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Tankless to Heat exchanger control

does anyone have a suggestion for a control to a pump between a tankless water heater and a 100 gal. heat exchanger tank (HX).  I want the pump to come on when the  HX is less than 120F and off when the HX equals 160F.  these temps are approximate.  Also only want the pump to run when the radiant pump is running.  In other words when the thermostat(s) are calling for heat. 

thanks for any ideas...this is the third time I have typed this..hope all this posts.


  • Ironman
    Ironman Member Posts: 7,119
    edited October 2013
    Tankless Control

    The use of the tankless for a heat source is gonna be your biggest obstacle. It's designed and controlled to have 50* inlet water with 125* outlet - a 75* delta T. It also has a very large head loss through the heat exchanger and is probably designed for 6 gpm or less. Its controls will fire and modulate the burner based on these design parameters.

    Some models will allow you to set the temp to what you're describing, but most won't. Also, at those temps, you won't get the full firing rate once the hot return water hits the tankless, even if it does allow the higher settings. Simply put, it's the wrong appliance for space heating. Unless it has an ASME "H" stamp, it's not approved for space heating.

    Even if you manage to get the setup to function, the tankless will have a very short life and have little or no warranty.

    This scenario comes up almost weekly around here because folks think they will save money by using a cheaper appliance than a boiler. My response is always the same: "If a tankless could take the place of a boiler, then there'd be no need to make boilers".

    That being said, if you still want to proceed, then I would use a Taco SR50* pump relay to control the radiant side pumps and use a Taco SR501 to control the tank charging pump. The SR501 is a single pump relay. Run its "T T" or "R W" in series through the tank's aquastat and on through the "X X" (isolated end switch) of the SR50* that's controlling the radiant circ's. Make sure there are no factory jumpers on the "X X" terminals.

    The SR501 terminals will have some different markings from what a multi zone SR50* will have. If you can use a single zone SR501 on the radiant side also, the use the "C" and "NO" terminals on the right side in place of the "X X" terminals that the multi zone has. Place the radiant circ on the left terminals in this scenario.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
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