Understanding Y-plan

[Wingedcat]

I would like some help to understand my CH/HW configuration.

My system is a totally standard open vented fully pumped Y-plan system with an indirect mains pressure HW cylinder and 5 radiators. However, there is one strange anomoly.

As you might expect the mid position valve is on the flow pipe from the boiler, and its two outputs branch off to the Hot Water and the Radiator loop.

However, there is a 2-way valve positioned between the HW port of the mid-position valve and the connection to the Flow port of the HW tank coil. This valve opens when the Call for Heat signal is given to the boiler, i.e. HW, CH or Both are demanded by the controller.

At first glance this extra 2-way valve seems completely unnecessary. Why has it been put here? What harm can it do? I initially thought it could not do any harm if I leave it as it is. However, as the 2-way valve is connected to the Call for Heat signal to the boiler and not the live signal from the boiler to the pump, it could present a problen.

For those that understand 3-way valves, if the valve gets left in the HW only position, and the boiler overheat stat fires up the pump, (which it does do for a few seconds every time the boiler has been on for a while then subsequently turned off), the pump will be asked to circulate the water in the HW coil, but won't be able to because the extra 2-way valve is shut, therefore straining the pump.

Is this worth worrying about?

Thanks for your help

Luke

[Christian McArdle]

It is an essential safety feature designed to prevent a major explosion in the event of a boiler fault. It would unlawful to remove it. It appears to be miswired. It should be wired to a stat on the cylinder and only open when the cylinder calls for heat.

It is not permitted for safety reasons to use a mains pressure cylinder as a boiler bypass as it can result in overheating of the cylinder. The separate valve enforces this restriction. If your system requires a bypass, then fit a separate automatic bypass valve to give the water somewhere to go.

Christian.

[Wingedcat]

Thank you for that Christian.

I understand what you say and see why it is used as a safety device.

However I can't immediately work out how to wire the 2-way valve correctly as you advise, i.e. to the HW On signal only. As you can imagine the HW on signal from the Tank Stat is wired to Call for Heat on the boiler. However, the orange output wire from the 3-way valve is also wired to Call for Heat on the Boiler (live when CH is ON). Therefore, I can't see how I could configure the 2-way valve correctly so the it opens ONLY when the HW is on without using some sort of logic gates. To be honest, it's probably not doing any harm being open when the CH is also on, as the 3-way valve will be in CH only mode.

Okay, the bypass valve. As I understand it, this caters for when the mid-position valve finds itself on CH only (i.e. grey wire - HW Off - is live) and the overheat stat activates the pump. It must be fitted between the pump and the mid-position valve and connect to the Return pipe to the boiler. Could you just confirm that this is correct.

Thanks again Luke

[Wingedcat]

Sorry I meant:

Okay, the bypass valve. As I understand it, this caters for when the mid-position valve finds itself in a relaxed state (HW only) and the overheat stat activates the pump. It must be fitted between the pump and the mid-position valve and connect to the Return pipe to the boiler. Could you just confirm that this is correct.

[Christian McArdle]

Yes.

Christian.

[Christian McArdle]

I'd fit an additional cylinder stat and run it off that. You should really get the original installation instructions and follow those, though. An alternative is to connect it direct to the original stat and have the stat also drive a relay than slots into your Y-Plan system instead. However, check that a second stat isn't required as part of the safety scheme.

Christian.

[Set Square]

It sounds as if it has been wired to the cylinder stat's 'call for heat' contact in the (mistaken) belief that it will only open when the HW temp is below the stat set point. But whoever did it was ignorant of how Y-Plan works, and didn't realise that it would also open in CH mode!

You need to isolate this valve from the rest of the Y-Plan electrics with one of the following:

• Use a double pole cyl stat (if such a thing exists) - with one set of contacts used in the normal Y-Plan fashion, and the other set driving this valve

• Use a second cylinder stat to drive the valve - maybe set slightly higher than the main stat - but well below the safe working temp of the cylinder

• Use the existing stat to drive a 2-pole relay, and use the 2 poles as per double-pole stat, above

subsequent post] (i.e. grey wire - HW Off -

Yes, that's where to connect it.

As an alternative, since you are fitting a by-pass anyway, why not get an additional 2 port valve and put it in the CH circuit and convert the whole thing to S-Plan. Replace the 3-port valve with a simple tee-piece. It then gets a whole lot easier to understand - and avoids the need for any of the 3 solutions listed above!

[Christian McArdle]

You would need to replace the 3 port with 2 valves for S-Plan. The existing

2 port valve must not be used for service use, but must be additional for safety only for when the service zone valve fails. It should be tested annually to ensure that it is operational, along with the other safety devices, such as the over temperature and over pressure blow offs.

Just to clarify this, there must be TWO zone valves between the boiler and the hot water cylinder. This also applies to the electric immersion (which is by nature more dangerous). There must be 2 thermostats between the electric supply and the element.

Christian.

[Wingedcat]

Thank you both Christian and Set Square for your extremely useful help.

Firstly it is not possible to put in a second stat tank because it has to be a special tank stat specially accommodated into the unvented cylinder. I don't fancy this anyway in case the safety stat fails in the open position then the safety two way valve will never open.

I think the way to go is to convert to S-plan and then I will have independent control over HW and CH making connecting up the safety two way valve much easier. Of course, it will involve a bit of modification to the pipework but nothing major. Bearing in mind I will have to rejig it a bit to take into account the bypass valve, I think this is the way to go.

I also understand that I need a further 2 two-ways valves leaving the safety in place.

Interesting point about the immersion heater requiring two thermostats. There is indeed an immersion heater fitted to the tank but whether it contains two thermostats internally I don't know. The cord to it simply terminates in a 13 amp plug which is kicking about on the floor of the airing cupboard not plugged in (there is no socket for it in the vicinity).

Thanks again to you both.

Luke

[Matt]

So why does the standard Y plan diagram thats been around for years never show this extra valve?

[Christian McArdle]

That would be the normal situation. You have one set to 55C-60C. The other is set to around 70-75C and should be a manual reset type that won't turn back on without pressing a button.

Christian.

[Wingedcat]

Because this is an unvented mains pressure hot water cylinder which requires the extra valve. Most hot water cylinders are gravity fed from a loft tank so this extra safety precaution is not required.

[Christian McArdle]

Because it is specific to mains unvented hot water cylinders. It is one of the reasons that you must possess a specific training certificate to install these commercially. An untrained spanner monkey plumber would just install them like a traditional vented cylinder and people would be blowing up all over the place, allegedly.

Christian.

[Christian McArdle]

PS. Whoever installed it wasn't worthy of the certificate, as the zone valve has been effectively bypassed by being miswired.

However, it isn't anywhere near as bad as some installation errors that could be made. This single miswiring would require a plethora of other failures and bad luck to present itself in an explosively dangerous fashion. That doesn't mean it shouldn't be fixed, though.

Christian.

[Matt]

Sorry misread your original post, saw the vented but not the mains pressure!

"My system is a totally standard open vented fully pumped Y-plan system with an indirect **mains pressure** HW"

[Ed Sirett]

The extra zone valve is correct. It is not permitted to use only a 3-port valve to control the heating to an unvented cylinder. The cylinder has been installed correctly.

If the boiler requires an a bypass for maintaining the flow after firing then the primary circuit should be fitted with a permanent radiator (e.g. towel warmer) or a smart bypass valve.

[Ed Sirett]

Reading the other posts I would like to clarify something. The only time this arrangement comes up is when an unvented cylinder is retro fitted to an existing heating system that previously used Y-plan. Anyone starting from scratch would use S-plan controls for sure. Unvented cylinders are (IME) supplied with a zone valve anyway.

For this arrangement to work correctly the additional zone valve on the indirect coil must be the sort with a a changeover micro switch (grey, orange and white wire) so that HW satisfied signal is available for the

3-port valve.

The HW demand is taken through the tank stat to the 2-port valve which then produces both HW demand and satisfied signals for the 3-port valve.

The boiler sounds like it really needs a bypass circuit which was lost when the unvented cylinder was installed.

[Set Square]

Ah yes - clever - why didn't I think of that?!

You mean 'lost' because of the presence of the additional zone valve, which no longer guarantees that there will always be one path open through the

3-port valve?

[Wingedcat]

Now Ed this is sounding really clever. Let me just take a moment to make sure I understand.

Tank Stat Closed (ie. HW Demand) goes to Brown on 2-way valve. Therefore, the only time the additional 2-way valve is opened is when the Tank Stat is demanding heat. (Totally correct).

Permanent Live is connected to Orange on 2-way valve. When the valve is shut, the White wire becomes live. This goes to the Grey wire on the

3-way valve (i.e. HW Satisfied).

Grey on the 2-way valve goes to SW Live on the boiler. Therefore when the valve opens (caused by Brown becoming live), boiler fires.

A side effect would mean HW Off from the programmer and also Tank Stat Open wires are no longer required.

This is sounding like the best solution so far as I just have to rewire the 2-way valve and fit the auto bypass. (Bypass Radiator would be rather too complicated from a plumbing point of view to be worthwhile).

Just to fill you all in, yes this mains pressure cylinder was retrofitted from a gravity system hence the re-use of Y-plan rather than a start from scratch S-plan. Thanks again for all your help

Luke

[Set Square]

Agreed. So you only need the C and NC contacts on the cyl stat - just used as an on/off switch to drive the 2-port valve motor - rather than as the change-over required by Y-Plan.

No, this is not quite correct (above 3 paras). The volt-free contacts on the

2-way valve need to exactly replicate what the cyl stat previously did. So common (orange) should be connected to HW demand from the programmer rather than permament live. The other 2 (when you've worked out which way round!) should be connected to grey on the 3-port valve + HW off on the programmer, and orange on the 3-port valve + boiler.

You *do* still need HW-OFF from the programmer - otherwise the system won't work in CH-only mode.

Yes - but make sure you get the wiring right!