Room thermostat to call pump only

[MNW67]

My boiler is a concord WCF 255. I struggled to find anything about it online.

I'm not sure how the runs can be separate when it all fills from the same fill point by the cylinder and drains out through a shared drain valve.

Originally the three port and zone valve were not there. I added those in to give me the ability to force the water either through the whole system or just the cylinder, and the zone valve to heat both floors or just upstairs.

I have found an old post from forum regular @ericmark which very helpfully shows the three incarnations of C-Plan. It looks like you originally had the middle one i.e. there was a cylinder thermostat, but there wasn't a 28mm zone valve on the hot water circuit. If a 28mm valve is added, I believe it has to be the specific version for C-Plan which has an extra wire.

C plan has changed over the years, first View attachment 266854 then View attachment 266853 and thenView attachment 266852 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.

This is the thread he was replying to:

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

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...

www.diynot.com

 

[glnovice90]

My setup doesn't even have a cyclinder thermostat. Like I said, the temperture must cap out to whatever the boiler thermostat limit is because the water is scalding hot.
Looking at those wiring diagrams though, it looks like I can have a direct call to the pump. I'll go ahead with my solution then and see how it goes. Thanks for your help.

 

[ericmark]

This was the start of my central heating wiring, not got the completed version on this PC, it had a fault in that without a relay, now fitted, I could not switch pumps independent of boiler I had expected to plumbers to move to a single pump, but they left it with two pumps.

In the original installation they thought, by selecting the pump, they could select main house or flat, but selecting just one, causes reverse flow in the other. My problem is floor panels which have needed repairs putting plywood on top, which means hard to access any wiring, and the three core and earth changes colour between house and flat, and one core open circuit. Which is why I selected Nest Gen 3, as all will go through 2 wires, however Nest has proved not to be as adverts said, and will not link to Energenie TRV heads, so now has a Wiser Thermostat in parallel so I can monitor both hall and living room temperature, and add bedrooms if required.

At the state of central heating on moving in, I assume the last people used the open fire.

 

[MNW67]

My setup doesn't even have a cyclinder thermostat. Like I said, the temperture must cap out to whatever the boiler thermostat limit is because the water is scalding hot.
Looking at those wiring diagrams though, it looks like I can have a direct call to the pump. I'll go ahead with my solution then and see how it goes. Thanks for your help.

I didn't appreciate that. Why not just leave it as it is and turn the boiler thermostat down so the maximum it reaches is 65C?

 

[glnovice90]

That's a fair point. I'll give that a go.

I do wonder though if seasonal changes and different insulation profiles will affect cylinder temperature accuracy throughout the year. The boiler and pipe work run through an uninsulated garage while the cylinder sits in the floor above and is well insulated.

 

[MNW67]

That's a fair point. I'll give that a go.

I do wonder though if seasonal changes and different insulation profiles will affect cylinder temperature accuracy throughout the year. The boiler and pipe work run through an uninsulated garage while the cylinder sits in the floor above and is well insulated.

That's a bit beyond me!

But I just wanted to mention that you're not supposed to run old boilers with cast iron heat exchangers too cool because it can cause the flue gases to condense and that can cause the heat exchanger to rust. So long term at 65C boiler flow might be a bad idea.

Also, under your plan in the OP, if you don't have a cylinder thermostat, how did you intend to limit the cylinder to 65C?

 

[glnovice90]

That's a bit beyond me!

But I just wanted to mention that you're not supposed to run old boilers with cast iron heat exchangers too cool because it can cause the flue gases to condense and that can cause the heat exchanger to rust. So long term at 65C boiler flow might be a bad idea.

Also, under your plan in the OP, if you don't have a cylinder thermostat, how did you intend to limit the cylinder to 65C?

I would hope the 1-6 range the manufacturer provided would keep the boiler operating issue free...

I'm planning on how to wire my modernised heating system at the minute, which would include creating a new wiring centre and adding the cylinder thermostat. Right now all the valves are operating passively to whatever I set them to.

 

[MNW67]

I'm planning on how to wire my modernised heating system at the minute, which would include creating a new wiring centre and adding the cylinder thermostat. Right now all the valves are operating passively to whatever I set them to.

That makes sense now. I'll be interested to hear how it goes.

 

[Madrab]

If the stat is a traditional banded one then that utilises a bi metal strip, over time these become less accurate as the metal fatigues. That coupled with the fact that the stat is usually located 1/3rd up from the bottom of the cylinder so in practice the top of the water column will typically be hotter than where the stat is located up to the point when the water at the stats location reaches its set point and limited to the max temp of the flow from the boiler. In practice it isn't hugely accurate

Modern pocket stats use a sensor - typically a thermocouple - which is much more accurate but it's still limited to its position in the water column (as far as the mean temp of the water in the cylinder is concerned)

 

[glnovice90]

In that case, if I had the boiler capped at 65 and the cylinder thermostat capped at 65, then all that should happen is over time any inaccuracy drift would just result in the boiler operating longer than necessary?

 

[MNW67]

I would hope the 1-6 range the manufacturer provided would keep the boiler operating issue free...

You might think so. But I've seen the problem mentioned in quite a few places. I suppose if setting 1 equates to a boiler flow temperature of 55C, then much of the time you will be getting return water into the heat exchanger at less than 45C. It seems logical that condensation is unavoidable at that sort of temperature because the dew point is often stated as 54C.

 

[Madrab]

Yes, until the water @ the stat reached the set point. The max water temp in the column would always limit itself to the flow temp into the coil

 

[ericmark]

I use oil to heat DHW in winter, as no way to turn it off, in summer however I use electric, as cheaper and easier.