Only get central heating if hot water turned off (Ed.)

[dptc1203]

Hi, just moved into a new house. I have a standard boiler British Gas 330+ with a hot water cylinder. There is a cold water tank in the loft too. The house has a hive thermostat installed. My hot water and heating systems are functioning. However, my central heating only seems to continuously work when the hot water is turned off. If I attempt to turn the CH on while the HW remains on, it seems to randomly turn off after a period of time (approx 15-30mins) and doesn’t come close to reaching the desired temperature set.

The only fix is to turn the HW off and all of a sudden the CH will come back on and heat the house up pretty quickly.
.

Any ideas on what it could be? PLEASE HELP!!

 

[ericmark]

There are a few different plans on how central heating is wired and plumbed.
In general there are exceptions.
No motorised valves means C Plan
A three port valve either W Plan or Y Plan, more likely Y Plan
Two or more two port valves S Plan

So what does your heating use?

 

[Stuckinarut]

Sounds like motorised valve fault if you have one or a possible wiring fault.

 

[ericmark]

I found some instructions and it has this diagram it does not seem to follow any of the plans. But to help we need to know what you have. Motorised valves have micro switched which do some times stick, but as it is too much guess work.

 

[dptc1203]

I have a 3 port valve next to the cylinder. I can hear it moving when I turn the heating on and off and same with the hot water.

 

[Stuckinarut]

Just because it’s moving, doesn’t mean it’s engaging the microswitch. Could you possibly and safely post a photo of the wiring at the Hive?

 

[dptc1203]

It’s probably not that helpful as the junction box is a complete mess! The dark grey cable goes directly to the hive receiver. The white cable behind it goes to the boiler. The bottom white wire looks to go to the pump. And the middle white cable goes to the hot water tank thermostat. Not sure any of this is helpful as it’s all over the place.

 

[dptc1203]

Just because it’s moving, doesn’t mean it’s engaging the microswitch. Could you possibly and safely post a photo of the wiring at the Hive?

reading up on this so probably im wrong… but The 3 port valve has 3 positions. HW, CH or both at the same time. Is there a possibility the valve is engaging for the first two separately but is sticking when I call for both HW and CH at the same time?

 

[RandomGrinch]

Can you check where the white wire goes from the middle white cable?
If it connects to terminal 2 (satisfied) on the tank stat, the loose white wire should go into terminal 16 of the wiring centre.

 

[dptc1203]

Can you check where the white wire goes from the middle white cable?
If it connects to terminal 2 (satisfied) on the tank stat, the loose white wire should go into terminal 16 of the wiring centre.

Wow yes didn’t even realise that…good spot. Do you think that could be related?

 

[Dereekoo]

Have you looked under the box there should be a lever that comes across. When nothing working it should be in its rest position, there is a spring inside that pulls it to this rest position. When you are just asking for hot water it should remain in its rest position. When you are just asking for central heating it should move across to the other side of the slot. When you are asking for both hot water and central heating it should be held in a central position.



PS proof read your original post. it doesn't make sense

 

[dptc1203]

Have you looked under the box there should be a lever that comes across. When nothing working it should be in its rest position, there is a spring inside that pulls it to this rest position. When you are just asking for hot water it should remain in its rest position. When you are just asking for central heating it should move across to the other side of the slot. When you are asking for both hot water and central heating it should be held in a central position.

PS proof read your original post. it doesn't make sense

With the hot water only on the lever is it’s rest position and is solid as if a spring was keeping it there. I turn the central heating on and it remains in the rest position and is loose to move. But I can feel hot water going through and down the radiator pipe. The lever remains the same with both HW and CH on.

 

[RandomGrinch]

Wow yes didn’t even realise that…good spot. Do you think that could be related?

Yes, the result would be exactly as you describe well in your first post.

Here's a good primer on how a Y Plan system works...

Y Plan central heating system

Y-plan central heating system operation and wiring diagram

flameport.com

If you you are calling for hot water and heat, the 3 port valve will move to the mid-position, as it is receiving a live feed on its white wire...

Once the tank is up to temperature, the tank stat would switch it's output to terminal 2; in doing so, the call to heat to the boiler (SL) is dropped, but the grey wire is now live.
With the white wire and the grey wire now live, the valve should move to the heating only position, and the orange wire from the valve will provide the switched live to the boiler...

In your case, terminal 2 (satisfied) on the tank stat isn't connected.
When the tank is up to temperature, the switched live to the boiler is turned off, but the grey wire does not become live; the valve isn't moved over to the heating only position - it remains in the mid position, but with no switched live provided to fire the boiler.

By switching the hot water off, the tank stat is bypassed, the grey wire is provided with a live feed directly from the programmer, the 3 port valve moves over to heating only, and a switched live is fed to the boiler from the orange wire on the valve.

As that one wire is loose, it's worth checking the tightness of the other terminals in the wiring centre.

 

[Dereekoo]

With the hot water only on the lever is it’s rest position and is solid as if a spring was keeping it there. I turn the central heating on and it remains in the rest position and is loose to move. But I can feel hot water going through and down the radiator pipe. The lever remains the same with both HW and CH on.

Cannot say definitely but sounds as if the actuator is buggered which unfortunately for you may mean major plumbing work to replace, I just replaced mine which fortunately was a Drayton 3 port valve which meant I could just simply replace by removing the complete actuator box off the valve by pressing a clip and rewire a new one in. Yours may be the same, try unclipping it off the valve.





Just to give you a better understanding of how it all works I found this quite useful, if you have a neon screw driver or multi meter and are comfortable working with electrics you could check whether you are getting the necessary feeds to the actuator wires at the junction box when calling for hot water or heat etc Good luck
AHI VOD Campaign 2023 HD

 

[ericmark]

The internals of the valve are complex, only worked on it once, and found it hard without the diagram shown, I bought a new valve and crossed fingers and it worked.

One thing I found was latching the lever kept it going while but it resulted in radiators getting slightly warm in summer, so only a temp cure even if it works.