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Hydronic Heating with Tekmar Control - Intermittent Problems

We have hydronic heating system and Tekmar 256 Boiler Control installed in our fourplex. The following are the current settings in the Tekmar:


Room occ


Room unocc


Outdr Design


Terminal Unit


Boiler Indr


Boil Design








WWSD unocc


Boiler trip


<span style="font-size:12pt">We have heating problem intermittently. The following is our observation:</span>

<span style="font-size:12pt">We have set the boiler target temperature to 45C (113F). Accordingly, the boiler flame should shut off when the boiler water reaches this temperature. However, intermittently, we note that the boiler water temperature reaches up to 65C (149F). Consequently, in such circumstance, the limit switch on the boiler trips off. Somebody at the property needs to reset it to restart the boiler to keep the heating system running.</span>

<span style="font-size:12pt">Has anybody got any idea,what is wrong in our case?</span>


  • JStar
    JStar Member Posts: 2,752

    What is the heating curve? What are the minimum and maximum supply temperatures? Do you have any thermostats controlling the "Demand" terminals?
  • zacmobile
    zacmobile Member Posts: 211
    target vs. design

    are you sure it's target temperature that you set? the only  place on the control where it says target is in the display menu which is non-adjustable. to set the maximum boiler temperature (which will be establishing your heating curve) you want to set "BOIL DSG" which looks like is set for 160F and with a differential of 9F you will see the boiler hit 165.5F when it is -5F outside, so that sounds like what is happening here. what is the outdoor temperature right now and where are you located?
  • rsuleman
    rsuleman Member Posts: 3
    Target vs Design

    We are in Toronto and outside temperature is 23F (-5C).

    Yeah, as I wrote earlier, Boiler Design is set at 160F.

    Do you recommend to change any of the setting mentioned in my earlier post? If yes, what it should be in?

  • zacmobile
    zacmobile Member Posts: 211
    a couple more questions

    I can give you my quick & dirty method for setting the 256 but first, what kind of emitters do you have? cast iron rads, radiant floor, baseboards etc. and what is your desired indoor air temperature? and what kind of boiler do you have? (condensing, cast iron, copper tube etc.)
  • rsuleman
    rsuleman Member Posts: 3
    a couple more questions

    Emitters are cast iron rads, desired indoor air temperature is 65F and boiler have copper tubes.
  • zacmobile
    zacmobile Member Posts: 211

    OK, based on your info I would do these settings:

    ROOM OCC = 65F

    ROOM UNOCC = Doesn't matter unless you have a timer connected

    OUTDR DSGN = -1F for Toronto

    Terminal Unit = 5

    BOIL INDR = 65F

    BOIL DSGN = I'll get to that at the end

    BOIL MIN = 130F to protect the boiler from flue gas condensation

    BOIL DIFF = I would do 'Ad' for adjustable, and the 256 will figure it out

    WWSD OCC = 65F

    WWSD UNOCC = Doesn't matter unless you have a timer connected

    C/F = whatever you want

    Now to adjust the BOIL DSGN I would probably start at 130F and turn it up a degree a day until people stop complaining it's cold in their apt. this setting will be the maximum required system temperature at the coldest day of the year (OUTDOOR DSGN -1 for Toronto) You probably should raise the high limit on the boiler to 160F or so as well, this will be well within a safe operating range for your boiler.
  • Randy Baerg
    Randy Baerg Member Posts: 26
    tekmar 256 boiler operation

    The tekmar control uses a split differential.  The split differential is 1/2 above target - 1/2 below target. So it will always fire above the target. The average temperature output will therefore be equal to the target.

     This "Automatic Differential" can also dynamically adjust itself. The control watches how fast the boiler temperature is changing (an indication of the system load) and will get smaller or larger. If the temperature is rising rapidly this indicates a small load on the boiler. Small loads can lead to short cycling so the control will expand the differential to help prevent this from occurring.The differential is constantly adjusting every cycle.

    The boiler aquastat / high limit is no longer the 'boiler control'. The settings on these should be set high enough they do not interfere with the operation of the 256 control. Set the operator at least 25°F higher than the boiler design temp entered in the 256 control. and the high limit above that. This should allow the tekmar 256 to control the boiler and prevent it from tripping on high limit.
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