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