"Tony Bryer" wrote in message
...
In article ,
Christian McArdle wrote:
I'm guessing a bit here, but
I suspect the taps actually control the input to the heater. The
tap spout is left connected and unvalved to the heater as a vent.
This would basically mean the heater is vented when the tap is off,
and pressurised when the tap in on. Obviously, proprietary taps must
be used for this.

When the water is being heated, some will drip from the tap as it
expands.

Exactly so: we have a notice on a PoU water heater in our church
kitchen explaining this. I don't know when they changed the rules but
the modern ones can be used with ordinary taps with the expansion being
taken up by the feed pipe if long enough, or expansion vessel if not.

The ones which use the mains pipe for expansion (and some combi boilers too)
will have a warning that they're not to be used without adding an expansion
vessel if there's a check valve or loose-jumper stopcock upstream of it. Of
course when someone comes along and fits a check valve or whatever they
probably don't think to check what's downstream of it ...