Replace Horstmann Diadem

[EdTurtle]

Hi, I currently have a horstmann diadem, which is working fine but we want to replace it with one that provides a bit more control.

It is wired as per the wiring diagram, terminal one goes straight to the boiler live, and terminal 4 goes to the thermostat, then comes back to the CH pump live. There are no other connections, only HW on and CH on. We do have a cylinder stat and a (I believe) a 2 port valve (I can get part number/photo if needed). Live goes to the stat, then comes back to the valve, but no connections are made to the boiler control unit.
The thermostat has only two wires.

The queries I have are
1)If i buy a replacement control unit (eg http://www.screwfix.com/p/danfoss-fp715si-programmer/63760) then can i simply change the two wires currently wired to HW on and CH on to the new terminals on the new programmer for HW on and CH on, and ignore the terminals for HW off and CH off since both the on functions are normally open?
2)Does it matter that the cylinder stat is wired in this way? It is actually wired into one of the 10 way junction boxes, just that all the terminals are unused other than the L N and E as mentioned above. It seems i have a C plan, just not wired that way?
3) Also want to replace the thermostat, looking at the danfoss ret2000b, which is 2 wire. The instructions for both the control units mention switch ratings of 3A or 2.5A depending on model, does the thermostat need to be the same ratings, or not as its just a one way switch in essence?

I actually want this one: http://www.screwfix.com/p/flomasta-25902sx-two-channel-programmer/3439g anyone have wiring diagrams for this, and if so, can i wire it as per question 1 above? Only info i can find is that this fits a standard UK backplate, so i assume this is 3 = HW on and 4 = CH on?

Thanks, i can provide any more info as required, but i have tried to include as much as possible this time!

 

[stem]

Hello and welcome to the forum. Your first post is very comprehensive, so I can comment as follows:

If i buy a replacement control unit (eg Danfoss FP715si) then can I simply change the two wires currently wired to HW on and CH on to the new terminals on the new programmer for HW on and CH on, and ignore the terminals for HW off and CH off since both the on functions are normally open?

Yes you can. It will replicate exactly what you have now. However,

From your description of the existing wiring, it would appear that the Hot Water 'on' fires up the boiler and heats the hot water, then when you put the Central Heating 'on' the pump starts and circulates the water around the radiators. This would mean that you need to have the Hot Water 'on' in order to get the Central Heating 'on' also. (I.E. You can't have the Central Heating 'on' on its own) Is this correct? If so, you will need a programmer suitable for a gravity system. The Danfoss says it is "Suitable for Gravity-Fed & Fully Pumped Systems"

We do have a cylinder stat and a (I believe) a 2 port valve (I can get part number/photo if needed). Live goes to the stat, then comes back to the valve, but no connections are made to the boiler control unit.

This is a bit odd. Your comment about it resembling a C Plan but not wired that way sounds about right. It sounds as if when the cylinder is hot, that the valve will simply close, but the boiler will continue to be powered from Terminal 1 of the Horstmann. Is this correct?

Also want to replace the thermostat, looking at the danfoss ret2000b, which is 2 wire. The instructions for both the control units mention switch ratings of 3A or 2.5A depending on model, does the thermostat need to be the same ratings, or not as its just a one way switch in essence?

Either will be OK for what you are switching with it. A central heating pump is rated at approx 50w, which is only 0.2A

I actually want this one: http://www.screwfix.com/p/flomasta-25902sx-two-channel-programmer/3439g anyone have wiring diagrams for this, and if so, can i wire it as per question 1 above? Only info i can find is that this fits a standard UK backplate, so i assume this is 3 = HW on and 4 = CH on?

Sorry I can't comment on the Flomaster, but the bottom line is that any programmer that offers a switching function for Hot Water and Central Heating, will provide you with the same functions you have now. Some don't have the facility needed by gravity systems to ensure that the Hot Water is also switched 'on' automatically when the Central Heating is required, but they will still work. However, but you would still need to make sure that the programs were set so that when you want heating the hot water is also selected 'on'.

 

[EdTurtle]

Hi and thanks for your reply, looked at a few forums, but this definitely seemed the most friendly and active, glad I wasn't wrong!



OK, I knew I'd forget something... yes, its a gravity system (baxi back boiler in case your interested) so the CH is locked to the HW, but HW can be on individually. I did notice that the controller had the option to lock the two together, hopefully will be clear in the instructions!



Yes (I think) that is the way you have described it with regard to the valve closing, but terminal one being live still. I could test this with a meter if necessary, I guess simply turn the cylinder stat right down to like 10deg or something which should shut the valve off and see if terminal one is live still? Would this be helpful/useful?



So, I have two choices... 1) simply replace the system as is, moving the wires around to their new locations in the backplate as needed. or 2) rewire the whole lot to correlate with a C plan using the 10 way junction box?



So, would it be worth doing 2) or not? If so, how would I know whether I need C plan, and that I indeed do have a 2 port valve?!



Thanks, again, I think I have covered everything?

 

[EdTurtle]

Forgot to mention, might be relevant?

In summer when we only have HW on, i set the CH to off... However, the radiators still warm up! To get around this i normally just turn the thermostat down to 10deg, (though i think the bathroom still comes on as it has no TRV, but cant remember)

Why is this?!

Thanks

 

[Steelmasons]

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.(16 position)

 

[stem]

So, I have two choices... 1) simply replace the system as is, moving the wires around to their new locations in the backplate as needed. or 2) rewire the whole lot to correlate with a C plan using the 10 way junction box? So, would it be worth doing 2) or not? If so, how would I know whether I need C plan, and that I indeed do have a 2 port valve?!

The advantage with a 'C Plan' is that when the hot water cylinder is hot, as well as closing the motorised valve, a boiler interlock is provided that switches the boiler off too. With the system you have now, it sounds as if even when there is no demand, the boiler will come on every now and again to keep itself hot which wastes energy.

A two port valve looks something like this;

Untitled

• stem
• 26 Mar 2010
• 6

It would normally be found near the hot water cylinder and controlled by the cylinder thermostat. However, C plans are relatively rare. Gravity hot water systems with pumped central heating without motorised valves are [or were] extremely common. Having said that, you do have a hot water cylinder thermostat, this could be just used to provide a boiler interlock, or maybe wired to a motorised valve. Only you can tell us.

In summer when we only have HW on, i set the CH to off... However, the radiators still warm up! To get around this i normally just turn the thermostat down to 10deg, (though i think the bathroom still comes on as it has no TRV, but cant remember)

Why is this?!

Even though the pump is not running, the radiators can still get warm because of natural circulation around the system. Hot water is less dense and so rises. Then it cools and cooler water being more dense falls, so a pattern of circulation is established.

I would recommend that you consider upgrading the entire system to fully pumped [S Plan or Y Plan] giving full motorised valve control of both the hot water and heating. At some point your old Baxi will give up the ghost or spring a leak and require replacing. New energy efficient boilers cannot operate with gravity systems because they need to maintain a minimum flow rate through them so you will have to upgrade the system then anyway.

 

[EdTurtle]

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.(16 position)

I cant program the HW and CH independently, if i have CH on then the HW must be on, but i can have HW on with no CH

The advantage with a 'C Plan' is that when the hot water cylinder is hot, as well as closing the motorised valve, a boiler interlock is provided that switches the boiler off too. With the system you have now, it sounds as if even when there is no demand, the boiler will come on every now and again to keep itself hot which wastes energy.

A two port valve looks something like this;

Untitled

• stem
• 26 Mar 2010
• 6

It would normally be found near the hot water cylinder and controlled by the cylinder thermostat. However, C plans are relatively rare. Gravity hot water systems with pumped central heating without motorised valves are [or were] extremely common. Having said that, you do have a hot water cylinder thermostat, this could be just used to provide a boiler interlock, or maybe wired to a motorised valve. Only you can tell us.



Even though the pump is not running, the radiators can still get warm because of natural circulation around the system. Hot water is less dense and so rises. Then it cools and cooler water being more dense falls, so a pattern of circulation is established.

I would recommend that you consider upgrading the entire system to fully pumped [S Plan or Y Plan] giving full motorised valve control of both the hot water and heating. At some point your old Baxi will give up the ghost or spring a leak and require replacing. New energy efficient boilers cannot operate with gravity systems because they need to maintain a minimum flow rate through them so you will have to upgrade the system then anyway.

Ok, i think i see... so, if i tell you how the motor valve is wired, you can tell what system i have? The cylinder stat wiring definitely goes into the junction box, i can find out how/where the connections are if you want? If so, what info will you need?

Replacement is on the cards, but not for a few years...

Thanks

 

[stem]

If you are just planning to swap over the programmer, and leave the system to function exactly as it is now, there is no need to give us any further information.

If you would like us to identify what system you have, the first thing is to determine if do actually do have a motorised valve or not, and where the wires from the hot water cylinder thermostat go. Also, do you really have a cylinder thermostat? I hope I don't insult your intelligence, but some have confused an immersion heater and a cylinder thermostat.

Should you find that you don't have a motorised valve, the cylinder thermostat could be there to provide a boiler interlock. If so, it would only really work properly during the summer when just hot water water was being used. The central heating would override it in the winter.

 

[EdTurtle]

OK, thanks again. Don't worry about my intelligence, it would be worse that you assume i know what i am talking about! Though having said that, i know i have a cylinder stat, and an immersion - just the valve i am unsure on.

In the meantime (since its so cold) i will just replace like for like, and in the summer when perhaps we don't need the heating, i shall provide further info and possibly go about sorting out a rewire.

On a related note, back to the thermostat... I am confused a little! I currently have a live coming into the thermostat (when call for heat is on) and then a switch live return to feed the pump (as said above). I have no other wires in the wall, no earth either. I want to replace this, I am looking at two, one is volt free battery powered, and has three terminals C, NO and NC. The other is 240V and has L N C NC NO connections. I assume i need the battery powered version because i don't have a permanent live in the wall? Then just wire call for heat to COM and then switch live to NO? Is this correct?

The units are Danfoss RET2000, there re three wiring diagrams below for clarity:

http://heating.danfoss.com/PCMPDF/44035v01.pdf

Thanks again

 

[stem]

I assume I need the battery powered version because i don't have a permanent live in the wall? Then just wire call for heat to COM and then switch live to NO? Is this correct?

Yes, that is correct. You don't have a neutral either.

The units are Danfoss RET2000, there re three wiring diagrams below for clarity:

http://heating.danfoss.com/PCMPDF/44035v01.pdf

I couldn't follow your link, but the Danfoss RET2000 (I assume you are considering the 2000B version) terminals are marked 'Com' 'On' and 'Off' you want 'Com' and 'On'. For other types 'Com' and 'NO' would be correct)

 

[EdTurtle]

Perfect, thanks again! Yes, no neutral either.

 

[stem]

When you said:

Live goes to the stat, then comes back to the valve, but no connections are made to the boiler control unit.

I thought you meant that no wires from the motorised valve go to the boiler, but reading your post again, I now wonder now if you are referring to no wires from the 'cylinder thermostat' go to the boiler. If you do have a wire from the motorised valve going to the boiler live (it's normally orange) then you probably do have a correctly installed C Plan.

Anyway as you say, that's for another time....

 

[EdTurtle]

No wires go to the boiler control except the L N E supply then the return from the wall stat (goes to pump) and the live that goes to the boiler then... So, no wire comes from the cylinder stat or the valve..

I know i said it could wait, but would you like pictures/a wiring diagram!?

 

[stem]

Go on then spoil me

I would be interested to know where all of the wires from the motorised valve go also.

 

[EdTurtle]

Right then, here we are. The three wires from the cylinder stat are - Earth, Brown = Common, Blue = NC, so this breaks when up to temp.


From there the wires go to the wiring unit: Brown goes to the live supply, and blue goes to black on the valve

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• EdTurtle
• 1 Dec 2016

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• EdTurtle
• 1 Dec 2016

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• 1 Dec 2016