The previous mention of the electric, four port heater valve linked to sources for the valve, but without any method to control that valve.
there is a valve sold by old air products that includes the necessary controls. I previously used one of these units and it performed fine for me.
Larry
FWIW, Thermotion sells a controller for the valve, but it costs another $69, so the Old Air Parts link Larry provided is a much better value.
https://www.thermotion.com/4-p...l-valve-pn-354-69694