Central heating won't turn on unless the hot water is on

mblndt

Hi there! I've just installed a starter kit in my home. It has an old school hot water tank with a simple two way motorised valve (DVA 2/5).

The system works fine if the hot water is set to "on", but if it isn't on we get no heating.

I had assumed the purple loop wire served the purpose of connecting the central heating to the hot water but I think I might be wrong.

How can I trouble shoot this?

policywonk

Hi. Welcome to the community. Would you do the following quickly?

1. Identify the boiler you have, the specific model. We need to understand how it is triggered.
2. Confirm whether there is another two port valve serving the central heating circuit.
3. Take photo of the wireless receiver and post here.
4. I presume you have to switch things back to get heating working again, to the old programmer. Would you photograph that layout and post it here, also identifying the model of old programmer?

When you post the shots, use a laptop or desktop browser. Mobile phone browsers do not allow one to post photos easily.

We'll come back as soon as we can. Note we are just volunteers - so our days are busy too but the people on this community do try hard to help.

mblndt

Thank you so much for replying and helping people!

Boiler: Baxi Solo 15 HE A
I can only see the one two port valve next to the pump.
The old thermostat wasa Centaurplus C27.

I'll send photos shortly as a separate post - I don't have access to a laptop right now.

Old wiring had 6 contacts: left to right:
1: 4 black wires
2: 1 red wire
3/4: empty
5: 2 red wires
6: 1 red wire.

It was rewired using what the app said to do for the Centaurplus C27 - I'll send photos shortly.

Thank you once again!

mblndt

I'll be able to get a photo of the wiring on the receiver to you tomorrow hopefully - I can't hit the fuse to expose it all safely tonight as other kitchen appliances are running - sorry.

mblndt

Here's the programmer wiring (4 black wires into N that I taped together temporarily).

policywonk

Your old programmer seems to be just a programmer, not a thermostat as well. I can see that by fitting the wireless receiver you replaced that and took charge of the heating timetable via the app.

However I wonder what happened with the wall thermostat. Did you also install a WIRED Tado wall thermostat, or a wireless smart wall sensor?

• I ask because without a thermostat, you cannot instruct the boiler to wake up.
• I suspect that the CH NO wire goes to the thermostat somewhere - and that is somehow disconnected from the authorisation sequence.

Can you locate it? Whats its model?

mblndt

Thank you for this - I had no idea we have a thermostat somewhere. I'll have a really good look.

mblndt

We're currently just using it as a wireless smart wall sensor, I believe - the one from the kit sold as "Tado V3+ Wireless Heating & Hot Water Smart Thermostat Starter Kit."

mblndt

Hi again! I think I've been using some words wrongly because I don't know what I'm talking about!

Having looked around the house I think we have never had a thermostat - just the programmer and the ability to turn the heating on/off (without any temp controls).

So now we have the new programmer where the old programmer was and the wireless tado thermostat in another room.

If we have another (old wired) thermostat I can't see it.

policywonk

This link: https://www.sparksdirect.co.uk/download/TC7-user-3.pdf presents the installation guide. According to that, going left to right:
N - Neutral - goes to N(eutral) on the wireless receiver.
L - Permanent Live - goes to L(ive) on the receiver.
1 - Ignore.
2 - Ignore.
3 - HW ON - goes to HW NO on the receiver.
4 - CH ON - goes to CH NO on the receiver.
Now it seems that your wires were correctly reassigned to the Tado receiver.

When you press the CH heat button on the wireless receiver does it light up and fire the boiler, when the Tado stuff is in place?

If not, would you contact Tado and explain that the way your home is plumbed suggests it is a C Plan with the CH side controlled by a zone valve. Explain you have no heat and get it escalated.

@hugbilly would you have a look at this thread please? Aside from a device fault, I don't see why the boiler won't be triggered on CH, unless the wiring follows a logic which isn't conventional.

mblndt

I just tested it with the tado stuff on. I:

-turned off CH and HW: the boiler went quiet

-turned on CH only: no sound (other than the click of a switch - no heat in boiler)

-turned on HW in addition to CH: boiler came on.

I'll write to Tado - thank you so much for your expertise and advice - I really couldn't get to the bottom of this myself.

mblndt

By the way - just in case I miscommunicated this earlier, so long as HW is on 24/7 (which isn't ideal financially) the CH does work - it just doesn't do anything if HW is off.

policywonk

Looking again at the instructions for your programmer (TC7-user-3.pdf) - given that you seem to have CH pumped but HW gravity fed - on page 3 of that guide it presents this diagram:

Would you check? PIN 3 of the old programmer should provide one red wire to give the COM supply to the thermostat. However yours has two red wires. Any way of identifying where they come from?

As this is partly gravity fed the wiring needs to change anyway. Will think about this tonight.

mblndt

Thank you again for all your thought here.

Sorry for the poor perspective here - this is a photo I took earlier. You're right: one red wire seems to be going in a different direction from the others. That said, unfortunately once it goes into the wall near those earth wires you can't see much more than this photo shows - it just disappears upwards and I'm not sure how best to determine its purpose. I have a multimeter and am happy to go around the house turning things off and on and testing the wire if that would help?

hugbilly

This system looks to be an unusual one, not exactly C plan perhaps because, from the photograph, the hot water loop appears to be pumped with the central heating being enabled when the motorised valve opens allowing the hot water to be "tee'd off" from the cylinder feed.

I think true C plan is only gravity hot water. But that's an aside really. So in this system hot water requires just the pump and boiler with central heating needing the valve to open as well.

To control the HW the wireless receiver would have to turn on the boiler and pump and then for the CH as well it would need to do the same but also enable the valve. I wonder if the valve's opening starts the pump and boiler via a microswitch in the valve as in a more orthodox system.

Incidentally there's no sign, in the 'photograph, of a cylinder 'stat so I wonder how the hot water temperature is regulated, just by the boiler's own thermostat perhaps.

To realistically say much more I think I would need a bit more information about the existing system's wiring and controls and what happens when HW and CH are selected.

Sorry not to offer more, perhaps a bit more research (maybe with a multimeter) will reveal what's going on.

mblndt

If there are any tests you'd like me to do or photos you'd like me to send just let me know - I'm not really sure where to start with testing things.

policywonk

@hugbilly was worried you would see that, thank you.
@mbindt. This is an uncomfortable suggestion for me. I believe the most reliable solution is, if you would consider it, to commission a builder to:
1. Add another 2 port valve pointing to the cylinder.
2. Add a wiring centre and rectify the wiring into an S plan.
The cost will be recovered in savings within 2.5 years. Unfortunately there are three major wiring differences between whats in your home and what the central heating industry would normally expect. Those three differences have resulted in your HW call turning on your pump and boiler, but the CH call not doing that. It is possible to put in a temporary bridge to fix it but am really worried that it might cause a short.

mblndt

Okay - thanks so much for your time both of you. It'll be a little while before I'm in a position to do that but it's very useful to have a long-term solution - thank you.

policywonk

If you are comfortable using a multimeter I could walk you through the diagnostics and help locate where things are - and should be. Let me know. Genuine offer. You'd have to do on a warm day however.

1. Go to the cylinder and try and locate it's thermostat. There seems to be a cable hiding back there. Trace it and work out where it goes. Label it as the HW STAT.
2. Go to the pump and trace the cable. It may go through that connector on the floor and then onto somewhere else. Probably the wires going to the programmer. But which one? Label it.
3. Goto the boiler and trace its wiring back to the programmer. Label it.

A cheap multimeter will help. If you are up for it let me know

mblndt

I'd really appreciate that if you have the time and energy. I can use a multimeter to check a wire against a known ground but that's all I really know how to do - I might be a bit underskilled to do a good job unless it really is as simple as one end of the multimeter on the wire and the other on a ground. I wouldn't know what to do if, for example, I couldn't find a nearby ground.

policywonk

If you are comfortable with this. The first step is to identify the primary power source for everything:

• pump
• boiler
• programmer.

On a warm day, turn of the power and double check it is all dead.

1. At the programmer: identify where the cables came from to power the programmer.
2. At the zone valve, trace the grey wire, orange wire, brown wire and blue wire.
   1. The brown (switched live) wire and blue (neutral) wire should power the small motor in the valve, when it is being asked to provide heat. That request, the call for heat comes via the brown wire. If you follow that brown wire it will go to a junction which eventually kicks of another wire which goes to the programmer (one of the red wires). You need to find out which one and label it.
   2. The grey wire technically provides a permanent live feed to the valve. It will also go to a junction, where it too connects to a cable. Find where that cable goes and label it: permanent live CH valve.
   3. Technically the orange wire is the one which, when the valve is fully open, hands the authority to the pump and the boiler to wake up and seize the day. Again it will probably go to a junction where the SWITCHED LIVE to the pump, and the SWITCHED LIVE to the boiler is then relayed on. Spot that junction and the cables. Should be easy to label the pump's switched live. The second wire at that connection should be the boiler.
3. At the cylinder- look for the thermostat and trace its origin. It is probably a two core plus earth cable. One of those cores will take its authority from the programmer. The other will pass it onto something. What exactly?

Let me know if this helps. Check your email here.