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Will a circulator on return damage a Amtrol Boilermate?

I am replacing a WH-41Z due to corrosion, otherwise it worked fine for over 10 years (age unknown). We've since adjusted the pH of the water and I'm in process of plumbing everything as it was. According to the manual (page 4) it indicates that "Be sure the return line is NOT plumbed to the suction side of any heating circulators. This may require moving the heating circulator off the boiler tapping on packaged boilers. Failure to do so will result in overheating and tank damage when the heating system is in operation."

Our oil hot water boiler has two return circulators on the return, one for the baseboard heat return, the other for the boilermate's return. The boiler's send (post expansion tank) T's out to the baseboard's two taco valves for the two heating zones, and the send for the Boilermate.

I can't see why this would cause damage to the boilermate. It appears to be a uniform coil inside the tank, and the unit worked fine for years before the Aquastat and it's well housing failed from corrosion. I've seen other systems setup this way, but not clear why Amtrol warns about overheating and tank damage.

Can someone shed some insight on this?
Much appreciated,


  • Ironman
    Ironman Member Posts: 7,376
    I think what they’re trying to say is “don’t pipe it into the suction side of any SPACE heating circulators”. In other words, don’t pipe it so that the space heating circulator is in series with it.

    That may require removing the space heating circulator from where it comes mounted on a packaged boiler and installing a Tee on its discharge side to accept the return from the Boiler Mate.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Ironman
    Ironman Member Posts: 7,376

    Also, you’ll need check valves on each circulator to prevent flow from one zone going into another when that zone is turned off.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Zman
    Zman Member Posts: 7,569
    The Amtrol tank coil has pretty high flow resistance. They are probably concerned about the boiler water flashing to steam and damaging the tank or coil. This can happen when the circ is pumping into the expansion tank with high resistance on the suction side. Where is the expansion tank in relation to all this?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    JUGHNE Member Posts: 11,062
    For the same reason you pump into ModCon boilers on their inlet/return.
    Keeps the pressure up inside the coils.
    EBEBRATT-Ed Member Posts: 15,534

    The manual does specifically state that the pump can't be on the boiler mate return line it must be on the supply pumping into the boiler mate.

    They show the expansion tank on the boiler supply in the usual spot.

    It must be due to the coil pressure drop

    If you have multiple pumps the only fix is to repipe and move both pumps to the supply pumping away from the expansion tank.

    The other fix would be to go to one pump with 2 zone valve 1 for heating and 1 for the dhw. You could cut 2 zone valves into the supply piping and move the expansion tank to the common return pipe so you wouldn't have to move the pump but it might cause the relief valve to open.

    Or take your chances and hook it up the way the old one was. I am sure there are thousands hooked up that way