No CH when hot water switched on

[ianb1965]

House is only 6 months old so hopefully will get this fixed under warranty. Noticed recently radiators not heating on morning (Honeywell ST9400C timer, Ideal Logic 24 boiler).

After some experimenting it seems that if the controller has HW on (green light on) then switching Heating on (either timer, constant or +1h) has no effect on boiler (even though green light comes on on controller). If I switch the HW off and then try switching heating on, it works fine.

Does this sound like a faulty controller?

Thanks.

Ian

 

[Dan Robinson]

Does the heating green light come on when the hot water is also on?

What room thermostat do you have?

 

[ianb1965]

Hi Dan - thanks for your quick reply. Yes - if say, running on timer, both green lights are on. Visually timer is working correctly.

Just had a browse through this forum - looks as if culprit could be the motorised valve (4043H)

Ian

 

[Dan Robinson]

If it were a problem with the zone valve then i doubt it would work regardless of timer setting.

Most likely a timer problem or a wiring problem.

Has it always done this, or is it a new thing?

You will need to get jiggy with a multimeter really.

 

[Agile]

When the hot water is up to temp the boiler should turn off! Does it?

If it does then does the CH work then?

The HW stat should be set at about 60 C ! Check it.

Tony

 

[ianb1965]

Dan - worked fine until recently.

As far as HW is concerned yes boiler shuts down when up to temp. But CH will still not come on unless HW is off at timer.

A few more tests today...
If HW and CH lights are both on, switching HW off does not make CH "kick in" - CH has to be turned off and back on.
If CH on (by itself so ok) and I then switch HW on, boiler shuts down (in this case hot water was only recently on so probably at temp)

So in summary CH will not signal boiler if HW on and switching HW on kicks off CH!

Wiring looks like S plan if that makes any difference.

Ian

 

[pampers]

Does this have a outside sensor (weather compensation)

 

[bhm1712]

it sounds like the changover relay within the controller is faulty to me. The deciding factor being that to get the CH on you have to cycle the CH switch on the controller to get it to reactivate once the HW is turned off, and the CH was already on. no other real way on an S plan that this could happen unless rewiring has gone on which it obviously hasnt...

Mental fault finding i have done for you with S plan wiring in mind:

CH on and roomstat calling = 240 volts to motorised valve brown wire. This will operate the CH zone valve motor, open the valve and change over the normally closed contact to normally open inside the valve head. This will allow current to flow through the grey live wire and onto the orange wire to the boiler ignite and pump run.

If HW is selected and cylinder stat is calling = 240 volts onto HW zone valve brown wire. This will operate the motor, open the valve and change over the normally closed contact to normally open and allow current to also flow through orange to boiler ignite and pump run.

If both HW and CH are selected and calling, there will still only be 240 volts at the boiler and pump run terminals (usually terminal 10 in JB) so in effect if HW is selected first, this will ignite boiler and run the pump, turning on CH after that will purely serve to open the zone valve for CH. Vice Versa for if CH was selected first.

now with that in mind, we can say that having HW on should not kick off the CH under normal conditions, and also vice versa. The boiler and pump will receive 240 V and current flow in both situations.

In this case, if having the HW on stops the CH that must mean that the selection of HW is somehow dropping the live feed to the CH zone valve motor. This live comes from the main incoming feed to the controller, through a set of relay contacts (when you select CH) in the controller, out and through the roomstat to the motorised valve motor live terminal.

Assuming that this wiring is standard S plan, this must mean that the feed is being lost from the controller. The relay is the most likely culprit. Having to deselect and select CH again to get it to work seems to point towards the relay losing its coil feed when HW is initially selected and only being re-energised with the deselect and reselect.

 

[ianb1965]

Pampers - no outside sensor.

bhm - very good point - I hadn't realised the significance of that. House builder sending someone next week - in meantime will stagger timed hot water and heating on the morning so they don't overlap!

Thanks

Ian

 

[bhm1712]

post updated Ian....

 

[ianb1965]

Wow - that's a very detailed explanation bhm and the most worrying thing is...I actually understand it! Makes perfect sense - thank you.

So does that mean it's the opening of the valve that actually triggers the boiler firing?

Ian

 

[bhm1712]

yes, the contacts in the valve heads serve to switch power onto the boiler and pump.

As stated in my above post, the first one to be selected will put the feed to the boiler. In the event that you have selected HW for 2 hours and CH for 1, when the CH switches off after 1 hour, the feed from the HW zone valve will still be on and there wont be any momentary loss of power to the boiler or pump.

For the sake of 30 quid (for a basic controller from screwfix) i would get one and test it out. If it turns out not to be that,which i doubt (although without seeing it myself its a little harder to say) you can always repackage it and take it back. if it fixes the issues, then its only 30 quid. Unless of course you can get it free under warranty.

 

[ianb1965]

Just to give an update on this....the builder sent an electrician today who confirmed the problem was indeed the programmer. But he couldn't supply a new one - the plumber has to do that - he just wires them!

Until then he's done something to the wiring which causes the heating to come on whenever the heating does.

Thanks to all for you help - greatly appreciated.

Ian