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Update: If you’re looking for a C Plan or similar central heating wiring diagram, click here!

One of things we noted when we first moved in was that it was impossible to run the central heating system to just heat some hot water without also heating all the radiators.

Not too bad in the winter, but definitely not a good thing in summertime when the last thing you want to do is heat the house!

We’d delayed doing this for a while as a “tame plumber” we knew commented when asked right back at the start that it was nearly impossible to do what we wanted to do with the system without completely replacing the control gear, adding valves, moving pumps, and about a week of his time…. along with the associated cost. Not immediately put off by this we parked the idea as something to get back to in the future, and promptly forgot about the issue as we’d replaced the hot water cylinder earlier in the year which solved the issue of needing to run the heating to get hot water.

As we’re in the process of clearing up the old kitchen (which contains the boiler & associated controls) we thought it was a good opportunity to have another look at this problem as we’re about to build a cupboard around the boiler, and figured it would be rather easier to make any necessary changes now rather than after the cupboard was finished.

After some investigation & a little research, it soon also became apparent that our system already contained most of the components necessary for independant control of our hot water & radiators – which rather flies in the face of our ex-friendly-plumber’s comments.

As it was looking more and more like a simple re-wire (!) I thought I’d have a go, and equipped with an appropriate diagram of what the wiring should look like got cracking…!

We have what is supposed to be a “C-Plan” system, consisting of gravity-feed indirect hot water and pumped heating/radiators.

First task was to identify the various cable runs, what they were connected to, and what each core in the cable was doing.

Cracked open the existing “junction box”, and was somewhat amazed to find what can only be described as a rats nest… Incredibly the diagram above and the “mess” should really match… Hmmm.

Think whoever installed this system was a subscriber to the school of creative wiring !!!

Without further ado, the wiring was stripped out, traced back & identified – including this rather bizarre way of obtaining power for the wall thermostat.

Looks somewhat more organised now 🙂

A new “wiring centre” / junction box was promptly mounted on the wall, and everything connected up to match the a Honeywell C-Plan wiring scheme.

To provide separate HW & CH controls we also needed to replace the system’s programmer, so upgraded from a Horstmann C17 single channel programmer to this Horstmann C27 dual-channel one, seen here under test before it’s mounted somewhere.

The wiring was quickly finished off, routing boiler & pump cabling via a separate junction box for tidyness.

As we’re going to reposition the programmer after the boiler cupboard has been built there didn’t seem to be much point in tidying up the wiring & mounting the programmer in its final position.

This looks a little messy at the moment, but works just fine as a temporary solution until the cupboard’s finished.

All finished – took about 6 hours start to finish, most of which consisted of tracing cabling through the house & testing each and every core as the previous “installer” evidently didn’t see any merit in using standardised wiring colours!!

Next stop, re-routing what used to be the hot & cold water supplies to the old kitchen sink. Possibly a job for tomorrow…

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