On Wed, 1 Mar 2006 14:37:41 -0000, "Roger Mills \(aka Set Square\)"
wrote:

In an earlier contribution to this discussion,
CampinGazz wrote:

i'm fitting a home style heating and water system to my motorhome, a
12 volt eberspacher 5KW water boiler, a 20 litre twin coil calorifier
(hot water cylinder with an extra coil to heat from the engine) 3
radiators, digital programmer, tank and room stats, and hopefully a 3
way valve to select hot water, heating or both to be heater when the
boilers running.
But i just can't seem to find a 12 volt DC powered 3 way valve, the
mid position type that's fitted to 99% of home central heating
systems,

You'd probably stand more chance of getting 12v on/off valves - rather than
a 3-port valve - and then use 2 of them, as in a domestic S-Plan rather than
Y-plan system. I don't know about secondary contacts which close when the
valves open - you may need to use relays to create that feature.


Even so, this is not quite as foolproof as two port S-plan valves
because the latter have a mechanically linked microswitch which can
only operate if the valve really is open; I suppose unless the
contacts weld shut or something.

With a relay across the coil of a solenoid valve, you don't know if
the valve coil is open circuit or the valve stuck.

Presumably a boiler should protect itself in case of pump failure so
one would hope that it doesn't all go bang or I suppose another way
would be to include a flow switch and use that to control the
boiler.... That would be fail safe at least (unless its contacts
welded shut of course.)





--

..andy