Thoughts on supplies to power thermostats, where they are taken from.

[ericmark]

There was a question, on changing to a thermostat with these connections, when the original used a battery for power. Three core and earth to original thermostat, but two cores, earth and grey, not connected. No more info, don't know if cable goes to boiler, or wiring centre, so the question is where to get power from. So I consider what we may have.
* The boiler may be room sealed, so maybe not a good idea to suggest looking in the boiler for the other end of the cable.
* Any wiring centre will likely have a lot of cables, and to direct to there, easy to select wrong cable.
* The Nest Gen 3 is often powered from a USB supply, which, is in turn, powered from some handy 13 amp socket, so why not power from a handy socket?
* The Meross does not connect to TRV heads, so may as well use Nest e which is smart and battery powered, so why use Meross?

The idea of supplying a thermostat from the boiler, is for when the boiler is 230 volts controlled, and you want the boiler's fuses to control the input from the thermostat, so the thermostat can not by-pass the boilers' protection, So taking from the same FCU does not really help with many boilers. It also means no 230 volt is supplied to the boiler, if the FCU fuse is removed. Also, if the boiler's FCU is supplied from an UPS, clearly want the thermostat to be also battery backed in some way.

But when advising over a forum, with limited info, doing a risk assessment, and likelihood of the method working or not, what is wrong with an independent supply?

I have looked at the cradle method, and it is so dangerous to start with, as nothing to stop a twin or triple channel thermostat from being put on a cradle wired for single channel using extra low voltage control, so any argument as to wrong connections made in the future, that does not really hold water.

So is there really any problem using an independent supply to power the thermostat? Another what does the team think.

 

[stem]

Blimey, how many times have you asked the same question over the years.....

So I'll give you the same answer:

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

 

[bernardgreen]

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.

I support that good practice.

 

[ericmark]

Vaillant and Worcester links don't work. To isolate the supply, one needs a two pole switching device, so a FCU will not count, the Ideal is clearly very old as refers to IEE the last one, well most homes only have one supply, I also not it asks for type A RCD, the external fuse rating of 3 amps is also interesting.

I look at my own boiler, the external equipment includes two pumps, two motorised valves, two wall thermostats, all supplied from on switches fuse connection unit with a 3 amp fuse, with no RCD protection, it is the only thing in the whole house not RCD protected, and I have a compliance certificate from last year. Not saying I agree with not having a RCD, but that's how the scheme member electricians wired it.

However, as I have said, if you are not going to supply it from any 230 volt supply other than the one through the switched FCU supplying the boiler, this must include when the step-down power supply is internal to the thermostat or external. So the FCU must also supply the USB power supply used with Nest Gen 3 thermostats.

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.

I can't see how, if the 24 volt is supplied from the boiler, then when the boiler is switched off, there is no 24 volt supply. And with a volt free thermostat, why should it matter how powered? Be it battery, USB or any other supply, that's whole idea of volt free contacts.

I would where possible use a single FCU for all power, however the last house also had two independent supplies, one to the boiler, and one to the Myson fan assisted radiator. There were no cables connecting the radiator to the boiler. If one considers the load if all radiators were fan assisted type, think one may have a problem with a single 3 amp fuse?

My question is which is the safer, independent supply or removing boiler covers, clearly you think removing boiler covers, maybe they don't need to be room sealed, and the room sealing is a hype about nothing?

 

[davelx]

Try finding an SFCU that is not double pole.

 

[ericmark]

Try finding an SFCU that is not double pole.

That can be locked off? I can put a lock through the fuse holder, but that does not isolate, there are add-ons,

but in the main there is only one isolator in the house, and that is on the consumer unit, today we are seeing RCBO's which are double pole, but the simplest method of isolation has to be the plug and socket. So simple, ⁣but the manufacturers still tell people to fit a FCU, clearly not for safety, or they would go for plug and socket, and before we had solar panels and battery, I had considered fitting a plug and socket, so something like this

can be used to keep the CH running. I note the drive for heat pumps which, if it being fitted to combat climate change, will also need to cool, so that means more fan assisted radiators, that is the only way a radiator can heat and cool.

I can see to use one plug to supply all, will allow it to be isolated and locked off, and also can easily be powered with emergency pack or generator. But what we are looking at, as said before, is which is the most dangerous, removing the boiler cover, or using an independent supply for volt free thermostats. From @stem reply better to remove the boiler cover.

 

[SUNRAY]

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.

Any boiler isolator must isolate all poles except earth. My boiler has 4 connexions: L, N, Enable & Pump. Argueably the pump line doesn't require isolation but the other end of it is in the wiring centre amongst all the other live wiring so good practice 'n all that.

 

[ericmark]

Well, I was looking at the manual for the Valiant Eco TEC Plus and noted

so it seems with some the wall thermostat is not powered from the boiler.

 

[SUNRAY]

Well, I was looking at the manual for the Valiant Eco TEC Plus and noted View attachment 350010so it seems with some the wall thermostat is not powered from the boiler.

That is a fairly common arrangement. However it should be taken from the same circuit

 

[stem]

Well, I was looking at the manual for the Valiant Eco TEC Plus and noted View attachment 350010so it seems with some the wall thermostat is not powered from the boiler.

Picking up the supply externally from the boiler is often what I recommend. That way there's no need to open up the boiler casing to access the fused connection unit and pick it up from there.

As @SUNRAY above it should still be the same circuit. Elsewhere in the manual you posted the link to states on pages 31 and 35:

 

[ericmark]

The problem with a 230 volt feed into a boiler is if it comes from another circuit one it can energise the boiler when it should be isolated, and two it can by-pass a safety device.

Over the years I and I am sure most electricians have seen wiring by those installing equipment involving plumbing which leaves a lot to be desired, including not installing earth wires, and using green and yellow for other than earth bonding. So I can see how telling plumbers all must be supplied from the same circuit as a good move to try and get them to follow standard electrical safety procedure.

However @stem pictures, the first two say same supply, since most homes only have one supply, that is hard not to comply with, and the use of a FCU rather than a plug and socket, does limit times when supply is removed from the boiler, when someone wants to use the socket, and it also with earlier editions of BS 7671 allowed the heating to be supplied without RCD protection, however that needs to be balanced against the ability to use an emergency supply, with a plug and socket arrangement it would have been easy for me to use my jump start pack to power the central heating.

The use of emergency power is another reason why having, all from a single supply, is the preferred method,

this clearly is not a good idea, although pressing the centre button

on the heat link will allow heating to run. So not too bad, but it really does not matter if the supply to a volt free thermostat is battery, remote power supply, or built in power supply, if volt free, no reason it can't be supplied from elsewhere, as long as there is a way to override the thermostat.

This does seem a problem, Nest with centre button, OK, but this is a problem, be it a fault, flat battery, or whatever, one can't override the thermostat without opening the base and bridging the contacts. Last week I had my boiler serviced, the flat where the boiler is located, is a walk up a flight of steps, and around the house, to access the thermostat inside the main house, not really what one wants to do when servicing the boiler.

Today, my boiler is on a different supply to the rest of the house, the grid supply can fail, but the boiler will still have a supply, so for me, it is important all which is connected with the central heating, will work with no grid supply. My Nest Gen 3 thermostat is supplied from the heat link, which is in turn supplied from the boiler.

I feel the universal cradle this to me is an accident waiting to happen, there should be a blocking part which stops one physically being able to fit the wrong thermostat/programmer. It is just too easy for someone to buy the wrong part, and as a result feed low voltage into an extra low voltage circuit.

With many boilers there are internal fuses, and so we need to read the instructions carefully, as even taking the supply from the same FCU it could by-pass the internal fuse.

However down to nitty-gritty, should one say as a blanket statement a thermostat MUST use same circuit (not supply, wording need to be correct) or simply the thermostat is BETTER if connected to same circuit when it has volt free contacts, but MUST be connected to the supply if the contacts are not volt free?

I think all boilers should have the electrical connections where one does not disturb and room seal accessing them, these should be checked as part of an EICR, so unless this is the case, any EICR would have FI code for the boiler, but since some boilers have an internal fuse which can be by-passed by taking the thermostat supply from the FCU, that practice should be discouraged.

 

[SUNRAY]

The problem with a 230 volt feed into a boiler is if it comes from another circuit one it can energise the boiler when it should be isolated, and two it can by-pass a safety device.

I always fit appropriate boiler isolator:

Any boiler isolator must isolate all poles except earth. My boiler has 4 connexions: L, N, Enable & Pump. Argueably the pump line doesn't require isolation but the other end of it is in the wiring centre amongst all the other live wiring so good practice 'n all that.

 

[securespark]

Some very early (and very cheap) FCUs were SP only. Dunno how many remain but for me DP switching on any hard-wired appliance is a no-brainer. This can of course be achieved with the use of a plug and socket but to be within the rules, the socket should be unswitched. This forces the removal of the plug to isolate.

The "3A maximum" fuse rule for gas appliances was something we were taught religiously, and it was one of those rules we never questioned but were never told why, or whose rule it was.

Googling the topic brings up a lot of discussion on this site, but I have never been able to ascertain whether this is a rule and if so, who implemented it.

Bold text edited to remove typo.

 

[ebee]

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.

Can I also mention that any person who intends to open up such things must be competent to do so in a safe manner and ditto to anyone jacking up a car or working on its brakes etc in the first place "Each Man should know his limitations!" .

you might have absolute trust in a brain surgeon to work on exposing and jabbing parts of your brain, very highly skilled.
would you have the same trust for that highly skilled person to slide down a pole, run into your home fully equipped to put a fire out?

 

[ericmark]

Can I also mention that any person who intends to open up such things must be competent to do so in a safe manner

This is a good point, in my homes we have had a number of boilers, one was room sealed where the electrical connections were, daft idea I know, but some manufacturers still do it, so all cables to the boiler need to be round, and enter through some sort of stuffing gland.

Signs don't help, authorised personal says nothing, and most electricians would assume they are authorised to open up electrical connections, the idea of having to have a gas and electrical inspection on the same day, so the electrician can inspect the boiler's wiring is daft, but that is how it is at the moment for some boilers.

As to fuse size, that would depend on what equipment is installed, I know my boiler has two pumps, and two motorised valves, some systems could have more, most motorised valves have a micro switch inside, but the wiring does not allow that micro switch to be volt free, as a result on my system I have relays, there are relay boxes made for underfloor heating, but to try and make a set of rules which suits all is never going to happen, there will always be an exception to any rule, in the main my system was re-designed to cope with the fact only two cores of the three core and earth were intact, so I looked for a method using extra low voltage to control the heating system (both CH and DHW) from the main house with two wires, I used Nest Gen 3. The other option was go wireless.

One has to work with what one has, and in my house lifting floors is out of the question, and to remove ceilings is a very big job. My heating on moving in used a plug for the pump, part supplied from a FCU in the flat from the consumer unit, and part supplied from the house from the fuse box, now all from one FCU, so I looked at the label saying for service contact and said to myself they are the last people I would use. Why anyone would put their name to such workmanship, not a clue.