Can you describe how this works. Just turn the thermostat down ? Is there a valve which reverses the water flow ?
Yup, just turn the thermostat down and if the 4 way valve shifts ok then you will have heat. I have a 1990 Carver 3607and have been comfortable when the outside temps were near freezing.
4 way valve Is this 4 way valve located in the unit itself or down in the hull ? Should i hear it click or make a noise when it activates ?
it's in the system, your A.C. is really a open loop heat pump. look those up online and it'll help you understand how they work, I use mine all yr in Virginia on the lower Bay, And they'll run you out of the cabin. just try them out and see how they work, Just don't try to go from cold to heat fast you'll trip the breakers..
There is a reversing valve located near the compressor, when you turn the thermostat up to the desired temperature the valve reverses the flow of refrigerant and the hot gas flows through the evaporator which is blown into the ambient space. You may also have a switch to engage the unit into the heat cycle.