I have just converted my Y plan to S Plan. All seems to work

However the water circuit has never been 'right'

There is an unvented OSO Cylinder in an airing cupboard (in a bathroom)
the other side of the house to the boiler.

It has a zone valve controlled by a different circuit to the boiler /
time switches etc.
There is no call back to the boiler.

So the time switch comes on for the hot water and pumps to that zone.
The zone valve cuts in and out (on a different circuit) and the bypass
valve deals with the fact that there is no call back from the valve to
the boiler.

SO

I thought I would run a wire from the time switch to the unvented tank.

Looking at the standard s-plan wiring for the zone valves the Grey wire
appears to be permanent live and the Brown wire is live when the timer
calls for the zone to switch on.

Two questions
1. Why does it need the permanent live through the grey wire?
Can I join the grey to the Brown?
(I can send four wires to the otherside of the house that way)

2. Do I need to get this 'certified' because of part 'P' ?

Thanks

Mark

On Sat, 11 Nov 2006 05:21:36 -0800, FGoogle wrote:

I have just converted my Y plan to S Plan. All seems to work

However the water circuit has never been 'right'

There is an unvented OSO Cylinder in an airing cupboard (in a bathroom)
the other side of the house to the boiler.

It has a zone valve controlled by a different circuit to the boiler /
time switches etc.
There is no call back to the boiler.

So the time switch comes on for the hot water and pumps to that zone.
The zone valve cuts in and out (on a different circuit) and the bypass
valve deals with the fact that there is no call back from the valve to
the boiler.

SO

I thought I would run a wire from the time switch to the unvented tank.

Looking at the standard s-plan wiring for the zone valves the Grey wire
appears to be permanent live and the Brown wire is live when the timer
calls for the zone to switch on.

Two questions
1. Why does it need the permanent live through the grey wire?
Can I join the grey to the Brown?
(I can send four wires to the otherside of the house that way)

2. Do I need to get this 'certified' because of part 'P' ?

Thanks

Mark

Assuming this is an industry standard type zone valve then ou should have
an orange wire. This is joined to the grey when the valve is open.

The orange/grey pair can be used to switch on the boiler of course.
They can also be used with 'volt free' switching.

HTH



--
/home/ed/News/Sig for uk.diy

In an earlier contribution to this discussion,
wrote:


Two questions
1. Why does it need the permanent live through the grey wire?
Can I join the grey to the Brown?
(I can send four wires to the otherside of the house that way)

2. Do I need to get this 'certified' because of part 'P' ?


I don't know about any peculiarities of your particular system, but standard
S-Plan works like this:

Each zone valve has two independent circuits - one to drive the motor to
open the valve, and one (volt-free contacts) to switch other things on once
the valve has opened. The motor circuit is driven by the programmer and room
or cylinder stat. The volt-free contacts of all valves are connected in
parallel - with a permanent live as the input and with the output connected
to the boiler and pump. That way the boiler and pump run whenever or or more
zones are calling for heat - but only the zone(s) whose valves are open get
any heat.

If you cross-connect the motor circuit to the volt-free contacts then *all*
valves will open in unison regardless of whether there's a demand in their
particular zone. This is *not* what you want!

AIUI, changes to CH control wiring *are* covered by Part P. But who's to
know when you did it or, indeed, whether it was originally installed as
S-Plan?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

It has a zone valve controlled by a different circuit to the boiler /
time switches etc.
There is no call back to the boiler.

So the time switch comes on for the hot water and pumps to that zone.
The zone valve cuts in and out (on a different circuit) and the bypass
valve deals with the fact that there is no call back from the valve to
the boiler.

This is one of the reasons why unvented cylinders must be installed by a
trained competent person.

The zone valve you are looking at is the safety override valve that was
supplied with the cylinder. You must not interfere with it in any way. It
should be lashed up to its own dedicated overheat stat. It is the equivalent
of an immersion heater's overheat stat.

What you need to do is add an additional functional zone valve to your
S-Plan-Plus system. This should be run from your programmer, through a
separate thermostat and with the boiler interlock connections used. The
unvented cylinder is, thus, separated from the boiler by TWO independent
zone valves.

DO NOT disable or interfere with the existing zone valve. It is an important
safety device and MUST NOT be used for functional zoning.

Christian.

Christian McArdle wrote:


This is one of the reasons why unvented cylinders must be installed by a
trained competent person.

The zone valve you are looking at is the safety override valve that was
supplied with the cylinder. You must not interfere with it in any way. It
should be lashed up to its own dedicated overheat stat. It is the equivalent
of an immersion heater's overheat stat.

What you need to do is add an additional functional zone valve to your
S-Plan-Plus system. This should be run from your programmer, through a
separate thermostat and with the boiler interlock connections used. The
unvented cylinder is, thus, separated from the boiler by TWO independent
zone valves.

I was looking at the wiring diagram from the manual of the cylinder
which shows the wiring diagram for an S-Plan using the valve from as a
normal zone valve.

I would have thought that using it in this way would be safer than it
currently is (I.e. if the power failed to the current zone valve the
boiler would continue to heat it up - if it is on the same circuit as
the boiler then it the zone failed - the boiler would also fail)

I agree an extra zone would do the same thing - but then you have 2 x
zone valves - is this really necessary? You also say 'through a
separate thermostat' - but there is no mention of this method in
the manual?

Is it 'normal' to have 2 zone valves on an S-Plan with an unvented
cylinder?

Thanks for any help

Roger Mills wrote:
Each zone valve has two independent circuits - one to drive the motor to
open the valve, and one (volt-free contacts) to switch other things on once
the valve has opened. The motor circuit is driven by the programmer and room
or cylinder stat. The volt-free contacts of all valves are connected in
parallel - with a permanent live as the input and with the output connected
to the boiler and pump. That way the boiler and pump run whenever or or more
zones are calling for heat - but only the zone(s) whose valves are open get
any heat.

If you cross-connect the motor circuit to the volt-free contacts then *all*
valves will open in unison regardless of whether there's a demand in their
particular zone. This is *not* what you want!

Sorry to sound dim - but I am still confused.

Forget about thermostats for now - lets assume there are 2 x heating
circuits

These valves are N/C so no power = Closed

Zone 1 - Live - Brown and Grey Wires - When the valve opens Live
goes onto the Orange Wire

Zone 2 - Same wiring - but is off

Zone 1 puts live on the Orange wire - but as Zone 2 is off (Closed)
it doesn't get to the Grey Wire (and hence doesn't get to the brown
wire)

What am I missing?

Sorry to be dim

Thanks for the help

Is it 'normal' to have 2 zone valves on an S-Plan with an unvented
cylinder?

Yes. It is a legal requirement. It is against the law not to have it. The
thermostat has to be non-self reseting (i.e. requires a "reset" button to
turn back on).

Building Regulations Part G3
3.3 To meet the requirement a directly heating unit or package should have a
minimum of two temperature activated safety devices operating in sequence:

a. a non-self resetting thermal cut-out to BS3955:1986 Specification for
electrical controls for household and similar general purposes, or to
BS4201:1979 (1984) Specification for thermostats for gas burning appliances;
and

b. (snipped description of temperature/pressure relief valves).

3.6 Safety devices listed in paragraph 3.3 (see also 3.4) for direct heating
are also required for indirectly heated units and packages but the
non-self-resetting thermal cut-out should be wired up to a motorised valve
or some other suitable device to shut off the flow to the primary heater,
that is:

(snip list of approval methods).

Christian.

Christian McArdle wrote:
Building Regulations Part G3
3.3 To meet the requirement a directly heating unit or package should have a
minimum of two temperature activated safety devices operating in sequence:


Now I am well confused.

I was trying to put it right as per the manual.

Surely those regs mean the two stats on all unvented tanks (I.e hi stat
and control stat)

I looked up 5 different wiring diagrams for unvented cylinders none of
them have 2 valves - all have one from the time switch - all of
them have a built in thermostat (and a high limit stat) .

Any web references to wiring diagrams with two valves?

On Mon, 13 Nov 2006, Christian McArdle wrote:

Is it 'normal' to have 2 zone valves on an S-Plan with an unvented
cylinder?

Yes. It is a legal requirement. It is against the law not to have it. The
thermostat has to be non-self reseting (i.e. requires a "reset" button to
turn back on).

Building Regulations Part G3
3.3 To meet the requirement a directly heating unit or package should have a
minimum of two temperature activated safety devices operating in sequence:

a. a non-self resetting thermal cut-out to BS3955:1986 Specification for
electrical controls for household and similar general purposes, or to
BS4201:1979 (1984) Specification for thermostats for gas burning appliances;
and

b. (snipped description of temperature/pressure relief valves).

3.6 Safety devices listed in paragraph 3.3 (see also 3.4) for direct heating
are also required for indirectly heated units and packages but the
non-self-resetting thermal cut-out should be wired up to a motorised valve
or some other suitable device to shut off the flow to the primary heater,
that is:

My S-plan system with system boiler and unvented DHW tank has only one
zone valve on the DHW primary circuit but surely the safety cutouts built
in to the boiler itself are sufficient as backup safety devices i.e. two
zone valves are not required.

In addition to the zone valve and associated cylinder stat, there is a
boiler thermostat which controls the maximum temperature of the output
flow of the boiler. There is also a second over-temperature cutout in the
boiler which requires manual reset and the usual over-pressure release
valve in the boiler.

Even if all these safety devices in the primary circuit failed, there are
the two safety over-temperature / over-pressure valves on the cylinder,
i.e. there are five independent safety devices in total.

Surely then it is not a requirement to have two zone valves?


--
Alistair Riddell - BOFH
Microsoft - because god hates us

My S-plan system with system boiler and unvented DHW tank has only one
zone valve on the DHW primary circuit but surely the safety cutouts built
in to the boiler itself are sufficient as backup safety devices i.e. two
zone valves are not required.

The wording requires the temperature measurement to be local. It
specifically states "if the unit incorporates a boiler the thermal cut-out
may be on the boiler". This suggests that an external boiler that is not
incorporated may not use its cut-out to meet the requirement.

However, I've just examined the Megaflo installation instructions in detail.
It appears that you can use just one motorised valve, if you use S-Plan and
ensure that the zone valve power goes through both the overheat and normal
thermostats. It is only necessary to have two valves if you use 'Y' plan,
where the 3 port valve can't reliably isolate the cylinder. The systems I
have seen have been 'Y' plan, so have needed the additional valve.

So it appears that you can just use the one valve for your S-Plan system.
You just connect one switch wire to permanent live and the other to your
boiler call for heat, in parallel with your heating valve.

Christian.

So it appears that you can just use the one valve for your S-Plan system.
You just connect one switch wire to permanent live and the other to your
boiler call for heat, in parallel with your heating valve.

Christian.

OK

So back to the original question

The wiring diagram for S Plan shows permanent live (on grey) and
switched live from the timer (on Brown)

My Question was is there any reason I can't join the Grey wire to the
Brown?

Anyone?

Thanks

Mark

On Mon, 13 Nov 2006 04:59:44 -0800, FGoogle wrote:

Roger Mills wrote:
Each zone valve has two independent circuits - one to drive the motor to
open the valve, and one (volt-free contacts) to switch other things on once
the valve has opened. The motor circuit is driven by the programmer and room
or cylinder stat. The volt-free contacts of all valves are connected in
parallel - with a permanent live as the input and with the output connected
to the boiler and pump. That way the boiler and pump run whenever or or more
zones are calling for heat - but only the zone(s) whose valves are open get
any heat.

If you cross-connect the motor circuit to the volt-free contacts then *all*
valves will open in unison regardless of whether there's a demand in their
particular zone. This is *not* what you want!

Sorry to sound dim - but I am still confused.

Forget about thermostats for now - lets assume there are 2 x heating
circuits

These valves are N/C so no power = Closed

Zone 1 - Live - Brown and Grey Wires - When the valve opens Live
goes onto the Orange Wire

Zone 2 - Same wiring - but is off

Zone 1 puts live on the Orange wire - but as Zone 2 is off (Closed)
it doesn't get to the Grey Wire (and hence doesn't get to the brown
wire)

What am I missing?

The grey should be connected to a permanent live supply independent of
the time controls. The oranges are then connected to together and sent to
the boiler's switched live input.

[Or alternative the 2 greys and 2 oranges from both valves are connected
together to form one pair which switches the boiler form the boiler's
supply.]


Sorry to be dim

you're not it's the installation that's wrong.





-
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

On Mon, 13 Nov 2006 10:18:38 -0800, FGoogle wrote:


So it appears that you can just use the one valve for your S-Plan system.
You just connect one switch wire to permanent live and the other to your
boiler call for heat, in parallel with your heating valve.

Christian.

OK

So back to the original question

The wiring diagram for S Plan shows permanent live (on grey) and
switched live from the timer (on Brown)

My Question was is there any reason I can't join the Grey wire to the
Brown?

Anyone?

Thanks

Mark


There is a good reason:

Firstly the Zone valves are often used in pairs (one for HW & one or more
for heating). If you connect the grey to the brown you will cause the
zone valve to latch in the on state until _all_ zones are satisfied.

It does not matter if the HW cylinder is nowhere near the heating zone
valve if the orange wire is live (because some zone valve somewhere is
open) then this zone valve will stay open because the brown wire (which
opens the valve) is connected to grey and (via the switch in the valve) to
the orange and so the live.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

My Question was is there any reason I can't join the Grey wire to the
Brown?

What you do is have the zone call for heat (i.e. output from programmer and
through any thermostats) to the brown wire. You have neutral on blue. You
have permanent live on one of the other two (i.e. grey) and the boiler call
for heat on the remaining one (i.e. orange?). It MUST be a permanent live.
You can't cheat and use the brown instead. It won't work, as the other zone
will then be able to turn on the other zone valve. This is dangerous and
illegal, as the unvented cylinder will overheat.

Christian.

On Mon, 13 Nov 2006 09:10:10 -0800, FGoogle wrote:

Christian McArdle wrote:
Building Regulations Part G3
3.3 To meet the requirement a directly heating unit or package should have a
minimum of two temperature activated safety devices operating in sequence:


Now I am well confused.


Time to get some pro help.


I was trying to put it right as per the manual.

Surely those regs mean the two stats on all unvented tanks (I.e hi stat
and control stat)

I looked up 5 different wiring diagrams for unvented cylinders none of
them have 2 valves - all have one from the time switch - all of
them have a built in thermostat (and a high limit stat) .

Any web references to wiring diagrams with two valves?


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

replying to FGoogle, allwaysaway wrote:
An old post but often thought the same thing, Can`t see what the problem
would be as your still using micro switch and proven open valve. the switched
live goes straight back to boiler via wiring centre as would permanent supply
regardless of other motorized valves

--
for full context, visit https://www.homeownershub.com/uk-diy...re-302592-.htm

On 20/02/2018 02:14, allwaysaway wrote:

replying to FGoogle, allwaysaway wrote:
An old post

Indeed it is. That means we have not the faintest idea what you are
talking about because we can't see the original posts you are replying
to; they expired from our news server long ago.

Please do us the courtesy of quoting some of the message you are
replying to.

(the web site you are using even lets you do it automatically - just
highlight a section of the message before you start typing your reply!)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

replying to John Rumm, GeoffB wrote:
I've recently replaced a faulty Honeywell V4043H motor. While investigating
the wiring etc. to see how the whole S Plan system worked, I found myself
asking the same question: why not connect the grey wire (normally permanently
live) to the brown wire, on each of the two (HW and CH) zone valves? If you
do that, the only connection between the two valves is the orange wires that
feed live to the pump & boiler. For each of the zone valves, the brown and
grey would go live together when switched on by the timer. But there must be
a reason why the grey wire is permanently live , otherwise Honeywell would
just have bridged the brown & grey in the valve.

--
for full context, visit https://www.homeownershub.com/uk-diy...re-302592-.htm

On 27/06/2018 17:14, GeoffB wrote:
replying to John Rumm, GeoffB wrote:
I've recently replaced a faulty Honeywell V4043H motor.Â* While
investigating
the wiring etc. to see how the whole S Plan system worked, I found myself
asking the same question: why not connect the grey wire (normally
permanently
live) to the brown wire, on each of the two (HW and CH) zone valves?Â* If
you
do that, the only connection between the two valves is the orange wires
that
feed live to the pump & boiler.Â* For each of the zone valves, the brown and
grey would go live together when switched on by the timer.Â* But there
must be
a reason why the grey wire is permanently live , otherwise Honeywell would
just have bridged the brown & grey in the valve.



Welcome to 2018

It is to stop back feeding through the programmers single pole relays.

If both CH and HW are calling and the programmer stops calling for say
CH whilst in demand the live demand from the HW will keep the CH 2 port
valve open.

--
Adam

GeoffB wrote:
replying to John Rumm, GeoffB wrote:
I've recently replaced a faulty Honeywell V4043H motor. While investigating
the wiring etc. to see how the whole S Plan system worked, I found myself
asking the same question: why not connect the grey wire (normally permanently
live) to the brown wire, on each of the two (HW and CH) zone valves? If you
do that, the only connection between the two valves is the orange wires that
feed live to the pump & boiler. For each of the zone valves, the brown and
grey would go live together when switched on by the timer. But there must be
a reason why the grey wire is permanently live , otherwise Honeywell would
just have bridged the brown & grey in the valve.

No, it wouldn't work. Consider the situation where both CH and HW call
for heat. Both valves motor on, opening the valves and closing both
microswitches to energise the orange wires, so the boiler runs.
Then, the HW stops calling for heat. The thermostat no longer supplies
power to the brown (and grey) wires. However, because the microswitch
is closed, power flows from the orange wire (energised from the other
valve's orange) to the grey. The grey is connected to brown, so brown
is still energised, and the valve stays open. The system continues to
heat the hot water when not required. If the CH stops calling for heat
earlier, the same applies in reverse.

Mike

On 27/06/2018 17:14, GeoffB wrote:
replying to John Rumm, GeoffB wrote:

But there must be
a reason why the grey wire is permanently live , otherwise Honeywell would
just have bridged the brown & grey in the valve.

The full S plan circuits are available he

http://wiki.diyfaq.org.uk/index.php/...on es:_S-plan

or download a full set of wiring diagrams:

http://www.diyfaq.org.uk/docs/Sundia...%20Edition.pdf


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

replying to Mike Humphrey, GeoffB wrote:
Many thanks for your reply ... I have been puzzling over this on my own during
the past week or so, and have come up with all sorts of theories, none of
which turn out to be valid as I delve deeper into the technicalities of the
system and wiring. So I had reached a dead end.

My S Plan system has worked fine for 30 years without a single hitch, but I
noticed recently that (1) the hot water wasn't getting very hot, and then (2)
that the pump & boiler kept cutting out (they would come on for 5 mins if the
water was completely cold, then go off for 30 or 60 mins, come back on for 1
min, or even just 10 seconds, then off again, and so on). I thought the most
likely cause was the timer, so I replaced it but that made no difference.
Then I began testing the cylinder stat, which appeared to be in order. In
desperation I replaced the boiler stat (the knob had become fixed in one
position, not having been touched for 30 years; I couldn't move it at all)
although I knew the boiler couldn't switch the pump off so it couldn't be the
cause.

So what else could be switching the HW off? I decided I had to really get to
grips with the thing, and so I analysed the entire system and the wiring
connections. I noticed that the wiring didn't correspond to the standard
S-Plan wiring which is shown in the timer (Honeywell ST699B1002) data sheet.
So that troubled me, but I carried on tracing a maze of wires through the
timer, junction box etc. At last I thought of the zone valve ... of course I
hadn't realised that the zone valve has a invisible micro-switch: I imagined
that it just opened and closed. So ... I tested the valve motor (resistance
cold about 2.35K ohms, which appeared to be ok, roughly) but while the valve
was closed the live feed from the cylinder stat was still at 240V ... so it
couldn't be anything else.

So I replaced the valve motor, and finally that fixed the problem; I
celebrated by having a hot shower. But by now I knew a great deal more about
the whole business, and wondered why the grey wire isn't connected to the
brown, for each zone valve separately. Hence my question.

Regarding your reply (which I do understand and is very useful), one point
still puzzles me. If the cylinder stat opens (i.e. HW is hot enough), that
cuts the live feed to the HW zone valve, so the valve closes under spring
pressure, opening the microswitch. Isn't that right? Why would the
microswitch stay closed when both brown and grey wires are no longer live?

The only reason I can see for the grey wire to be permanently live, is to
separate the timer/valve circuit from the pump/boiler circuit, so that the
pump/boiler current doesn't flow through (and possibly overload) the timer
switch. But that doesn't make sense, because the timer switch is rated at 3A
and the valve microswitch is rated at 2.2A. And the whole system is subject
to a 3A fuse at the mains switch.

So I would appreciate any additional wisdom you may be able to offer.
Thank you.



--
for full context, visit https://www.homeownershub.com/uk-diy...re-302592-.htm

replying to Mike Humphrey, GeoffB wrote:
PS I have studied your reply and thought about it some more. I would like to
add the following note: let's assume that the grey wire from the HW zone valve
is connected to the brown wire, but before the thermostat. So in the case you
describe, the brown wire from the timer to the thermostat would be live, I
agree. But not the wire from the thermostat to the HW zone valve, because the
thermostat switch is open.
The more I think about this, the more befuddled I become.

--
for full context, visit https://www.homeownershub.com/uk-diy...re-302592-.htm

replying to ARW, GeoffB wrote:
Thanks for your reply.
Do you mean that there is some sort of connection between the HW and CH
switches inside the programmer? If so, then your explanation begins to make
some sense. For the case you describe, if the CH switch is still live, owing
to 'back feeding' from the live HW switch, then the CH valve would stay open.
But how does 'back feeding' work?


--
for full context, visit https://www.homeownershub.com/uk-diy...re-302592-.htm

replying to GeoffB, GeoffB wrote:
I've got it, finally. Once you understand it, it's simple and obvious, but
before that it seems incomprehensible. I now see that there is a 'crossover
feedback effect' which keeps the valve (although switched off by the timer)
open while the other valve is also open. I was fixated by the idea that the
timer would cut power to the valve, which would then close, thus opening the
microswitch. What I didn't grasp is that the feedback effect is
instantaneous, so the valve wouldn't have time to close.

Regarding my PS about connecting the grey to the brown BEFORE the cylinder
thermostat, I have thought about how things would work out if you did that.
After all, the thermostat provides a secondary level of HW control, after the
timer. It appears to me that these are the possibilities:

1. If either HW or CH are on, but not at the same time (no overlapping
either), there is no feedback effect.
2. It HW and CH both switch off at the same time, there is no feedback effect.
3. The effect arises when both HW and CH are on at the same time, then one of
them is switched off by the timer.
4. In case 3, if the HW is switched off, the HW valve stays open (feedback
effect) until switched off by the thermostat.
5. In case 3, if the CH is switched off, the CH valve stays open (feedback
effect) until the thermostat switches off the HW valve, so ending the feedback
effect and so switching off the CH valve.



--
for full context, visit https://www.homeownershub.com/uk-diy...re-302592-.htm

replying to GeoffB, GeoffB wrote:
In case 4, all that would happen is that the HW would continue to run until it
reaches the temp set by the thermostat -- which is what you would want, anyway.
In case 5, the CH would continue to run until the HW reaches the temp set by
the thermostat, then both CH and HW go off.

My conclusion from all this is that the practical effect of wiring grey to
brown (for the HW, before the thermostat) seems to be minimal. Ok, the CH
would run on for a bit until the HW was hot enough (which it probably would
be, well before the CH was switched off), but that's a fairly minor
consideration. Overall, you really wouldn't notice any difference in the
operation of the whole system.

Anyway, I've wired up the whole system in accordance with the standard S Plan
diagrams, grey permanently live. Plus notes attached to the wires so I know
where they come from and go to. I've also discovered why the installer (30
years ago) didn't follow the standard wiring -- he cut the wires too short, so
couldn't reach the correct terminals (this had baffled me for weeks until I
decided to change it).

So that is the end of the story, hopefully for another 30 years.
Many thanks for all the helpful replies I've received.
GeoffB.

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