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Lochinvar sheild HIgh limit problem

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Hello,



I have a Lochinvar Shield Commercial Water heater giving me problems. Model SNA400-125, installed three years ago.



It keeps locking out on manual reset high limit when demand is at its lowest at night.

I have replaced the inlet and outlet temperature sensors, problem still there. Doent happen every night, it will run good durring the day, then it will start locking out for a couple of days, then it will run fine for the rest of the week.

Any ideas on what the problem would be?

Thanks

Comments

  • Zman
    Zman Member Posts: 7,579
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    Flow or temp

    Having the setpoint to close to the high limit temp could be causing the issue.

    Is the circulator working properly?

    Does the boiler have a postpurge circulator setting?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,271
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    could be the hx

    is getting limed up, check the temperature drop across the HX, see if it matches factory specs.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • operating_eng
    operating_eng Member Posts: 5
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    hi limit problem

    Set point is 148 deg F , Heat exchanger locks the unit out at 203 deg F, I dont have a sensor on the boiler heating loop so I cannot trend the boiler outlet temp at time of lock out.

    The pump is functioning correctly, seems to run hot but have yet to see it tripped out on its thermal overload, but that resets automatically. This boiler does have a postpurge cycle, the blower fan and the pump run during this time.

    It was reset yesterday morning then it locked out again around 3:00 PM, I reset it and now the boiler has made it through the night with no hi limit lock out.

    I am getting alarms from the boiler via the Building Automation System that only last a second then normal for for 15 min. This keeps repeting and does not show up at the boiler
  • operating_eng
    operating_eng Member Posts: 5
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    high limit problem

    I checked the delta T and its 15 deg. Inlet temp 147 deg F, outlet temp 162 deg F.

    Manual say anything above 25 delta T is bad
  • operating_eng
    operating_eng Member Posts: 5
    edited July 2014
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    High limit lock out

    The DHW heater was doing fine all night then at 11:00 am it locked out again. There seems to be no pattern for this high temp.

    Anyone have ideas?

    I tried to attatch a trend of the contant alarm to the BAS that does not show up on the unit so I have no way of knowing what the alarm is.
  • RobG
    RobG Member Posts: 1,850
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    Changes

    Have there been any changes to the system? Is it possible that any other maintenance personnel have "tweaked" any settings? What is the high limit aquastat set at? What is the manual reset high limit set at? Are there any leaks in the system allowing make up water?



    Rob 
  • operating_eng
    operating_eng Member Posts: 5
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    re changes

    The System has not physically changed since it was installed 3 years ago, the temp at the water heater was changed months ago before this problem has started. Because of the storage tank and bacteria concerns and code the DHW heater loop was temp was increased to 148 deg F. The heater maintains this loop temperature, the high limit for this loop is 203deg F.

    The building also has a loop that get heated by an injection valve that opens to add heat to the loop from the heater loop.The building loop has a set point of 116 deg F. This loop has a manual reset high limit aqua stat set at 140.

    Not sure what you mean about leaks, every time someone opens a hot water tap cold make up water is added to the loops.

    I have reset the heater again but changed the differential to 18 deg F from 5 deg F. so this will lengthen the time between cycles. Maybe my pump motor is getting too hot??
  • RobG
    RobG Member Posts: 1,850
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    Make Up Water

    Sorry about the make up water question, I was thinking about boilers. Have you tried contacting Lochinvar tech support? It could be a number of things, as I am not familiar with the unit the first thing I would do is contact the manufacturer if I went on a service call for the problem



    Rob
  • WMH2402
    WMH2402 Member Posts: 1
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    I'm having the same issue. Have you found out what your problem was? Thanks
    Jason_Henry
  • Jason_Henry
    Jason_Henry Member Posts: 1
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    I am also having these same issues. Has anyone found the correct repair. I have changed the heat exchanger inlet and outlet sensors. It is only 2 years old. Delta T is good. Circ.pump works good. Right now I have a digital camcorder on it with a test light on the circ. pump power to see if the control board is intermittently not sending power to the pump. Any other ideas?
  • hot_rod
    hot_rod Member Posts: 22,271
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    Did you contact the local reps or the factory tech support?

    If it is a product glitch there should be a technical bulletin on the possible cause and solution.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream