In an earlier contribution to this discussion,
Set Square wrote:

When I get time, I may look in more detail as to actually
what will happen. But you're likely either to end up with the boiler
not on when it should be - or on when it shouldn't be. It may not
actually do anything dangerous - just undesirable!

Having looked into this a bit further, my considered opinion is as follows:

3-port valves come in 2 flavours - diverter valves, and mid-position valves.
A diverter valve is relatively simply, having only 2 positions. The incoming
water is directed either to the HW circuit or to the CH circuit - but never
to both at the same time. Some valves have secondary contacts which switch
when the valve moves from HW to CH. If the 3-port actuator in question had
identical wiring colours to the 2-port one which it replaced, the chances
are that it came off a diverter valve rather than off a mid-position valve.
If this is the case, it has a very good chance of working satisfactorily.

An actuator for a mid-position valve does *not* have exactly the same wiring
colours as that for a 2-port valve. In addition to orange, grey and blue, it
has a single brown/white wire rather than separate brown and white wires.
Assuming that this is connected in place of the 2-port valve's brown wire,
the following will happen:

When hot water is demanded by the programmer and cylinder stat, the valve
will start to open. When it gets to the half-way stage, the first
micro-switch will throw - connecting the motor to permanent live on the grey
wire. This will cause the valve to continue to open - and it will remain
fully open for ever thereafter. The second micro-switch will also throw
(just after the half-way point) and will connect the HW demand to Orange -
causing the boiler to fire, and the pump to run.

When only HW is required, the system will work ok - with the boiler and pump
going on and off according to the demand. However, whenever the boiler and
pump are turned on in response to a CH demand, the HW will get hotter and
hotter (to approaching boiler temp) even though its demand is satisfied -
because the HW zone valve is *always* open regardless.
--
Cheers,
Set Square
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