Replace Horstmann Diadem

[stem]

OK so the black cable is to the motorised valve. Is looks as if there a grey wire coming from the black cable, I can't see it properly because its hidden by the other cable, where does it go?

Of the two white cables, which one goes where?

 

[EdTurtle]

Yes, the black is for motor valve.

The only wires that connect from the valve are the blue which goes to supply Neutral, earth which goes to supply earth and black (could be brown) which goes to the cylinder stat (the NC connection)

The two white wires, i assume you mean the flex? Top one is cylinder stat and bottom one is from the FCU. In fact, the FCU feeds a socket, this then plugs in to that socket, but the same thing, basically the supply.

Thanks!

 

[stem]

Ah OK, then as we suspected; the thermostat is just used to close the valve, it hasn't been wired up as a C Plan, using the white, orange and grey wires for a boiler interlock, which is a shame because the valve is suitable for it if it was wired in correctly as below.

 

[EdTurtle]

So i assume the valve wiring colours are standardised? Or have you got the colours from a diagram somewhere and put them in that C plan you've attached above?

Would it be worth me doing this? Would it mean I use less gas?

So, my controller obviuosly isnt the same as that one, but unless I am mistaken, i would wire the cylinder stat 1 (which i assume is NC) to terminal 6, and then also wire HW on to 6, and then room stat 1 (which i assume is NO) to terminal 4 and this would connect to CH on on the controller?

I do understand the logic of wiring the valve into the cylinder stat and boiler to control the wiring, but why would you also wire the valve into the CH side of things?

I will study the diagram a bit more, see if i can fully understand it before i change it all!

Thanks, you've been more than helpful!

 

[stem]

So I assume the valve wiring colours are standardised?

Yes they are; but not all 2 port valves are suitable for a 'C Plan'. Many two port valves don't have the white wire that yours does. I think the plumber installed it as a 'C Plan' using the correct valve, but the electrician didn't fully understand the concept and made the wiring up as he went along.

Would it be worth me doing this? Would it mean I use less gas?

Yes, you would save some gas. How much depends upon how long you presently have the programmer is set to having only the hot water 'on' when the hot water cylinder is already up to temperature. With 'C Plan' In the summer when you have just the hot water 'on' and the cylinder is up to temperature, the motorised valve will close and the boiler will go off too. [Presently the valve closes but the boiler remains 'on', so if the programmer is still set to hot water 'on', the boiler will fire up every now and again just to keep itself hot] Another advantage is that you would be able to operate the heating and hot water independently of each other.

Unfortunately, it won't stop the radiators completely from warming up a bit in the summer because of the gravity circulation. However, it might reduce it a bit because the boiler won't be in operation for so long because of the boiler interlock.

Here's how C Plan works. The cylinder stat powers only the motorised valve (Brown wire) when the motorised valve is opened, it closes an internal NO contact, between the grey wire (Live supply) and the orange wire connected to the boiler. The orange wire becomes live and the boiler fires up. When the hot water cylinder is at the temperature set on the thermostat, it turns off the power to the valve which closes, releases the contact and the boiler goes off too.

So, my controller obviously isn't the same as that one, but unless I am mistaken, I would wire the cylinder stat 1 (which i assume is NC) to terminal 6, and then also wire HW on to 6, and then room stat 1 (which I assume is NO) to terminal 4 and this would connect to CH on on the controller?

The cylinder stat contact closes when hot water is required. The same as the central heating contact closes when the heating is required.

I do understand the logic of wiring the valve into the cylinder stat and boiler to control the wiring, but why would you also wire the valve into the CH side of things?

This is the clever bit. For heating you require both the pump and the boiler to run. [As opposed to just the boiler for the hot water] So, when the motorised valve is closed (hot water not required) the motorised valve's internal microswitch changes over position and the white and orange wires are now connected together. So, when the room stat calls for heating, terminal 5 becomes live. This is linked via terminal 9 to the pump and the pump starts. Because terminal 9 and 10 are now connected together via the motorised valves internal microswitch the same live is also fed to the boiler and it fires up as well.

The boiler and the pump can't be permanently wired together because otherwise when the boiler runs for just hot water, the pump would run too, and the heating would come on. The motorised valve's internal switch isolates the pump from the boiler when just hot water is required, but connects them both together when heating is required.

Sorry the description is a bit long and waffly. Hope you can understand it OK.

 

[EdTurtle]

Many many thanks, with your description and the diagram i fully understand how the C-plan works. Thanks for taking the time to explain!

One thing i didn't understand was when you said

Another advantage is that you would be able to operate the heating and hot water independently of each other.

I don't understand what you mean here though? Surely i cant control the two separately if i have to have HW on if i want CH on?

And..... just to complicate things! I 'think' i have a mid position valve on the CH pipework, but this has been removed fully and "unwired". Would you like a picture of this, or is not relevant? Also, before i change the wiring system to C plan, would it be necessary for me to show you which pipes go where on the system, to confirm that the valve is indeed on the HW and not C, and whether the valve is on flow or return (if this matters?)

Thanks, i think i have got there with this now, other than the q's above, and am ready to wire it all up! I will of course let you know when i am successful ( i am confident!)

 

[stem]

At the moment you have to have both 'on' because only the hot water controls the boiler, & the central heating only controls the pump. So, to have central heating, you need to have the hot water on to operate the boiler, and also the pump to circulate water around the radiators.

The microswitch in the motorised valve that I explained in the description of the 'C Plan', allows the heating to control both the pump and the boiler, thus allowing you to have the heating 'on' without the hot water.

The unused mid position valve is very odd. If it is not wired up it will stay in the position where one port is permanently open and one is permanently closed, and I'm not sure what you mean by

on the CH pipework, but this has been removed fully and "unwired"

I don't see how it can be "on the pipework" and "fully removed"?

Unfortunately photographs rarely allow us to follow the pipework through the system to determine what it is connected to.

The motorised valve you photographed earlier is a Honeywell V4043H, which is suitable for a 'C Plan' but it does need to be in the pipework between the boiler and the hot water cylinder.

 

[EdTurtle]

IMG_20161204_214120

• EdTurtle
• 4 Dec 2016

This shows where the pipe goes from the motorised valve - it goes into the bottom of the boiler, and it doesnt have a Ch pipe on it, so i assume its the hot water and not CH!

The other valve is also shown on here, which seems to be on the CH side of things, but i cant be 100 percent certain ( I assume the connections to the boiler are documented somewhere? Baxi bermuda i think)

Also, when i say the valve has been removed, i mean the front part:

IMG_20161204_214205

• EdTurtle
• 4 Dec 2016

this will fit onto the valve behind, but the cables dont go anywhere, the electric part wasnt even in the cupboard...

At the moment you have to have both 'on' because only the hot water controls the boiler, & the central heating only controls the pump. So, to have central heating, you need to have the hot water on to operate the boiler, and also the pump to circulate water around the radiators.

The microswitch in the motorised valve that I explained in the description of the 'C Plan', allows the heating to control both the pump and the boiler, thus allowing you to have the heating 'on' without the hot water

Ok I think i get this, the HW water will be on with the CH because of the way its wired, but on the control unit i could just select CH on, and not both as i do at the moment?

Does this mean that when i change the programmer i dont need to select one for gravity heating and have the contols locked together as the c-plan systwm will control all this for me? Or have i missed the point!

Thanks, you definitely explain all this well, and i really appreciate it

 

[stem]

I can't really help you with the motorised valves without being able to see how they fit into the system as a whole. My qualifications / experience are electrical, but I have a reasonable understanding of central heating systems from working with my plumber father as a teenager, but it was over 30 years ago and motorised valves had only just started appearing.

It maybe that because the radiators get warm when just the hot water is on due to the gravity circulation, someone fitted that valve into the heating circuit to stop it. Why it has been decommissioned who knows. The locations for motorised valves need to be selected carefully so that they are not fitted in any pipes that have any expansion / vent pipes which would be restricted when the valve was closed.

If you have a genuine C Plan, and the wiring to go with it, then the controls do not need to be interlocked together they will work independently.

You may remember that Steelmasons said this at the outset.

It appears you have a C plan....zone valve will be the 6 wire type (28mm) that utilises an auxiliary switch to power boiler when cylinder stat is satisfied. CH can be programmed independently of HW and vice versa.

 

[EdTurtle]

Ok, sorry for that! I too have experience in electrics and not plumbing, hence me understanding fully how the wiring etc works, but not 100% certain on whether the valve is indeed fitted to the flow/return water/CH etc!

I also like to 100% understand things before I do them, in case of problems etc

I think i didnt understand the statements made by both you about the independent control because all the information i have found/read says that gravity systems need the CH to be locked to the HW. Just to clarify (which is where i think the confusion comes from) the CH pumps hot water through the boiler and then round the system, completely independently of the hot water tank upstairs? If so, then why is the CH normally locked to the HW?

Sorry to ask so many questions that are probably obvious to you, just want to understand fully!

Thanks again

 

[stem]

I will try to break it down a bit, hope this helps. It's all to do with the way the system is controlled.

Fact 1. To have the hot water 'on' you need to have only the boiler on
Fact 2. To have the central heating 'on', you need to have the boiler 'on' and the pump running

WITH YOUR SYSTEM NOW
As your system is wired now the hot water control switch at the programmer controls only the boiler, and the central heating switch controls only the pump. So to have heating 'on' you need to have the hot water 'on' to operate the boiler, and also the central heating 'on' to operate the pump.

WITH A PROPERLY WIRED C PLAN
A 'C Plan' when installed properly will allow the hot water channel to control only the boiler as now. But, it will also allow the central heating to control the pump and the boiler so you can switch only the central heating 'on' and it will start the pump and the boiler.

If hot water is not required, the motorised valve in the pipe to the hot water cylinder remains shut, so only the central heating operates.

all the information I have found/read says that gravity systems need the CH to be locked to the HW.

This is correct for a standard gravity system, which is what you have now, and so this is what you have to do. BUT if you were to convert it to a proper 'C PLAN' you would get separate control of the heating and hot water. Which is what it was designed for.

Just to clarify (which is where i think the confusion comes from) the CH pumps hot water through the boiler and then round the system, completely independently of the hot water tank upstairs?

Correct. Both are connected to the boiler, but they have separate circuits, the heating circuit is pumped, and the hot water circuit relies on natural convention for the water to circulate around the hot water cylinder.

If so, then why is the CH normally locked to the HW?

Because at the moment, the hot water is wired such that it controls only the boiler. If you switched only the heating 'on' just the pump would run and would circulate the water, but nothing would get hot. So, you also have to have the hot water on as well, so that the boiler operates.

 

[EdTurtle]

That makes perfect sense, thanks for taking the time to explain it

One final question!?

I am replacing the thermostat too, currently wired as a two wire setup. The instructions explain how to replace like for like, but also explain how to wire for three wire control... Is it worth me adding a neutral into the system and having three wire control, or just stick to two wire?

its a drayton rts8 thermostat

thanks

 

[jackthom]

Looking at the rts8 installation leaflet, the wiring diagram in fig 5 would be the ideal setup using a neutral connection. The thermostat will work fine in the two wire mode shown in fig 7 but without an anticipator in circuit it may be a bit slow to turn the heating off when nearing the desired room temperature.

 

[EdTurtle]

Ok, thanks, wont bother with a neutral connection then

Will let you know once i have got round to doing this!

Thanks again, I really appreciate it

 

[stem]

I am replacing the thermostat too... Is it worth me adding a neutral into the system and having three wire control, or just stick to two wire?

That all depends upon your choice of new thermostat. Some need a neutral to work, some don't.