In an earlier contribution to this discussion,
Andy Johnson wrote:

Hi,

I know very little about plumbing, and am looking for some advice on
diagnosing a problem.

We have what I suspect is a fairly average heating system. A
Potterton Neataheat boiler and controller box downstairs in the
kitchen and a hot water storage tank upstairs in the bathroom. The
pump is next to the tank and feeds into the tank and radiator system
via a couple of motorised three-way valves.

A few of days ago we stopped getting hot water through the taps.
Here's what I've obsrved:

- If we turn on only the heating then the boiler fires up, the pump
starts, and the radiators heat-up.
- If we turn on the heating and hot water then the boiler and pump
star and the storage tank fills with hot water. The 'lever' on the
back of the motor controlling the heating valve moves, so I think we
get hot water in this case because the heating-circuit valve is
opening on command and the pump is moving hot water through both
valves. - If we turn on only the hot water then the valve motor
appears to open the heating circuit valve, but the pump does not
activate. Neither does the boiler.

I'm hoping for some advice on what could be causing this. The valves,
pump, and boiler appear to be okay. I suspect that the thermostat on
the hot water tank is faulty, and the pump and boiler aren't starting
because the system thinks that the tank is full of hot. Thats just a
guess, though, because I don't know much...

I'd be grateful if anyone can suggest things I should check to solve
this. In particular, how to test the tank thermostat. I'm fairly
proficient with electrics, and can get access to a test meter if
necessary.

Thanks, Andy

I'm slightly confused by your description - because you would normally have
ONE three-way valve or TWO two-way valves, but not TWO three-way valves. So
I will assume they are two-way valves - each having just an inlet and an
outlet for the water.

If I am right, you have an S-Plan system. You can see both a schematic
diagram and a wiring diagram by looking at S-Plan in
http://content.honeywell.com/uk/homes/systems.htm

The principle of operation is that the motorised valves are controlled by
the programmer and thermostats (room stat for the CH valve and cylinder stat
for the HW valve). Each valve has an additional set of contacts which close
when the valve is fully open. These contacts are totally separate from the
motor wires, and are used to turn on the boiler in pump. So as soon as
either (or both) valves open, the boiler and pump start up.

From your description, it sounds as if your HW valve is opening (so the cyl
stat is ok) but it isn't turning the boiler and pump on. However, if the
boiler and pump are turned by the CH valve, the hot water gets hot because
the HW valve is open.

So there appears to be a problem with these so-called volt-free contacts in
the HW valve. Either they're not closing, or something has become
disconnected.

Armed with the wiring diagram from the above reference, you should hopefully
be able to isolate the problem.
--
Cheers,
Set Square
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