wiring central heating systems

Postby fisha » Fri Jul 25, 2008 2:54 pm

hi i was wondering could anyone give me advice on wiring central heating sysytem.after your supply is brought to a spur is a switchwire and neutral then brought to the time clock and after the timeclock does a switchwire go to the valve and then boiler .any advice would be greatly appreciated .
Posts: 17
Joined: Fri Jul 25, 2008 2:46 pm


Simply Build It

Postby ericmark » Fri Jul 25, 2008 4:33 pm

There are many types of central heating systems with non combi systems most follow one the the Honeywell plans C, W,S Y etc. You will find these on Honeywell web site.

Often thermostats do not directly connect to the central heating boiler but to a valve which once set to correct place then in turn starts the boiler.

Try http://www.ericmark.talktalk.net/boiler.html click on C to get to Honeywell web site.

All best Eric

Postby fisha » Fri Jul 25, 2008 5:04 pm

thanks alot thats alot of help.i was always curious about how the different systems work i only want a basic system with an upstairs and downstairs stats that feed the pump .so i presume that the clock feeds the boiler directly .im premusing that if the temperature is above the temperature set on the stat that hot water wont go to the radiators because the pump valve is closed.
Posts: 17
Joined: Fri Jul 25, 2008 2:46 pm

Postby TOPSPARK » Fri Jul 25, 2008 9:52 pm

I agree with ericmark but just a point to consider as i found out earlier this week if you have a sime combi boiler then you will need a relay unit in order to use a room stat as the call for heat connections are of the no voltage type
all the best
Posts: 97
Joined: Fri Jul 25, 2008 9:44 pm

Postby ericmark » Sat Jul 26, 2008 8:27 pm

Many years ago with very little money and new house with no central heating I tried to fit my own. Thought I would used thermo siphon for hot water and pump for central heating. Spend ages working out radiator sizes. All fitted then found mistakes.
Although I had planed on bedrooms being cooler than down stairs the bed rooms were boiling as heat goes up. The central heating would just heat the hot water unless pump was switched on then even if switched off again thermo siphon heated all up stairs radiators. I also fitted a myson radiator which was noisy when watching TV late at night. So extra radiators fitted down stairs, extra boiler fitted to do hot water mainly to be able to use airing cupboard as bedroom but also cured control problem. Then last fitted thermostatic radiator valves. Been in 28 years now just about got it right.
And most of it now old hat but not worth changing.
Some things worth thinking about though.
Sealed systems need annual check open vented don’t.
Heat storage tanks have moved on from my day with the hot water going through a hot coil to domestic so domestic hot water at mains pressure opposite way from when I fitted mine.
The tank has extra hot coils to use solar power.
Combi systems are very good with limited space but will not allow multi fuel heating i.e. solar power.
All boilers are now condensating so water flow rates are far more important than in my day if too hot on return it will shut down boiler. The boiler does not switch off and on any more but has variable output. Incorrectly sized or throttled radiators can mess up whole system.
Because it is condensating and/or combi does not mean it has to be closed loop. Closed loop if it fails has limited amount of water it can leak but where mega flow and the like is used has to have annual check. Open system does not need annual check but should it leak it will continue to leak until you turn it off.
There is no right or wrong system all have advantages and disadvantages and every heating guy will try to sell you something different. In the main most heating guys have their favourite boilers and want you to use what they like fitting. Although there may be better systems if you don’t use what they recommend the chances are if it goes wrong you will have real problem getting it fixed. There are also some real odd systems like the Sterling engine which produces both heat and electricity from gas or oil sounds great but if it goes wrong how will you get it fixed. I have seen many electricians fitting central heating systems but if it does not work they can’t fault find they can only re-wire from scratch you really do need to pick carefully.
Because of the problems getting people to understand how central heating is wired the combi has become popular as all it needs is a supply all controls are done by manufacturer so not relying on electrician knowing what he is doing. Common now not to have room thermostat but rely on radiator TRV’s may be centrally heated but not centrally controlled. Sometimes a thermostat is placed in a non heated part of the house to turn off system in summer.
A lot to think about. Knowing what I do now I would not install my own system again.
All best Eric

Postby sparx » Sat Jul 26, 2008 8:59 pm

Hi all.
a couple of other points to bear in mind,
1] part L of building regs insists that where control is by TRV's one area must have rad without one and a room stat must be fitted in that area to prevent boiler cycling on & off, this can be combined with timer ie programmable room stat.
2]not only SIME boilers that need relay. All BOSCH Worcester combi & system boilers need the controls external to boiler to be fed from boiler ie
not correct to take perm. live and switched live & Neutral & earth from external controls direct to boiler. It will work but it bypasses all the boilers internal safety circuits!!!
Correct way is to fit relay contacts in place of internal link Ls-Lr, then use sw.line to relay coil and coils N to spare N term on connector plug.
This confirmed by B-W tech. help line, if you connect these boilers you will know the set up,
regards SPARX
Posts: 2166
Joined: Wed Mar 21, 2007 8:33 pm
Location: The fifth continent.

Postby TOPSPARK » Sun Jul 27, 2008 6:24 pm

Hi fisha
The heating system your are describing is an s-plan system with an extra room stat and two zone valves.One zone valve is for downstairs and the other for upstairs as is the two room stats. Goto the honeywell website and the s-plan diagram is on there but all the room stat and zone valve cables and a fivecore cable from the combi boiler also the supply from the switch fuse spur goto to a ten way box which is the wiring centre for the whole control system. If you dont understand from the s-plan wiring diagram how to connect the system up i suggest you call in a spark who knows how to wire the system up for you.
all the best
Posts: 97
Joined: Fri Jul 25, 2008 9:44 pm

Display posts from previous
Sort by
Order by


  • Related Topics