Y-Plan wiring theory advice (replacing programmer)

Diggory · Sep 29, 2012

Hello,

I have a question about replacing the programmer in my central heating controls.
I currently have a very basic Honeywell timer/programmer connected to the heating system via the Y-plan.

I would like to replace it with a USB controlled relay. So that I can control the heating and hot water via a computer.
(I would link to a page about the relay, but because I'm new here the board won't let me. But you can google for UDIN-44 if you want more info.)

I am not an electrician, and I will be employing an electrician to do the work, but I would like to understand the required wiring before I employ them, so that I can explain what I would like. Please could I ask your advice? (I'm not familiar with A/C electrics, but do have some experience with 5v electronics theory.)

So - as far as I can work it out in the y-plan at the programmer you have Neutral, Live and connections 1,2,3,4 (DHW Off, HTG Off, DHW On, HTG On). Obviously the On and Off Pins for each system are mutually exclusive - e.g. either DHW On (pin 3) is connected to Live or DHW Off (pin 1) is connected to live - but never both (and the same for the HTG pins.)

What I don't understand is how the neutral connection is connected - from the internal wiring diagram of my programmer and the y-plan schematic it appears that N and L are basically connected together inside the programmer - wouldn't this be a short circuit? Or is that just to provide power for the programmer itself?

Basically what I'm asking is - in my four relays can I just connect live to the central poles and pins 1-4 to the four contacts, or does neutral need to be connected too?

Thanks for any advice.

HandySparks · Sep 29, 2012

Diggory said:
What I don't understand is how the neutral connection is connected - from the internal wiring diagram of my programmer and the y-plan schematic it appears that N and L are basically connected together inside the programmer - wouldn't this be a short circuit? Or is that just to provide power for the programmer itself?

Basically what I'm asking is - in my four relays can I just connect live to the central poles and pins 1-4 to the four contacts, or does neutral need to be connected too?

Welcome to the forum.

The diagram shows L and N connected to "Clock" so, yes, the neutral is just required to power the programmer's internal timer.

And, yes, connecting the two common central poles (2 and 5 from the left in the bottom row) to live and the four relay outputs to the CH on, CH off, HW on and HW off would work. If it's a Y plan you'll probably find that one of the "offs" is not required and not currently used (can't remember which).

You would perhaps have been better to have used two single pole, double throw relays rather than four single pole, single throw relays; you'll have to make sure that you don't energise an "on" and a corresponding "off" at the same time.