new along with low voltage harness so this only place left to look. Cant find any tech data on logic tree or sequence diagram to understand what is supposed to happen. We think all the other sensors OK or ignition sequence would never start (temp and H20 pressure...there is NO duct pressure sensor on this Rev 1 unit built 5/2004).
    Problem isolated. After talk with HTP rep, looked for fault to ground on one of the sensors. There was no fault to ground but high resistance (10's of ohms that drifted around in the vent high temp limit. This is the resettable device located on top the heat exchanger/boiler near the exhaust and intake pipes. Resistance should be less than an ohm. Why this produces the strange symptoms and doesn't generate a fault is a problem for the Munchkin engineers to figure out, but it is not their explanation that a short to ground pulls down the 24 v supply. All 24 v test points good.

    A simple way to diagnose sensor faults on this unit is to progressively remove sensor connections one by one, short together (NOT to ground). WHen the system lights normally, that's the bad sensor. Be sure to reconnect and confirm the fault returns before replacing. You can measure the resistance also to confirm. Of course, don't leave the jumper in and fail to replace the sensor!!

  5. #5
    Join Date
    Dec 2007
    Posts
    4,698
    Quote Originally Posted by paulhoupt View Post
    A simple way to diagnose sensor faults on this unit is to progressively remove sensor connections one by one, short together (NOT to ground). When the system lights normally, that's the bad sensor. Be sure to reconnect and confirm the fault returns before replacing. You can measure the resistance also to confirm. Of course, don't leave the jumper in and fail to replace the sensor!!
    this is the simple way to diagnose any type of similar issue.

    knowing the resistance is another thing.
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  6. #6
    Join Date
    Jul 2010
    Location
    wisconsin
    Posts
    270
    Are you sure the gas valve plug, which is black, is plugged in.

  7. #7

    Gas Valve Connection

    Quote Originally Posted by tiger man View Post
    Are you sure the gas valve plug, which is black, is plugged in.
    Thanks for the suggestion. This was an early-on suspect (but not the problem), since as you know, the connector contains a circuit board with a full-wave rectifier in it so that DC can be applied to the two valve coils (at least in Honeywell valve on this unit). Apparently the Dungs valve does a similar trick. Not sure why they go to the trouble of AC to DC since at least the Honeywell valve spec sheet says it will work with AC also. The new low voltage harness also included a new connector. Replacing the harness, gas valve and control board were not needed as the problem was the hi temp limit sensor (at least its running fine now). What I still don't understand is why the failure that results in the vent limit temp to go from ~ .1 ohm to 10's of ohms produces the failure to start sequence, AND IS NOT DETECTED as a fault by the control board. If anyone knows the input circuit on the 926 controller, I'd like to see it and maybe I can suggest a circuit design change to catch this on the next board revision (or get HTP to do it in software). Does anyone know a live person at S.I.T. controls (who builds the board) who might address this question? Paul

  8. #8
    Join Date
    Jul 2010
    Location
    wisconsin
    Posts
    270
    The water pressure sensor will also give odd codes if it gets corroded, and wont give a pro, low water, condition.

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