This is probably something glaringly obvious so I hope that you don't
mind me posting this question.
my son moved into his first house on Monday and the CH heating only
works when the water in the hot water tank is below the tank
thermostat which is currently set at 90, when it reaches this he has
to run the hot water to cool it down so that the CH will come back on.
He has a potterton EP2000 sp with a baxi boiler and honeywell valves.
In the airing cupboard there is a hot water tank with the thermostat
that runs the heating as well, potterton EP2000 sp,
a pump, two Y valves and a white 'control???' box
In the lounge there is a wall thermometer - it clicks to show under or
over temperature but does not start the boiler
The boiler is in the kitchen and starts up when you run hot water off
of the tank or turn the thermostat about the current temperature.
My son thinks that it is a wiring problem but he hasn't any plumbing
or wiring experience.
To recap - the heating only works when the water in the hot water tank
is below the temperature set on the hot water tank thermostat.
Not sure about the *two* Y-valves, but ignoring that it sounds like he's got
a Y-Plan system with a 3-port mid-position valve which directs the flow from
the boiler either to the radiators or to the coil in the hot cylinder or to
both at the same time.
Have a look at Y-Plan in http://content.honeywell.com/uk/homes/systems.htm
and see if it matches what he's got.
From the symptoms, it's a pretty safe bet that it's a wiring fault. With
this setup, the cylinder stat *must* be the change-over type, and the NO
contact must be connected to the grey wire of the 3-port valve. For good
measure, there also needs to be a Hot Water Off connection to the
When the hot water demand is satisfied, the boiler is turned on by a switch
in the 3-port valve's actuator. If there's no feed on the grey wire, you get
no boiler - simple as that. [When the HW is being heated, the boiler if fed
through the tank stat's NC contact - which is why he needs to keep the HW
below the stat setting to get any CH].
[If the HW OFF connection is correct, but the cylinder stat isn't, the CH
should work if he turns HW off at the programmer. Has he tried that?]
Just a guess but if the valves are zone valves (two port not three port)
then you have an S-plan system and the symptoms suggest the heating valve
motor has failed, the valve has been set to manual open (actually about half
open so water passes but the microswitch is not operated) thus when the hot
water is demanded that zone valve opens and operartes the microswitch in its
head giving power to pump and boiler and heat goes to both hot water
cylinder and heating radiators.
If you are electrically competent determine which (brown)wire in the white
"control terminal box" is the supply to the motorised valve serving heating
and see if it is live when heating is called for. If it is you should be
able to hear the motor drive, if you can't the motor has in all probability
failed. Spare motors are available for a very modest sum.
Update- son has just checked
the valves he has are two open shut valves (both appear to be working)
not y valves.
The thermostat downstairs seems to open and shut the valve but it
doesn't trigger the boiler and pump on.
The honeywell room thermostat in the lounge has 2 x 2 (2 larger wires
which contain a red and a black in each) be he can't see where these
should connect in the airing cupboard.
There should be a control terminal box in the locality of the valves where
the main interconnections are made. IIRC you said there was in an earlier
Both Honeywell water and heating valves should be connected into this via
five wire flex.
Each valve will have a opening signal to its motor via a (usually brown)
wire and if the valve is opening then it must be present ok. Also each valve
will have a neutral (blue), an earth (green/yellow), a permanent live feed
for controlling the boiler and pump (grey) and a switched or controlled
signal wire (orange). The greys will be joined together and connected into
the live supply, the oranges will be connected together and joined to the
wire(s) feeding the pump and boiler. The valve heads contain microswitches
which close their contacts when the valve is fully open. The grey and orange
wires are simply connected to these.
With a suitable test instrument (and we don't need a clamour about not using
neon screwdrivers here thanks folks) check if the live feed is present to
the grey wire to the valve serving the heating and if it is, continue
checking to see if it arrives at the orange wires in the terminal box. Note
here that manually operating the valve with the trigger will not cause the
microswitch in the unit to operate so energise it via the programmer and
I suspect from afar that you may have either a cocked up wiring scheme of
one of the wires described is not making contact somewhere along the route.
possibly one of the greys or oranges is broken off under a screw in the
terminal. Honeywell valves don't usually suffer from microswitch failure but
the same cannot be said about a lot of their competitors.
I come across a LOT of wiring centres/junction boxes which do not conform to
the industry standards but you may find it helpful to google for honeywell
or S-plan and have a look at a wiring diagram for how it should be wired
Refer to the S-plan wiring on the Honeywell link posted by Set Square.
If the valves are moving, the fault is probably with the micro-switch
(2 grey wires from terminal 1 live to microswitches, 2 orange wires
from switches to terminal 10 on the Honeywell schematic; terminal 10
provides a live feed to the pump and boiler). Your terminal numbers
won't necessarily correspond to the arrangement shown.
From your initial post, I assumed it was a Y-Plan system. I now see that it
is an S-Plan system. Have a look at the S-Plan wiring diagram at
You will see that the stats control the opening and closing of the valves,
and that each valve has an additional set of contacts which close when the
valve opens and switch the boiler and pump on. It's the wiring to and from
these additional contacts which you need to check very carefully. Both
valves should have a permanent live feed to their grey wires, and both
orange wires must be connected to "switched live" on the boiler (via
terminal 10 in the junction box if the standard diagram has been followed).
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