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Lochinvar KBN-106--Numerous Hard lockouts with Outlet Temp Diff----Hopefully Resolved!

JUGHNE Member Posts: 11,058
edited February 2021 in THE MAIN WALL
KBN 106 installed in 2013, no problems until last 2 years.
First got Outlet MRHL, red screen. message would indicate temp that were impossible for the basement location.
Changed 4 wire outlet sensor....seemed OK for a while.
The next change was control board 3 months ago....again OK for weeks.

Now getting "Outlet Temp Diff" red screen....message included "Sensor 1 at 86 degrees" and "Sensor 2 at 86 degrees".
With this message I fail to see the temp diff problem.
After reset is pushed and system started, system temp was 50 degrees (as was office) and ODT of 9 degrees.

Lochinvar TS suggested checking outlet temp sensor ohm readings.
These are two sensors in a single probe, one is a 10K and the other 20K.
All the sensors have shown almost exactly double for the higher one than the lower one at a variety of temps from 68 to 86 degrees. The readings have been consistent from the original to the first replacement and now to the 3rd new replacement, which was suggested by Lock TS.

Almost 7 years of trouble free operation. Old gravity CI rads piping, PS piping, pumps set on high speed, 3" PVC inlet and exhaust well within lengths. Pump strainers clear.
Standing NG at 7.08" WC....4.63" WC firing. Meter clocked at 118,125 BTUH.
Never a loss of flame or failure to fire.

Any ideas are welcome. Thanks.


  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    The definition of insanity... have you checked all the wiring and connectors? If the same sort of thing keeps happening -- exactly the same problem -- with three different replacements... it's probably not the replacements. However, those sensors are -- by design -- very sensitive to resistance, so problems with the wiring or connectors should be on the radar.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 11,058
    My wife agrees with the definition stated above and often insinuates that it does apply to me on an almost daily basis. :*

    However, as far as wiring connection integrity, I did check all sensors at the Molex connector going into the control board. This includes 5 sensors and I moved things around possibly enough to produce some failure of continuity.

    Then tech support did insinuate ever so slightly that sometimes the internal wire connections may partially fail.....never happens...but is possible.
    It was also suggested to mount the sensor a 1/4 turn different than it had been, so as to change the water flow pattern around the probe.
    This is on the 1" discharge of the boiler that has the factory supplied boiler circ of Grundfos UPS26-99F pushing water thru the water coils.
    JUGHNE Member Posts: 11,058
    Surely someone here uses Lochinvar???
  • Zman
    Zman Member Posts: 7,569
    Is there a sensor wire that has been crushed, stretched or is running in parallel with line voltage?
    What are your actual delta t's when the sensors are both reading the same?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    JUGHNE Member Posts: 11,058
    I had checked the sensor values at the molex that plugs into the control board, so those wires are fine for all sensors.
    I believe all of the line voltage is contained in the JB or control board. With the exception of the fan power which is quite aways from sensor wiring.

    When the unit is running the delta T varies but the max shown is 18 the diff setting is 20.
    (only SH1 in use)
    SH1 set point is 160
    Minimum SH set point is 32
    Max SH set point is 190
    SH offset is 10
    SH diff is 20
  • Matt_67
    Matt_67 Member Posts: 286
    Just a guess but assuming the original MRHL temp was crazy high I would be checking for a rubbed through gray wire on the harness to the outlet sensor that is intermittently making contact somewhere.
    JUGHNE Member Posts: 11,058
    This post is almost 30 days old and there has not been a hard lockout/manual reset since.

    One thing I did change was the post burner pump operation time.
    When the new board was installed months ago, the factory default settings of pumps run on time were left in place....30 seconds for system pump and 30 sec for boiler pump.

    30 days ago I set the system pump for 40 minutes run time after heat call ends.
    The boiler pump was set for 10 minutes delay.

    This is a two story house with previous gravity piping that starts out with two 3" mains and then 638 EDR of CI radiators connected.

    I am thinking that residual heat in the iron mass was affecting the inlet/outlet temps by causing the boiler to overfire.

    Any ideas concerning that theory??
  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    You have hit a strong possibility there. All that mass in the radiators is going to take a long time to heat up -- and cool down. If it's working...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ironman
    Ironman Member Posts: 7,373
    edited February 2021
    I would think just the opposite would be true with p/s piping: residual heat in the HX was causing the temp to climb tripping the MRHL. By increasing the post purge time, you corrected this.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    Agreed, @Ironman -- I think that that is exactly what made the difference -- hence the comment on taking along time for that iron to cool down.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 11,058
    We will see as time passes, I should have realized this could be the problem.
    This was the priest's rectory, then became the parish office.
    Years ago when it had a CI boiler, the lady would come in at 8 AM, turn the heat up and it would get warm by noon.
    Then turn it down to 55 at 4 PM as told to.

    So added a programable tstat and aquastat to the piping to run the pump.
    Pump would run for nearly 1 hour after shut down.
    Lady was grateful to come in to a warm office, she processes the checks and I learned long ago to keep these employees happy.

    The CI unit was snooting up, throwing CO. Bad chimney, little draft. With 3 stories to the roof the ModCon was the solution.