MaterTemp/Max-E-Therm 3-wire control question

[CA_geek]

I am replacing my heater with a Max-E-Therm 400k BTU heater. (Same electronics, etc as a MasterTemp 400, but was a tiny bit cheaper and DW likes the rounded shape better than the boxy shape)
As part of that, I have plumbed in a bypass valve so that I can bypass the heater. And I put a valve actuator on it and hooked it up to the heater control board.
I have an EasyTouch 8 automation system (with ScreenLogic so can control from phone/tablet)

I've found that EasyTouch doesn't speak RS-485 to the heater.
I didn't realize that until after I had everything hooked up and was searching online to try to understand what's going on.

So I've switched over to the 2-wire (firemans switch).
With the fireman's switch, it basically powers on/off all the heater controls - the heater's screen goes blank when the fireman switch is off.
And that means the bypass valve stays with the water going through the heater after the heater turns off.
If I use the heater control panel, pressing the "Heater off" button, the heater goes into "Heater Cooldown" for a while, then moves the valve.

SO:

The heater has a "3-wire control" option as well as the RS-485 and 2-wire.
I read the 3-wire control basically replaces the spa/pool heat on buttons.
My understanding is that there are 3 wires - a "pool", "spa", and "common".
If you short "spa" to common, it's like pressing the spa-on, if you short "pool" with the common, it's like pressing the "pool-on" button.

My question is - is it a continuous connection or momentary?
Do you have to keep "spa" and common connected for it to keep going on the spa heat?
Will the heat turn off if you break the connection? (And go through the cooldown phases, etc?)
OR is it really like pressing the spa button? ie. you connect common and spa for a moment and it will keep heating until it hits the temp requested or someone hits the "heater off" and only then will shut down.

I'm hoping it's continuous - that making the connection is like hitting the "Spa" button, and breaking the connection is like hitting the "Heater off" button.
IF that's the case, then I think I can hook up the "Spa"/"Common" to the "Heater" connector in my ET8.
And that will provide me with everything I want. The bypass valve will move properly (plugged into the heater circuit board) And the water temp will be controlled by the ET8 closing/opening the "heater" connector.

 

[ajw22]

It is easy enough to connect a few wires and test the heater’s behavior when you touch the spa control wires together.

 

[CA_geek]

So I tested shorting the "Spa" to "Common" on the 3-wire control.
And it behaves as I hoped and somewhat expected.
While they are connected, it starts going through the cycle as if you had hit the "Spa" button. And when you disconnect, it's like you hit the heater-off button. So I connect the wires, It starts moving the bypass valve, then after a certain amount of time, starts the blower, then starts the fire. And it will continue to run so long as the wires are connected (and that it hasn't hit the temperature set on the heater). Then when I disconnect, it goes through the shutdown sequence - shutting off the gas, then waiting some amount of time for the heater to cool down before moving the bypass valve back to the bypass position.
So it seems to me like using the 3-wire control interface is much better. Don't know why the manual has you replace the jumper by the fuse instead of using 2 ports of the 3-wire control.

 

[ajw22]

Don't know why the manual has you replace the jumper by the fuse instead of using 2 ports of the 3-wire control.

The actuator bypass valve is a recent addition to the heater board. Before that it didn't matter if the heater powered down immediately when turned off. The "fireman's switch" has been used as the standard automation heater controls for many years.

Installation manuals are not very good at guiding the various automation control options - firemans switch, 3-wire control, RS-485.