Combi wired up to a unvented cylinder

[Drew7779]

Hi does anyone have a decent diagram for wiring a combi boiler to an unvented cylinder, nothing on net .

Thanks

 

[tamz]

I take it you want to make the combi work on an S plan supplying one hw tap from the combi and the rest from the unvented?

If so wire it as a std S plan but dont connect the gray and orange wires as you would normally.
Instead connect the gray and orange wires from the valves through the roomstat connection on the boiler. This is vital if the stat connections on the boiler are volt free. but will still work the same if 240v.
When the hot water or heating is switched on at the programmer it will send the power to the brown wire of the valve and open the motor. This will make the microswitch on the valve (through the gray and orange wires) and ignite the boiler.

 

[aggis]

Hi,
If working on unvented cylinders you need to hold current certificates G3, & electrics Part P. both the installation & electrics are notifiable to local building control.
Sorry if you are registered, but these cylinders are a potentially very dangerous if not installed or wired correctly.

 

[Drew7779]

Sorry should have mentioned I have full 17th edition and part p , g3 and gas safe .
planning to use a vaillant boiler and connect orange and grey wires from valves to terminals 3and4 removing the red link wire for room stat.

 

[Drew7779]

Thanks for info tamz . Much appreciated

 

[Puddle]

Sorry should have mentioned I have full 17th edition and part p , g3 and gas safe .
planning to use a vaillant boiler and connect orange and grey wires from valves to terminals 3and4 removing the red link wire for room stat.

Take it you mean you are putting a system boiler in not combi

Your high limit thermostat on unvented cylinder should be on that circuit also (orange/gray wires form valve,usually gray feed,orange supply) ,directly cutting boiler/heat source off as if you put on valve opening circuit and valve sticks open,boiler will stay on even when over heat operated

You mention two valves so make sure both valve feeds to boiler connect before you loop through high limit for cylinder,so in effect ,if cylinder over heats will stop operation of heating side as well

As obviously if hot water valve stuck open or by passing and unvented cylinder over heats ,you do not want heating working either 🙂

 

[aggis]

If you contact Honeywell, they have a booklet on wiring diagrams & fault finding for c/h systems.

 

[fuzzy]

?? grey wire is a permanent live, orange is the signal wire from a two port.
why did you do 17th edt and part p?

why use a combi with an unvented cyl??

 

[tamz]

The grey and orange wires are the microswitch. The grey is usually a permanent live but it is immaterial it is just a switch. That is why it will work volt free (low voltage).

There are many instances where it make sense to fit a combi and an unvented.
For example: say the boiler is fitted in a utility room of a 3 bath house with the kitchen through the wall from the utility.
Normally you would fit a system boiler and a cylinder as the combi will never feed the demand. But say the bathrooms and the cylinder are miles away from the utility and kitchen. This would mean waiting for ages for the hot water to come through to the kit/utility.
So you fit a combi and supply the kit/utility, a 24 would do fine as it is only supplying 2 sinks, and fit the rest of the system as an S (or Y) plan.
Combi's are also usually cheaper than system boilers. ie a 24i junior costs around £700 whereas a 24i system (same boiler with some bits missing) costs around £750. 😕

 

[fuzzy]

yeh, i can never understand why a system boiler is more expensive and there less in it?????? i presume its becuase they make less?

i realise there are instances when it makes sence but its certainly not the norm is it.

i know what you mean about the microswitch, i have been to jobs when people have adjusted wires to suit themselves, sometimes it even works! problem is for the person following, best to keep as close to the design as possible

 

[tamz]

i have been to jobs when people have adjusted wires to suit themselves, sometimes it even works! problem is for the person following, best to keep as close to the design as possible

That is why you should learn what switches what and not depend on the standard wiring diagrams.
There are many ways to wire a heating system depending on who does it. Heating systems on new build where it is wired by the site electrician will definetly not follow a std diagram. You will open a joint box to find a pile of three cores. Easy enough to trace if you know what to look for.

The combi and unvented is not a normal thing you will come across often but they are fitted depending on the layout of the house.

 

[fuzzy]

i agree that people can learn to wire by just looking at the diagram and therefore not really know whats going on!! its like painting by numbers. that ok when first fitted but they should know what it does as well as following a simple diagram. that said why do electricians not follow a diagram, they should as often i will follow an electrician and theyve just done it their way and its difficult to suss out, and often its wrong

 

[tamz]

why do electricians not follow a diagram,

Because they (should say some) know what swiches what. It can be wired any way they like as long as the end result is the same.

An S plan is easy. Just learn this.There are a couple of minor variations but basically
HW on at clock through cylinder stat common, call to brown (motor) of valve, miroswitch makes and sends power to boiler (SL)/pump. The heating is the same but through the roomstat.

A Y plan is a bit different and harder to explain as there are 2 microswitches and a resister and diode.

 

[fuzzy]

i disagree, they shouldnt wire it anyway they like, they should wire it the correct way, thats why we have standard ways of doing thing, its not everyone please theirselves.

i know perfectly well how to wire systems, ive been doing them years, there appears to be more wires on an s plan but your correct they are pretty straight forward. Y is only hard to explain for those who struggle to understand it, i find it pretty easy

 

[Drew7779]

A combi or any boiler is just a heat source , it can be wired up in many different ways. The combi/cylinder option is good as you get downstairs hot water instantly and economically and good showers upstairs most combi boilers are not great when another tap is opened or a toilet flushes etc .

 

[GrahamM]

i disagree, they shouldnt wire it anyway they like, they should wire it the correct way, thats why we have standard ways of doing thing, its not everyone please theirselves.

i know perfectly well how to wire systems, ive been doing them years, there appears to be more wires on an s plan but your correct they are pretty straight forward. Y is only hard to explain for those who struggle to understand it, i find it pretty easy

With you on that fuzzy. It is a simple process to follow the wire diagram and then the next guy that comes along to a fault can suss it out in minutes without having to trace all the wiring back before starting to look at what he is there for in first place.
Makes life easier on everyone!
An old contracts manager I know would insist that all the systems fitted under his watch followed the wire diagram. Would not accept it any other way as he new fine well would save his lads time in long run trying trace faults later on.

 

[tamz]

They have been trying to get heating systems wired by numbers since the old ACOPs days. It will never happen. Electricians (especially on sites) will always wire it to suit themselves.

 

[fuzzy]

there have been many attempts to simplify the systems and standardise them, yes electricians seem to want to just do it their own way. we should standardise will it happen? prob not but i always insist on following the common procedure

 

[migoplumber]

forget about microswitches and complicating your life. i presume you have a seperate valve for your ch side.
use your unvented cylinder stat to control the s plan valve direct to the cylinder. (brown and orange linked via cylinder stat)
nice and simple.
shaun