Y-plan wiring questions

[Geronimo W. Christ Esq]

Hi all,

Further to discussions a few months ago in the group I've followed the recommendations of the experts and bought the following components to upgrade my simple CH/HW setup to a fully controlled Honeywell Y plan system :

- Honeywell wiring centre (not a terminal block)

- V4073 22mm mid-position valve

- L6400C full programmer

- L641 tank stat

- CM67RF wireless room stat

These will be connected to my existing Baxi Solo 3PF pump overrun boiler and my current pump. The plan is to install all the electrical bits in the airing cupboard relatively close to the cylinder so that I don't have to extend the wires.

In advance of actually installing everything, I am installing the components screwed into a piece of wooden board with the pump/boiler simulated by a standard 240V lightbulb, so that I can make sure my understanding of how the components all work together matches reality. The questions are :

- does anyone know how to wire a CM67 to a Honeywell wiring centre ? I don't have a schematic for it. The room stat terminals on the wiring centre are marked E, 1, 2 and 3. The terminals on the room stat itself are N & L plus A, B and C. I could buzz it out with my meter but I'd feel more confident hearing from someone who'd already done it :)

- do I need a bypass valve ? My bathroom radiator has no TRVs fitted, so I assumed there would always be a route between the flow and return irrespective of the position of the three-port valve.

[Geronimo W. Christ Esq]

The answer to this is, that room stat terminals A and B go between wiring centre terminals 1 and 3. I was pleased to find that I'd wired it correctly the first time. The obvious hint was the note in the instructions to short terminals 1 and 3 if no room stat was present.

Don't forget that if you are using Y Plan and a two-channel programmer (like the ST6400C) to snip link L1. The instructions on the wiring centre don't mention this inside the lid. If you don't do this then you get strange behaviour on the boiler side.

Nobody piped up to answer this :) I guess I can get away without one.

[Roger Mills (aka Set Square)]

In theory, you shouldn't need it with a Y-Plan system - because there should always be an available path for the water to take during the pump over-run period (unlike S-Plan, where all zone valves are closed at this time). The only proviso is that you need at least one reasonably sized (but not massive) radiator which *cannot* be turned off - which means that it should have 2 lockshield valves, and no TRV. There will then still be a path even if the other rads all have TRVs, and they are all off. It sounds as if your bathroom rad could meet this condition - but make sure that neither valve has a handwheel on it!

[Geronimo W. Christ Esq]

Thanks for that reply Roger.

The bathroom radiator is relatively small, but I am considering moving the TRVs from the radiator in my living room into the bathroom - since that is where the room stat will be and I understand that the system will not behave correctly if the room stat fights the TRVs. I may as well have the living room radiator act as the bypass.

I assume that when the three-port valve is set to do hot water only, that the cylinder coil will suffice as a bypass.

The pump overrun thing is a bit tricker than I thought. I had planned to follow one of your recommendations and run a five-core cable between the new wiring centre in the hotpress and the boiler downstairs, but it turns out that there is not conduit all the way (it goes across a floor as well as between walls) and tracking walls to install cables is not something I want to get into. Instead I'm going to re-use the existing three-core cable, which up until now has supplied mains to the pump, to carry the SL and PL signals to/from the box, and then provide the boiler with it's own permanent live from the existing mains connection in downstairs. I guess that should be easy, just a matter of putting new sleeves on the wires to ensure that it is clear they are live.

[Roger Mills (aka Set Square)]

That's fine - but make sure that it can't accidentally be turned off - preferably by having two lockshield valves on it.

Yes - as long as any balancing gate valve in the HW circuit isn't turned down too much. If you can contrive to run the HW heating and CH at

*different* times, you won;t need to do any HW/CH balancing.

That will work, but it's very bad practice! It is highly recommended to have the *whole* system powered from a *single* FCU so that anyone working on it can be *sure* that nothing is live. Powering it from two separate places will not achieve that - and you may get sued for electrocuting any unsuspecting BG (or whatever) person who comes to work on it!

[Geronimo W. Christ Esq]

The plan is to have the HW running relatively seldom, so I hope to balance the system such that it discriminates in favour of the HW whenever both are running, ie such that the gate valve is up a good bit. I only ever use HW in the morning and briefly in the evening, so it would seldom be running long enough to make the house cold :)

I accept your point and the note of caution, but running a five core is going to be too damn expensive to do, I'll have to take up boards and remove my present bath to do it.

As a related point, surely a reasonably knowledgable person would use a phase tester before risking his life on the assumption that things had been wired the way they appear. I've seen/heard of examples of far, far more dangerous things around houses than what I'm thinking of doing, undoubtedly you have too. I bet you'd never stick your finger near mains wiring without testing it first :)

[Roger Mills (aka Set Square)]

Hopefully - but it's not unreasonable to assume that the system is dead once you've turned it off at the FCU.

I suggest you fix prominent notices at each supply point making it clear that *both* switches must be off for the system to be isolated.

It's even worse if the two connection points are not on the same ring - because you can then turn a ring off at the CU, but it is still live!

[Geronimo W. Christ Esq]

Good advice, and I plan to do just that. No harm in being cautious.

This will indeed be the case unfortunately. The control gear upstairs is being taken off the circuit where the immersion heater runs; the boiler's switched live will be on the kitchen ring. At the minute everything is on the kitchen ring.