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Two expansion tanks

I just finished reading Dan Holohan's Pumping Away and have a couple of questions.
I have a recently installed Monitor MZ 25C condensing boiler that has built in Grundfos 1542 3 speed circulator and an expansion tank. It appears that the internal expansion tank is before the circulator on the return side.
When they installed the unit (two zones w/ zone valves), hot water baseboard and a dhw tank using 3 way valve) with an Amtrol mod 30 expansion tank on the supply side.
#1 - Pumping Away talked about the different in psi from discharge and the suction side. With my system, I have the circulator just after the internal expansion tank but before the external expansion tank. What if any effects does this setup have on the PSI?
#2 - this being a condensing boiler it wants the return temperature @ 130 in order to attempt acheiving max efficiency. My return temps are averaging above 145-150. How do I cool my return temps to below 140?


  • Ken_40
    Ken_40 Member Posts: 1,320
    The MZ's internal circ.

    was never intended to provide loop circulation, merely primary circulation. The expansion tank is fine as is for everything but huge gravity style radiators with gallons of water.
  • Brad White
    Brad White Member Posts: 2,398
    More from the World of MZ's

    So, ****, Barbara- How is it going with your MZ?

    Your last question on how to get your HWR below 140 (really below 125 if you want to condense):

    This all goes to your heat loss and the amount of radiation you have. If you have X amount of radiation and your heat loss is Y, then your design water temperature is Z.

    The higher your radiation area (X) is, the lower your design water temperature (Z) can be.

    If your current radiation ratio requires 170 degree water and your flow rate is as low as you can get it (for maximum extraction), then your return temperature "is what it is".

    You would need to add radiation such that it would serve the heat loss when supplied with (my recommendation) 140 degree water on a design day or a 130 degree average water temperature. Thus, on the coldest days, my return water comes back about 120 or even less. When it is warm out, my outdoor reset drops this further.

    Do you have OD reset?
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
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