Is there anyone out there who can tell me how this circuit would in any way regulate the kW that make it to the stoves heating elements? Maybe it is that all regulation of the 240v 1pH dual pole connection is a just a simple function of the heating elements in the stove?

The board is a WX361 controller board from Harvia and uses contactors (ZSL-940) to open and close the power to the stove as far as I can tell, and it seems to pass the power straight through the contactors to the stove.

I think there are two contactors there simply to allow the overheat protection (A1 and A2) to cut power on overheat. Is this true if you can tell? (opinions welcome!).

There are settings in the controller to set the control unit grade, but I can not see how that changes how much power would be sent through the contactors. Bonus points if you can explain how this setting works, "CONTROL UNIT GRADE SETTINGS" and double bonus points if you can explain the grade that would be best chosen for a 9kW unit that has 3 elements that will be grouped.

If I use this board, it will now be hooked up 240v 1Ph with a neutral.

I have included the wiring diagram for the 9kW stove also at the bottom.

Harvia WX361 Diagram: Harvia  WX361 Diagram

Harvia WX361 Board: Harvia WX361 Board

Harvia ZSL-940 Contactor: Harvia ZSL-940 Contactor

Harvia 9kW Heater Wiring Options: Harvia 9kW Heater Wiring Options

Edit to add context to the "Control Unit Grade Settings":

Typical back of heater tag with settings (mine does not have this): Back of heater chart

Control Panel Setting Option: Control Panel Setting Option

  • Typical heating element control is on/off. Rather than control the KW when on, the thing that is usually varied is how much on time .vs. how much off time. Sometimes described as "bang-bang" en.wikipedia.org/wiki/Bang–bang_control (well, SE formatting breaks the link no matter what I try, but you can see what to type...)
    – Ecnerwal
    May 18, 2023 at 18:07
  • As with most modern products, the control logic isn't on the diagram, it's going to be in software running on the microcontroller.
    – Ecnerwal
    May 18, 2023 at 18:14
  • So, if i'm understanding correctly, the logic board probably flips on the contactor (rapidly or every once in a while?) based on the CONTROL UNIT GRADE SETTINGS? 2nd question is, do you think that the second contactor is strictly needed or is it simply to cut power if the overheat protector is tripped? I guess I can't understand why there would be two other than that. @Ecnerwal, Thank you so much already!
    – Scott M.
    May 18, 2023 at 19:28
  • Preventing overheat is not different than strictly needed. Sometimes contactors weld their contacts shut. Hopefully two don't do that at the same time. If it's there, it's because it needs to be there to meet safety requirements, so deleting it would be risking broiling people or lighting the place on fire. You've provided nothing to inform any response about "control grade settings" which are not shown on any of the pictures you posted, as far as I can see.
    – Ecnerwal
    May 18, 2023 at 19:34
  • I have aded the portion of the manual for the controller that pertains to the unit grade settings to the original post for context.
    – Scott M.
    May 18, 2023 at 20:28

1 Answer 1


Is there anyone out there who can tell me how this circuit would in any way regulate the kW that make it to the stoves heating elements?

Probably using Pulse Width Modulation; essentially turning it on and off. The room has thermal mass, so turning it off doesn't have an immediate effect; some time will pass before it's noticeable.

Turning it on and off over a cycle of minutes can give whatever average power output you want. If you couple that with a somewhat intelligent controller (e.g. PID) you can get it to keep a fairly stable temperature in the room.

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