It's got all of the components of a C-Plan system, but it's not wired up like that!

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M51

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I've spent quite a few hours trying to fathom how my boiler is configured.

At first sight it appears to have all of the components of a c-plan system, 28mm 2 port valve, hot water cylinder thermostat, central heating pump and room thermostat.

Firstly the controller, a British Gas UP2 (Drayton 722 equivalent possibly) has the P-G jumper set to G for Gravity, rather than P for pumped. This means that hot water is always demanded (connection 3) when the heating is on (connection 4), I would like to control heating and hot water independently.

The wiring is then done in a very strange way, there is a constant live through the hot water cylinder thermostat to the 28mm 2port valve, so whenever the cylinder is below temp the valve is open, irrespective of boiler operation (the 3 wires for the microswitch have been cut short and are not used). Connection 3 (HW) of the controller provides the live feed directly to the boiler, and connection 4 (CH) provides the live feed to the central heating pump via the room thermostat, I've chopped together a diagram that tries to show this...
Boiler - Control.jpg

It's quite an old system that's been modified over the decades by the looks of it, perhaps the heat leak radiator was from an old solid fuel boiler?

The question is, (assuming I can get to the 3 microswitch wires on the 2 port valve) can I just rewire this with standard c-plan sundial wiring, set the P-G jumper to P and have independent hot water and heating?
Boiler - C Plan.jpg


Electrically it looks to me as if the answer could be yes, but from a plumbing point of view my concern is the location of the 2 port valve, I've read conflicting opinions about where the valve should and shouldn't be but I've not managed to find a fully justified reason for any particular location.

Thanks for reading this far, any advice is greatly appreciated!
 
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Sounds like the system may have been bypassed at some point? It may be feasible to just add another 2 port valve and renew existing/replace head. Can you post photos of the wiring center, 2 port valve, and possibly the programmer wiring?
 
The "wiring center" is just a round 4 terminal junction box with the common E L N and the connection between the cylinder stat and the 2 port valve.
boiler wiring - Copy.jpg


I've attempted to sanitise things a bit and show what the existing wiring would look like if done in a standard 10-Way Junction Box...
Boiler - What Plan.jpg
 
Certainly looks doable from everything being close.
 
Yes, I thought plenty of scope for re-wiring too. But is the 2 port valve in the correct (safe?) place to allow for full c-plan wiring?
The Honeywell Plan C Installation docs state...
Valve - The zone valve should be fitted in the DHW primary circuit so that, when closed, it does not close off either the cold feed or vent. It is preferred that the valve is fitted in the return to the boiler.
But I have seen unclear/conflicting diagrams/advice and I've not managed to find a fully justified reason for any particular location for the valve being in the flow or return etc.
It looks like this valve is fitted just before the return into the boiler, but doesn't that effectively stop the cold feed to the boiler?
 
Sorry, I meant to advise, I would change it to an S plan.
 
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Ah, yes, you did mention "just add another 2 port valve" I didn't make the connection between that and S plan as I wasn't familiar with any of this until a week ago.
Thanks very much for taking the time to look at this and provide your advice Chris, I do appreciate it.

If anyone has advice on the 2 port valve location with c-plan wiring I'd be pleased to hear that too.
 
C plan has changed over the years, first C-Plan_old2.jpg then C-Plan_old.jpg and thenC-Plan.jpg seems you have latter, since you can turn off the thermo syphon you can control DHW temperature summer and winter, and it can work either CH only or DHW only.

However it does depend on the house, I found even with CH pump turned off some radiators would thermo syphon, main problem is when CH is turns off but boiler remains on for DHW then since circulation started for CH it continued.

I have C Plan but basic first one shown, I do have motorised valves but to select with part of house is heated, or more to point not heated. I found although two pumps one for flat and one for main house, turn off one pump and the water flowed in reverse direction.

Plumber did point out if the TRV's close you should not get any flow, but flat TRV's were simple wax type and never seemed to be fully off.

He advised me not good for boiler fitting a motorised valve to DHW as no run on to cool boiler.
 
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Thanks for the reply ericmark,

Yes, from the hardware installed it does look like it should have the 3rd set of c plan wiring that you've shown,

Your last comment about the 2 port valve on DHW is the thing that I'm concerned about.
I realised that my diagrams didn't show the valve where it really is in my system, it's on the return just before the boiler, like this..
Boiler - Control 2.jpg

It sort of fits with the Honeywell Plan C Installation docs that state...
"Valve - The zone valve should be fitted in the DHW primary circuit so that, when closed, it does not close off either the cold feed or vent. It is preferred that the valve is fitted in the return to the boiler."
But I'm just not sure if the valve was installed in the wrong place and possibly that's why the wiring is very non-standard for c plan, of if it was wired like that due to lack of knowledge
 
I am not a plumber, OK worked for SLD pumps at one point so understand pumping to an extent, but my system also has the motorised valves and actually also the pump on the return.

The gas boilers always seemed to have pump and valve on the supply side, plus a by-pass valve, but it seems I don't have a by-pass valve, well would not really work on the return in the same way.

I have worked on oil with hot air, but not with water, but I have in main house 10 TRV's 9 of which have electronic heads and I think it is highly unlikely all my TRV's would be off completely to cause the pump to cavitate. At least not for long enough to cause a lot of damage.

My pipe work is wrong some where, I know I can get hot water return when all radiators are cool, so boiler cycles before it should, but the garage under house has been converted into 4 rooms, and all walls have been lined and not a clue what is behind the plaster board, I know one cable starts upstairs as red, yellow, blue but arrives down stairs as brown, black, grey, with one core disconnected. So I use Nest Gen 3 so two wires are enough and they only carry 12 volt DC.

So it is down to near enough engineering. I know there are faults, but it works reasonable well and I don't want to rip plaster board off the wall. This is my circuit diagram C_Plan_My_House-theroy.jpg as yet not fitted second relay, on my to do list, but don't really use flat so no real hurry. I found making the circuit diagram is important one to see errors, and two so some one else can work on it if required.
 
Ok, I think this thread is a bit messy with my incorrect diagrams and misunderstandings.
I've realised that what I'm really asking in a nutshell is...

On a C Plan installation is it acceptable to have the 2 way valve on the DHW return just before the boiler?

I'm now guessing it is, as that's where it's been for the past decade or so.
Although it is currently wired differently (as the DHW control is always on when the CH control is on) but there is the situation where the DHW is up to temperature and the valve closes due to the cylinder stat and the boiler counties to fire where there is still unmet CH demand.
That was the scenario that I was concerned about.
Perhaps I've answered my own question but I'm sure there's a textbook answer on the location of a C Plan valve that would disagree with me!
 
Razor900 said:
@ericmark combis and system boilers all have the pump on the return as well ;)
Click to expand...
I did look at the idea of by-pass valve, but bit which needs by-passing is the bit containing radiators, one atmosphere is 14 pounds per square inch, so any by pass would need to be a lot less, and the plumbed turned up with 15 pounds per square inch by-pass valves which clearly would not work.

I suspect there is a hidden by-pass valve as when I tested the pump pipe work was hotter than the radiators, so must be a way for water to by-pass radiators.

Previous owners hid things behind plaster board, including a full fuse box, so very likely some bits unaware of.
 
All rewired to the correct C-Plan scheme in a proper 10 terminal box and the Pumped/Gravity jumper on the controller set to Pumped.
Now have independent control over heating and hot water.
All working for the last 2 weeks without any issues and saving a fortune by not being forced to maintain a tank full of hot water when heating is on.
 
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