Using same supply for all central heating where does it say we must?

[ericmark]

Clearly we try to have a single point of isolation, and so the thermostat is normally supplied from the same supply as the boiler, however where the boiler has 24 volt or variable voltage control there is a worry running 230 volt and 24 volt in the same cable you can get capacitive or inductive linking, with some boilers actually putting the low and extra low voltage junction boxes on opposite sides of the boiler.

So where does it say a thermostat with volt free contacts should be supplied from same low voltage supply as the boiler? Or is this not a regulation, but some legacy from days when thermostat did not have volt free contacts?

I have 10 supplies to my central heating, one is low voltages and nine are extra low voltage i.e. pairs of AA batteries, but if I want to use a battery eliminator what stops me?

 

[AndyPRK]

Do you have any links to a battery eliminator.

Would love one

 

[JohnW2]

... however where the boiler has 24 volt or variable voltage control .... So where does it say a thermostat with volt free contacts should be supplied from same low voltage supply as the boiler? Or is this not a regulation ...

If, by "low voltage" you mean voltages above ELV (the IEC definitions say that ELV is "LV"), then I can't see how any regulation could have such an impossible requirement - if the thermostat is switching 24V, then I can't see how it can have any "low voltage" (>ELV) supply", can it?

I have 10 supplies to my central heating, one is low voltages and nine are extra low voltage i.e. pairs of AA batteries, but if I want to use a battery eliminator what stops me?

I think the only thing which would "stop you" would probably be unnecessary concern about imagined regulations, rather than the application of common sense!

Kind Regards, John

 

[stem]

I guess the requirement is where there is a receiver is powered by 230V that is supplied from another 230V supply that is not isolated when the boiler FCU is isolated could be dangerous. Even if it's only switching 24v. Someone working on the system may turn off the boiler isolator and then get an electric shock via the connected receiver if it is fed from elsewhere.

Using a single supply source is good wiring practice, and boiler manufacturers state in their instructions that only one mains supply should be used for the whole heating system. I found the four examples below in a couple of minutes.

Vaillant

Worcester

Glow Worm

Ideal

 

[securespark]

I've always understood it to be thus as well as using a maximum fuse rating of 3A, but are these Corgi regs?

 

[JohnW2]

I guess the requirement is where there is a receiver is powered by 230V that is supplied from another 230V supply that is not isolated when the boiler FCU is isolated could be dangerous. Even if it's only switching 24v. Someone working on the system may turn off the boiler isolator and then get an electric shock via the connected receiver if it is fed from elsewhere. ... Using a single supply source is good wiring practice, and boiler manufacturers state in their instructions that only one mains supply should be used for the whole heating system. ...

Yes, that all makes sense but (and maybe I misunderstood) I didn't think that was what eric was talking about - given that he talked about cablles, I thought he was talking about a wired-thermostat which was just switching 24V, with no other supply.

The 'good wiring practice' to which you refer is not restricted to heating systems. In any situation where there are closely-related electrical items (even if just a number of sockets in a room), it is wise that they should all have the same supply (and a 'single point of isolation').

Kind Regards, John

 

[stem]

Indeed, I think you are right. I was showing that the only info I know of is where sharing supplies was discouraged when they are 230V supplies. probably didn't explain it very well. I knew what I meant

 

[ericmark]

English is a problem when writing instructions with little or no ambiguity, and to ensure the device has volt free contacts and enough isolation is a problem.

However in some cases we know there is a problem running AC at low voltage and DC or AC at extra low voltage close to each other, so in some cases it would be better to get a local supply.

I have to agree this house when we bought it having the central heating supplied from two independent distribution boards was bad, and if following the instructions @stem has kindly shown then it would not comply as two independent double pole isolators, one on the fuse box and one on the consumer unit, however that is rather rare to have two distribution boards with no common isolator, in the main to lock off with a FCU we put a lock through the fuse carrier, but that only locks off the line, I don't know if the switches on a FCU are always double pole or not, and common not to have a switch on a FCU anyway.

So we know that putting low voltage AC with other supplies within a cable can mean inductive or capacitive linking, and some boilers actually put the low voltage and extra low voltage on opposite sides of the boiler, clearly so one uses two independent cables.

The writers of BS 7671 often change the wording as clearly they feel it is being miss read. There is no where near that care taken when writing boiler instructions, so when we read them, we do need some common sense, so "Any additional components that are connected to the appliance that require 230 volts must be connected to the same supply as the appliance." would be very hard not to comply with, yes I do have inverters, and we do have solar panels etc. But to run some items off and inverter or generator and others from the DNO supply would take a lot of doing, so that would simply not normally happen anyway.

So we are looking at English, it does not say same circuit, it says same supply, had it said "Any additional components that are connected to the appliance that require 230 volts must be connected to the same circuit as the appliance." then that would mean same fuse/MCB/RCBO but same supply means not having part of it running off a generator.

The Ideal saying the isolator should be near to the appliance makes some sense. I have never studied if all switched FCU switch both lives, but to be frank I would prefer a plug and socket, once you have fitted one of these

no one can turn it back on, and your not relying on a switch contacts. With a FCU you can put the lock through the fuse holder, but that is only isolating the line.

The

Type A RCDs

must be employed where additional protection is required.

is interesting, I would assume that means if a TT supply rather than TN supply, but why not say "Type A RCDs

must be employed where the supply is TT?

It does seem central heating uses Chinese whispers as main instructions, I would say any motorised valve makes an area a zone unless linked, I know the eQ-3 bluetooth TRV heads can be linked to make whole room one zone, but a TRV is a motorised valve, be is a motor powered by the expansion of wax or a pair of batteries, it is a motorised valve. But it seems many people only think a large 15 mm piped valve is classed as motorised, however some people do seem to think if no electric powered it not a motor, the number of air motors and activators I have found which still work when the e-stop is pressed I have lost count.

But now we only have to take manufacturers instructions into account, and don't need to follow blindly, why should a supply for a volt free thermostat be taken from the boiler?

 

[JohnW2]

But now we only have to take manufacturers instructions into account, and don't need to follow blindly, why should a supply for a volt free thermostat be taken from the boiler?

What exactly do you mean by "a supply for a volt free thermostat"?

Kind Regards, John

 

[SUNRAY]

Again the silly folly of installing battery operated devices within mains powered system has reared it head.

In large commercial systems it is very common to have sections of heating systems fed from different circuits, boards, phases and even different substations.
So the simple answer is it's all down to design, competence and labling.
A thermostat and zone valve in an area is commonly fed from the fuse board for that area and a contact on the actuator then wired to the boiler system. This is often a deliberate design feature to automatically avoid heating an unused/redundant area.
The main point to consider is generally the isolation between the devices supply/internal components and the volt free contact operating the boiler, this information is usually found in MI's.

 

[ericmark]

Thank you @SUNRAY you have confirmed what I thought, nothing wrong will multi supplies as long as done in a planed safe way.

What exactly do you mean by "a supply for a volt free thermostat"?

Kind Regards, John

Some thermostats the supply to the electronics and relay contacts are common, like the duel channel Hive, others the relay contacts are not connected to the supply to the electronics like the single channel Hive.

Personally I think the way Hive uses the same back plate for single and duel channel is asking for problems,

If a duel channel is used to replace a single channel you could easy send 230 volt down 24 volt cables, different when some one actually needs to swap the cables, but just too easy for some one to fit wrong version. Nest it would need some one with at least some skill to change so unlikely anyone would mix up 230 and 24 volt. But even the boilers keep the extra low voltage and low voltage apart the OpenTherm contacts are other side of boiler to mains supply. Be it keeping 24, 40 and 230 volt separate or two independent 230 volts separate the same applies, one has to work out what you are doing, and not wire by trial and error. However I have cringed when I have read threads where some one tells the posters you need to fit a link L to 1 without first making sure the boiler uses 230 volt control.

 

[bernardgreen]

Personally I think the way Hive uses the same back plate for single and duel channel is asking for problems,

I agree with that. Terminal 1 can be floating ( single channel Hive ) or connected to Live ( dual channel Hive )

I wonder how many boilers with 24 Volt controls have been damaged by being connected to a dual channel Hive

 

[SUNRAY]

Thank you @SUNRAY you have confirmed what I thought, nothing wrong will multi supplies as long as done in a planed safe way.

Some thermostats the supply to the electronics and relay contacts are common, like the duel channel Hive, others the relay contacts are not connected to the supply to the electronics like the single channel Hive.

Personally I think the way Hive uses the same back plate for single and duel channel is asking for problems, View attachment 214795 If a duel channel is used to replace a single channel you could easy send 230 volt down 24 volt cables, different when some one actually needs to swap the cables, but just too easy for some one to fit wrong version. Nest it would need some one with at least some skill to change View attachment 214796 so unlikely anyone would mix up 230 and 24 volt. But even the boilers keep the extra low voltage and low voltage apart View attachment 214797 the OpenTherm contacts are other side of boiler to mains supply. Be it keeping 24, 40 and 230 volt separate or two independent 230 volts separate the same applies, one has to work out what you are doing, and not wire by trial and error. However I have cringed when I have read threads where some one tells the posters you need to fit a link L to 1 without first making sure the boiler uses 230 volt control.

Sadly this is not a Hive issue.
This convention has been in place for many years, possibly 50 and I have come across some very wierd symptoms where a time clock has been replaced with the other format. In the day of all 240V control there was unlikely to be any damage so the format went unchecked.
But as you very correctly describe, there is now real danger involved with mixed voltages in modern systems. To make it worse It is an actual shock risk as the 24V side is/can be classed as SELV and SELV equipment may not be built to the low voltage [mains] standards.

 

[ericmark]

I have this argument with my son, (also an electrician) should we wire houses so idiots are unlikely to make mistakes, or should we wire in a safe sensible manor and assume those who come after will do the necessary checks to ensure it stays that way? Be it the wiring of central heating or splitting rings side to side rather than up/down.

It seemed the central heating was the most complex wiring in a normal house, and there has been a move to include as much as possible inside the boiler, so less errors are made.

But we still have pumps, motorised valves, by-pass valves, wiring centres, programmers, and thermostats in some houses, and if anything it seems to be getting worse not better, with 3 zones or more instead of just CH and DHW, and with analogue controls we have a right mess up, with people trying to use a mixture of analogue and digital (on/off) and the results often means the boiler does not run efficient. We have a modulating boiler designed to use the latent heat, and then thermostats are added which in essence stop it modulating (turning down output).

However not reserved to central heating installation we have a load of tradesman who are either unwilling or unable to move with the times, and yes guilty, first time I saw a PLC instead of a cupboard full of relays I did not like the idea, however I did realise I needed to learn, and went back to collage. But I have worked with so many who have never upgraded for the old star/delta starter and dash pots.

It is the same with central heating, my house is oil fired, and the old cast iron radiators have a place with oil fired, as they store the heat and reduce hysteresis, but have no place with modern modulating gas boiler (except for looks). Some times it seems we are going backwards.

So back to same supply, my old house had a Myson fan assisted radiator, it worked very well, but the supply was from a local socket, it had it's own thermostat, and other controls and was not connected to the boiler in any electrical way, and in summer could be switched to circulate cool air. Not sure how it would work with modern modulating boiler? But it worked very well with the old type. The same in this house, most radiators have electronic TRV heads, which are not electrical connected in any way to the boiler, so 12 midday on a Saturday even in the heart of Summer I hear the TRV exercising the valve to stop it sticking.

I see no reason why all devices in a heating system should be from same supply, be it 3 volt or 230 volt makes no difference.

 

[sparxxxx]

All parts of a wired control system should have a common power source and isolation point. Sometimes this requires additional poles on an isolator or relays that are clearly labeled and shielded to prevent incorrect connections, ie Fire alarm interposing relays. There may also be warning notices of other voltages sources. All these are safety requirements, but also assist in fault finding, and I can't see any reason why wireless controls/interfaces should no be treated in the same way.

It could make control fault finding very difficult if there was an unknown wireless control switching something on and off.