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This might be a bit of an XY problem and I'm open to alternatives.

I have a vacation home with a boiler that powers both heating and DHW with an indirect water heater tank. Everything runs through a 24v zone controller with priority for DHW.

I want to be able to remotely turn the water heating on and off, but I can't power down the boiler or thermostats because I need to run the heater.

I think the easiest way to do this is using a common 120v smart switch that will activate a 24v contactor.

If the aquastat in the indirect is always disconnected it cannot call for DHW heat. But that thermostat is 24v, which goes to the zone controller.

Thus I can't simply switch the circulation pump because then the water heater will never satisfy and it will permanently take priority from the heating.

So how might I use a 24v contactor on a 120vac smart switch?

I'm thinking I can use something like OONO f-1022 https://czh-labs.com/products/dpst-1no-1nc-8amp-power-relay-module-ac-220v-230v-240v-control-voltage-1312

Connecting the control wires to the output of a lutron caseta switch

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    Your idea is reasonable, and that relay or similar type would be the easiest and safest way to achieve that goal, i.e., shutting heat request only for hot household water. The alternative would be to open the smart control and connect the internal relay directly to wires, without 220 VAC hot line -- but a mistake could create a shock or fire hazard. Commented Jul 10 at 2:47
  • There are "smart" relays which can be used from low voltage up to house current; the one I have needs to be wired into house current to power it but there are almost certainly versions that run on a low-voltage supply or batteries. Shop around.
    – keshlam
    Commented Jul 10 at 5:12
  • use a smart thermostat
    – jsotola
    Commented Jul 10 at 6:21
  • @jstola do you know of a smart aquastat?
    – Matthew
    Commented Jul 10 at 14:32
  • Seems like you've got the problem solved already. FYI, in case the lack of safety approvals or its AC input voltage range are problems for you, there are other vendors. Functional Devices' "relay in a box" RIB line is one. Also, your Caseta switch could power a receptacle, where a wall-wart power supply is plugged in, which in turn controls the coil of a relay.
    – Greg Hill
    Commented Jul 10 at 18:13

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