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  1. #14
    Join Date
    Jan 2006
    Posts
    32
    You've probably hit the nail on the head there. It would be about 124 F for a 7 deg design day, or 12 degrees over where I've set it. Of course the temperature doesn't stay at the target+1/2 differential temp, but drops down 24 degrees over the cycle. But since the air handler delivery efficiency is biased towards the higher temps since the heat transfer rate drops on the lower end of the water temps, your suggestion makes perfect sense (and would be borne out in practice since it dropped to 7 F just after the sun set and lo and behold the target temp is 124 with the RTU sensor set to off).

    Mystery solved ! Design day boiler temps set on the Tekmar need to account for the boiler differential.

    Thanks, this has been a useful exercise.

  2. #15
    Join Date
    Feb 2003
    Posts
    1,996
    The 26X's aren't perfect controls. I wish Tekmar offered some optional settings. I can't speak for the Buderus Logamatic controls, but for half the price Tekmar does a nice job. At least we're saving more then enough fuel to pay for the controls very quickly.

  3. #16
    Join Date
    Jan 2006
    Posts
    32
    Yes indeed, for the price they are good. Things I wish I had on the 262.

    1. A settable differential for the DHW. It's hard coded at 6 F. One cycle of my boiler doesn't quite do 6 degrees, so it takes two cycles and the water ends up much hotter than the 120 F setpoint. Comes on at 117, runs to 122, somes on again at 122 and runs to 128.

    2. Adjustable DHW purge. On the warmup from the DHW night time setback (114F), the controller thinks it is taking "too long" to heat the DHW, so it runs both the DHW and the Air handler circ pumps at the same time. This makes the DHW take even longer to heat up. So I use a relay that is an interlock and while the DHW pump is running, the AH pump circuit is interrupted.

    Both problems may be related to my very small boiler. If it was 100kbtu I think neither of these would be an issue. But of course it would be 5 times oversized for my design day losses in space heating !

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