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Hello everyone
I have recently converted my sisters old open vented semi gravity central heating system to a sealed fully pumped Yplan with Vaillant ecotec system boiler. I was full of enthusiasm for this project before i started. The installation of the boiler and re-arranging the pipework all went very smoothly. Now i am at the wiring of the system heating controls stage things are a very different story, I’m no longer enthusiastic and am actually starting to lose the will to live. My aim is to get the whole system installed except the gas pipe and mains electricity connection which will be done by a corgi/part P man before he commissions it. I believe this should be easily achievable if someone could kindly guide me in the right direction. I have looked at many wiring diagrams online but none with a wiring centre and a system boiler.......Its now time to admit defeat and ask you guys for your help. Below i have tried to paint you as bigger picture as i can without over complicating things. Take a look............

The Components
1 Vaillant ecotec 618 system boiler
2 Honeywell Sundial plan wiring centre 42005748-001
3 Honeywell ST6400c Programmer
4 Honeywell V4073A Mid-Position valve
5 Honeywell L641A Cylinder Stat
6 Honeywell Room Stat (Unknown) Its the dial type, brown and beige installed in the mid 80’s, has 4 wires red to 1 blue to 2 yellow to3 Earth is left loose. Previously wired to a Randall time clock on a pumped heating gravity hot water system.
.All the above components are new with the exception of the existing room stat
The Plan
To wire all the above components for a Y plan configuration fully pumped system boiler.
What I have done so far. . .
1 Replaced old boiler with the Vaillant ecotec, arranged pipe work and fitted the mid-position valve.
2 Screwed wiring centre into the wall in the vicinity of the Switched fused mains, boiler and mid-position valve ( all in same compartment)
3 Run the heat resisting twin and earth cable from cylinder stat to wiring centre (not connected)
4 Run the wiring for programmer. Five wires are needed for a Yplan configuration [NL1 34]. I have run 2x Twin and earth, is this acceptable? (not connected)
5 I have an existing wire connected at the room stat. Red-1, blue-2, yellow-3, loose earth run to wiring centre (not connected)
6 I have wired the mid-position valve to the wiring centres mid-position block as this was relatively easy.The terminal block accommodates a brown wire which I don’t have on the mid-position valve. Should I be concerned?
I’m confused
The wiring centre gives wiring instructions for pump overrun boilers and basic boilers, pump overrun requiring 5 wires and basic requiring 3.The vaillant ecotec being a pump overrun. The boiler installation manual gives instruction for a 3 core flexible cord only. Although the boiler does have 3 more terminals for " EXTERNAL ELECTRICAL CONTROLS . . . The boiler terminals 3,4 and 5 are for connecting external electrical controls such as a time switch and/or room thermostat. Terminals 3 and 4 are linked when the boiler is supplied. If external controls are used, this link must be removed, and the controls connected across terminals 3 and 4. Terminal 5 is an additional neutral connection for external neutrals such as from the anticipator of a room stat. If a room thermostat is to be connected in addition to a time switch the wire between the time switch ON terminal and boiler terminal 4 should be broken by the contacts of the room thermostat" I don’t know how relevant that is?? The boiler manual also says "CIRCULATING PUMP. . . The boiler incorporates a built in circulating pump that is fully prewired (no additional wiring necessary). The pump incorporates an automatic overrun period after the boiler switches off". What would i do with the wiring centres pump block?The boiler manual gives no info for wiring via a wiring centre.
I suppose, in a nutshell, my question is. . apart from what i have done so far ( which isn’t a lot) How do i do the rest? Which colour wire goes to which terminal number, do i need to make bridges anywhere, do i need to remove or cut links etc. I’m aware i need to cut links 1,4,5 and 6 in the wiring centre.
I know this is a big ask and i am fully aware that information on these pages comes from the goodness of people’s hearts so in advance i would like to give a massive thanks to anyone giving advice. . . it really is appreciated..........THANKS.
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  • Thread Starter Thread Starter
  • #2
First thing to do is to write smaller this size is fine and easier to read ,now im not having a go at you but i think youre out of your depth on this ,best thing to do would be to ring round and find an electrician who des this kind of work ,as it has to be right ,sorry i cant help you more but i know hyow tricky this stuff is from past experiance,
good luck


Not wishing to be picky BUT apparently fitting an unvented water heating system is as regulated as oil, gas, electric, (ok maybe not gas). The supplier of the cylinder or zone valves usually IME supplies wiring diagrams for this. All I can offer is take your time. programmer calls for energy for DHW, so sends feed to cylinder stat. If stat isn't satisfied it feeds through to "boiler" to ask for heat. Same with CH. Programmer sends out a"live" to the boiler but it has to go through the room stat first, then if hating is still required the boiler gets a belt (sorry , it's friday that should read "If heating is required and enough about my ex wife). If still no joy with this I'll try and dig out a diagram and PM it to you.


  • Thread Starter Thread Starter
  • #5
from memory last time i did an eco the link between 3 + 4 comes out and switch live from 3 port valave(orange) goes into pin 4.dont worry about 5 thats just an extra neutral feed. Dont think the 1 i did had internal pump aswell as the external pump but it was an s plan.the boiler has a good diagnostic and gives a code as to what is goin on on the lcd, codes are given in the manual

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