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showing the coldest water to GB-142

I believe that the Vitodens 200 is the only mod-con currently available with built-in facilities to control a variable-speed circulator.

Variable-speed circulators are now available in the U.S. but I have no idea if they can be used in the primary loop of any mod-con. Perhaps delta-t modulation will work, but I have a feeling it might mess with the brains of mod-cons unless they expect this to happen. Check with the manufacturer before you attempt.


  • archibald tuttle
    archibald tuttle Member Posts: 1,085
    showing the coldest water to GB-142

    Looking at a GB-142 for a job but I'm wondering if the primary pump can be modulated to keep the primary flow in line with the load in order to maintain condensing efficiency.

    Although it is handy in a way that they supply the boiler pre-piped with a primary loop, if the small boiler loop is circulating a greater volume than one the load loops running at any given time, then I'm going to be getting bypass that might return to the boiler at a high enough temperature to frustrate condensing, especially with only a single zone calling.

    Does the onboard pump and or onboard controls have any modulating feature, manual or automatic and/or should I just be sizing loop pumps with a higher volume in mind. Presumably this could be handled to some extent by the modulation of the gas input but I'm not sure if that modulates according to flow rate or some design awareness of loads that is input via some means into the onboard controls or???



    PS- I imagine this may have been hashed over in the more technical primary/secondary discussions as well as particularized disussions of these Buderus units but I imagine I would get a million hits on a search for those terms, but maybe you recall a recent thread addressing some part of this question and I would obviously be as glad of that link as of the great direct input I often find here.


  • scott markle_2
    scott markle_2 Member Posts: 611
    primary pump

    Brian, the primary pump on the gb is a 3 speed grundos 15-58. If your secondary flow rates are low you may experience a primary flow which is greater than secondary (even with the primary pump set on low) and some undesirable mixing that dilutes (warms) the return water will result.I have first had experience with this. The boiler output can be capped so it seems to me that as long as flow thru the boiler is sufficient to move a limited output a smaller circ could be used. However I don't think Budarus views it this way, they want a primary flow which can move the FULL output of the boiler and probably a bit more.

    If this was my own system I would consider using a up 15-10 for the primary and capping the btu output slightly below the (btu equivalent) flow that this circ. could produce thru the hx. But I'm not comfortable experimenting like this on other peoples systems.

    Ultimately the loss of efficiency that a few degrees of undesirable P/S mixing may produce is probably not that significant,and the operational reliability factor that this primary loop over-pumping affords may be worth the cost.
  • archibald tuttle
    archibald tuttle Member Posts: 1,085


    thanks for that highly intelligible explanation. I assume that the pump sizing of the 15-58 means it will handle the full BTU output even on low speed.

    Maybe one kind of answer would be to gang loads in series to increase the delta "T" (maybe including larger pumps to move the BTU's through more pipe loss of series load). I'm think, this system has hot water radiators upstairs and radiant tubing under the first floor installed with aluminum pans (whens that miracle glad wrap for warm floor coming that was depicted in FLIR photos here last week??). Maybe IF things were balanced nicely I could run boiler water through the rads and then through the floor and just use a setback mode for one or the other load depending whether it senses feed or return temp.

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