hot water / central heating problem

[Elizabeth Regan]

Please can anyone help.

I have a 7 day timer for my hot water and central heating. When I set the timer for the central heating to come on, it does, but the hot water gets hot...very hot (over 80 degrees)as well. No light on the timer comes on to show the hot water is on. The hot water will come on with the timer when it is set.

Having the hot water heat up everytime I have the central heating on is causing the hot water to get very hot and the tank thermostat is not stopping it. (For some reason there is a permanent live going to the thermostat on the tank as well?) I have now burnt out 2 thermostats!

An electrician came to check the wiring in my timer and said it was all fine. My plumber came several months ago and fitted a new motorized valve - he put in a 6 wire one instead of a 5 wire (to replace the old 5 wire) - there were no 5 wires available and he said it would make no difference....BUT could this be the cause of my problem? OR could there be a crossed wire between upstairs where the tank etc are and downstairs where the timer is? Would a crossed wire cause the hot water to come on everytime the central heating does?

Please help me someome. My plumber and electrician are completely stumped, my daughter burned her arm badly in the boiling water and my gas bill is rising!

[John Stumbles]

Do you have a 3-port valve - something like this

2 2-port valves, like 3-port valve probably means you've got a 'Y' plan system, 2-port valves are usually in an 'S'-plan system

There should only be a live when the programmer is calling for hot water

Honeywell have wiring diagrams for many heating/HW configurations at

However you may have to interpolate between these diagrams and the actual components you have if they're other makes than Honeywell. Unfortunately this is an area which falls between the traditional skill areas of electricians and plumbers, some of whom willbe thrown by something that's not straight out of the book. It doesn't help that some CH systems seem to be pretty much thrown together with wiring on choch-blocks crammed into a tiny wiring box with no labelling which makes verifying the correctness of the setup, or fault finding, very laborious. If this is outside your own competence you'll have to find yourself a heating engineer who does understand such things. It's not rocket-science, but as I say not every plumber groks it.

[John Stumbles]

...

Aside from what I said in my other post, if you turn down the thermostat on your boiler it will reduce the temperature the hot water gets to in your cylinder (and therefore your taps). Obviously this is only a stop-gap until you get the system fixed properly. It will also reduce the temperature of water in the radiators which may mean you don't get enough heating when it's very cold outside, so you'll have to find a balance.

[John Rumm]

OK there are several ways for this to be setup. but from your description it sounds like you have a Y plan system:

water comes from the boiler to a three port valve, and then goes either round the heating circuit, or the hot water circuit, and possibly the option being split between both depending on if the valve can work in a "mid position".

It sounds as if your valve is stuck[1]. Since you say the rads and the hot water get hot then that would suggest it is stuck either in the mid position on somewhere close to it.

Where is the valve? Is it easy to get at? If so, have a look at the leaver on the side (most have one):

leaver can be used to lock the valve into the mid position (handy for when the motor on it fails) so check it is not locked in that position.

[1] Stuck can mean many things:

a) The actual valve body (i.e. the water carrying bit) may be seized and hence the motor on top is not able to turn it.

b) the motor may be dead

c) the wiring may have a fault

d) the thermostat on the cylinder may have a fault

A bit of detective work on your system will help identify which is at fault. Can you set your programmer so that it asks for heating "Off" and hot water "On". Do the rads then cool down? Is the hot water still getting heated (a quick fondle of the pipe leaving the middle of the cylinder should tell you).

If the rads do go off, then that tends to eliminate (a) and (b) since the valve must have moved from mid to HW only position. If the system used to work and nothing has changed, that makes (c) less likely. So my money would be on (d) possibly with a bit of (c) mixed in!

If the cylinder stat was permanently calling for heat (and the wiring neglected to put the programmer in series with the stat such that it can override it and turn the HW heating off) then that could give you the symptoms you describe. If you turn the dial on the cylinder stat do you hear a click from it at any point?

What you do next depends a bit on how comfortable you are with a multimeter and prodding at wires that may have mains on them.

[Roger Mills]

valve which refuses - for either mechanical or electrical reasons - to move to the CH-only position, the thought occurs to me that it might *just* be a C-Plan system. This is a gravity HW/pumped CH system with a motorised valve in the HW circuit to stop gravity circulation once the HW is up to temperature. If the valve is wired incorrectly it could certainly cause the reported symptoms.

To the OP: How many water pipes connect to the boiler, and what size are they (22mm, 28mm etc.)? Where is the motorised valve? How many water connections does it have? Where do the pipes go which connect to it?