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Pump Question

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Chester
Chester Member Posts: 83
in Gas Heating
I have a new Greenstar boiler installation with a total of three pumps: the circulator inside the boiler; a Grundfos Alpha that drives two heating zones; and a separate circulator for the indirect HW tank. My technician suggests leaving the Alpha on Autoadapt. The display says its pushing 4-5 gal/min when the larger of the two zones is calling.

The installer didn't change the settings on either of the other pumps and I just want to make sure I have them optimized. The Bosch manual says I should set the internal circulator to "the lowest possible setting" but doesn't provide any guidance about what that means. The factory default is 3 (out of 4). Is the goal to make sure the boiler pump is just moving some minimum amount through the heat exchanger or should the volume be matched in some way to the secondary side?

I think the circulator for the DHW tank is set to its highest setting. Is that where it should be?

I've also noticed that there's a purging function after the zones stop calling for heat. When the zone valve closes the Alpha and the internal circ keep running for a few minutes. This confuses me because there's really nowhere for the circulation to go -- the display on the Alpha drops to 0 gal/min and I guess the internal circ is just moving water through the primary/secondary loop. Is that a necessary safety function or something?

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  • SWEI
    SWEI Member Posts: 7,356
    edited October 2015
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    Adjust the internal circulator when the boiler is firing at full flame. Look at the supply and return temps and set the circulator speed so that you maximize the difference, but still stay about 5°F away from the limit.
  • Chester
    Chester Member Posts: 83
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    That SWEI. Will that technique work when the boiler is charging the hot water tank? That's the only time I see it operating at full flame, at least at this time of year.
  • SWEI
    SWEI Member Posts: 7,356
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    Not sure exactly how you have it piped, but as long the primary circ runs during a DHW call it should get you in the ballpark. After you've made the changes, be sure to check the event history for any high-limit shutdowns or very fast "xx call satisfied" followed by another call, etc. If you see those, move the pump speed up a notch.

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