On Thu, 20 Nov 2008 21:20:55 +0000, Adrian
wrote:

If all that your switch is doing is to short out the relay contacts then
it shouldn't do any harm.... but do you know how the power to the motor
is cut when the door is at 'full open' or 'full shut'. If this is done
by the control box then adding a manual control across the relay could
be very dangerous...

I was going to use a momentary switch so I could stop it when I could
see it was at the desired height, but I will think more about that. I
will ask the seller but I expect they buy the boxes from someone else
and will not know about the circuitry.

If you're seeing sparks then they are coming from the contacts
themselves - which are on the mains side of the relay. Don't fit diodes
to these contacts - they won't last very long..... There might be some
point in fitting a high-voltage capacitor across the relay contacts....
but make sure it's one that's rated for mains use.

I'm not sure where the spark was coming from; I just know when the
relay switches off, it lights up blue for a fraction of a second.

It is a company selling doors, not someone just flogging an old one.
They have said they will post a new control box, though I worry will
the relay fail on that in time too?