HW works ok but no CH

[Rod1956]

Hi - please can anyone offer any help on this.

On friday I had a power cut (maybe unrelated) ever since I have no central heating.

I have a Goloworm Hideaway. I have a HW/CH controller which I use on timer for HW and permanently on for CH. A room thermostat controls the heating.

The hot water is fine - the boiler fires up. But the boiler will not fire up when room thermostat clicks in when set above room temperature (currently 15 degrees).

I have checked batteries in thermostat. There is water in header tank. I have reset main controller.

Although this is an old boiler - virtually every part has been replaced during last 6 months - including the heat exchanger.

Please can anyone help on this?

Thanks

Rod

 

[jackthom]

Hi, is there a 3 port motorised valve in the system. If so is it motoring right across to the H side when only central heating is required?

Untitled

• jackthom
• 19 Dec 2008
• 18

 

[Rod1956]

hi

there is a valve but it seems to only have 2 connections. There's a sliding lever which is in the middle position - if i move it towards me, there is a small amount of resistance and a meshing sound - then it goes back to the middle position.

Cheers
Rod

 

[jackthom]

When the room thermostat is ON (demanding heat) does the valve move at all? Usually it should move to the open position for central heating.

At that point it should operate a microswitch inside the head which sends 240V to tell the boiler to fire up.

 

[Rod1956]

hi

I've had another look in the airing cupboard and found another valve - the same as the other on - they both have only have 2 ports. The lever positions are marked AUTO and MAN,

When I switch on the HW one of them moves itself to the MAN position and when i switch off the water it returns to the middle there is a motorized sound while it's moving.

The other valve doesn't seem to do anything it just stays in the middle position - it's covered in fluff and dust so i'm going to try cleaning it.

 

[jackthom]

It could just be sticking. It's also possible (but less likely) that the room thermostat isn't actually supplying power to the valve motor.

It all depends on how it has been wired, but by moving the valve right across manually you may find the boiler fires up.

 

[Rod1956]

I've cleaned round the lever - and moved it from AUTO to MAN a few times - it just returns itself to the middle each time.

I've tried switching the CH on and off a few times but nothing happens.

The other motorized valve seems to work when the HW is switched on.

Can I test the room thermostat ?

 

[jackthom]

There are mains voltages on the thermostat and the motorised valve connections so only if you have a multimeter and are confident working with the electrics.

The thermostat is just a switch which supplies 240V to the valve when the central heating should be ON. The older mechanical thermostat would have 3 connections:
Live in, Switched Live to the valve and a Neutral connection as well.

A modern digital thermostat might not have the neutral connection.



I should say there is another possibility, ie that the timer is no longer supplying power for the central heating circuit.

 

[Rod1956]

There are 3 pins on the back of the Thermostat which just locate in 3 connections on the back plate - It's a Drayton Digistat - you're right there are only 2 wires connected. One is live I tested it with a pole tester.

The middle has no wire attached and the right side is not live - i have a circuit tester i'll try it across the pins on the back of the thermostat to check it's switching.

Unfortunately about 2 years ago a British Gas engineer who had just started, was trying to cure a fault (which was a seized ball cock on the header tank) but he diagnosed an electrical fauly. My airing cupboard is now full of junction boxes where he cut loads of wires and connected them all together - so my wiring is not simple. His boss sorted it all out but i still have the junction boxes and stacks of hanging wires.

 

[Rod1956]

It's not the thermostat - i tested the circuit and when the switch is on there is power across the 2 connections.

 

[jackthom]

It's not the thermostat - i tested the circuit and when the switch is on there is power across the 2 connections.

Just checking that when you say there is "power across the 2 connections" you mean they are both Live?

From what you say about the state of the wiring, it sounds like you will have an awkward time tracing this live wire to the motorised valve.

 

[twgas]

You need to check for 240v at brown wire to CH valve.

 

[jackthom]

This diagram might help. http://octaveblue.co.uk/c_heating/sp_ch.htm

If you're feeling the cold it should at least be possible to get the boiler fired up heating the water tank while opening the CH valve manually, assuming there is a locking tab to hold it open against the spring.

This should work without the hot water being on but something here isn't quite as it should be.

 

[jackthom]

Finger trouble, post deleted.

 

[Rod1956]

Just checking that when you say there is "power across the 2 connections" you mean they are both Live?

I used a circuit tester - a battery circuit with LED. I took the thermostat off the wall and put the tester across the 2 pins with the thermostat in the off state. The 2 pins had no circuit across them.

When I switched the thermostat ON there was a circuit across the 2 pins - the switch was working.