New Boiler = Room Stat = Part 'P' & Part 'L' question (long)

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(B)util kitchen din rm (st) liv rm ______________________________________________ 22mm gas

I I I I I I I I I I I I I I I I I I I I I I I I XI I I I I I

(B) = Boiler (st) = room stat. X= gas meter, the rest is self evident

so I went ahead and bought the biasi condensing combi and flue during B&Qs last 20% off bonanza. less than £500 all in, which was nice. then I read a bit more into the new installation regs and discover I need TRV on ALL the rads rather tham all but one, and a room stat into the bargain.

fair enough on the TRVs, I'm replacing one rad and adding another when the system is drained down and I'm hoping that the rads which didn't have TRVs aren't siezed.

the main question I have is regarding the room stat and how to wire it back to the boiler. 10 years ago when the house in question was refurbed (toshed out) the 22mm gas pipe was run along the wall at skirting height, through the living room, dining room and kitchen where the crap plumber dropped it down to 15mm* and continued into the utility rom. a few months later, the boiler was installed in the utility room and all was well.

the 22mm gas pipe was boxed in behind the 7" skirting, clipped onto the brick and follows through under the kitchen units into the utility room.

here's the part P question. how can I run the connecting wire for the room stat from the boiler to the living room ? I'm ok in the utility room as there are no units, the kitchen will be a right pain as there is no way I can remove the units, access under the kitchen will be by the plinth only. the dining room skirting can be popped off as we are redecorating that room anyway, same goes for the living room.

I plan to run the cable with the gas pipe, behind the skirting, the full length of the house as it would be one really simple cable pull of about 12m but I'm guessing there will be issues with part P and possibly part L ?

the big Q: does a simple 2 wire stat connection come under part P OR part L ?

can anyone see any regulatory problems with what I plan to do ?

the boiler work is being commissioned and signed off by my brother in law who is corgi regd and has done part P and part L but he has got it wrong in the past and I'd like some second opinions as the house will be on the market soonish and I don't want any legal hiccups.

thanks in advance

• being changed to 22mm

[Mike Harrison]

Room stat wiring will usually be low voltage, hence outside scope of Prat P.

[Christian McArdle]

You are mistaken. Fitting TRVs on the room with the room stat is not permitted by regulations and will make energy consumption worse, as it defeats the boiler interlock.

If you wish to have all rads with TRVs, then you'll need to abandon the room stat and fit a flow switch interlock and possibly rewire the pump.

Use an RF thermostat. No fixed wiring required. No redecorating. As the central heating is on the other side of an FCU, you can probably get away without Part P.

You can get cheaper ones than the following, but it is the best. (Honeywell CM67RF).

[Christian McArdle]

That is not true. Room stat wiring is usually at full mains voltage. Some combi boilers (but certainly not all) may have low voltage signalling, but almost all system boiler installations will not be. With a combi, you certainly can't assume that it is low voltage without investigating the installation manual.

Christian.

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the only reference to stat voltage that I can find in the inatallation manual / service manual is "DO NOT CONNECT LIVE WIRES TO TERMINALS TO WHICH THE ROOM THERMOSTAT MUST BE CONNECTED"

from which I deduce that the boiler /needs/ a stat and is low volatge ?

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I'm not, well, I wasn't, it was the brother in law who said /all/ rads need TRVs and the interweb said all but one. I was firmly with the web on that one but he disagreed. I thought that having two stats would confuse the boiler.

I /must/ have a room stat so the front room will be the rad without the TRV, the only issue now is the cable run through the kitchen and behind the skirting. if it is low voltage I shouldn't have a problem with parts P & L

is that right ? I'm only using a cheap £5.99 bimetallic room stat.

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fingers crossed :-)

[Christian McArdle]

Well the good news is that it will be low voltage. The bad news is that it will probably be 230V "low voltage".

Seriously, throw away that w*nky bimetallic stat and fit a proper RF programmable thermostat. It will be a fraction of the work and it will probably earn its value back through optimum start and more accurate control. You can also program it in the comfort of your living room, rather than requiring a torch in a cupboard to an old style programmer. With an RF stat, you can normally program it from your chair, as you can unclip it from the wall.

Christian.

[Christian McArdle]

Nope. You may deduce no such thing from that statement.

Christian.

[Doctor Drivel]

To have TVRs on all rads one method is have an auto by-pass valve fitted between the flow and return. Have a boiler which can maintain a constant flow temp. Have a it set to 80C. Balance the system to 10C across the rads and boiler, if an existing system. Install a pipe stat on the boiler return, so that the burner is off at 71C.

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eh ? low 230V as in not 415V ? elaborate.

thanks for that. the bimetallic stat is being fitted in the front room, close to the dining room door for even temperature sensing and1.5m (thereabouts) directly above the line I wish to run the single 12m cable for which the holes are alread drilled. no torches or cupboards will be used in this installation.

£5.99 vs at least £20 for a digital version, it's cost effective for me and any future cost effectiveness can be dealt with by the new owners. all I really need to determine is the cable run in relation to the various regulations.

keeping it simple and cheap is the prime mover, adding a £100 RF stat does not make any sense within this context and as the walls have to be re decorated anyway, chasing the walls out and running themover with a trowel full of jollop isn't really a problem.

it's the cable run that's at issue.

tia

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how about the cable run ?

what's your thoughts on that in relation to the various regs ?

tia

[Christian McArdle]

P.S. A bimetallic thermostat REQUIRES a 230V connection, or it will work very badly...

A "volt-free contact" requirement from the boiler will mean that you need some sort of electronic stat with built in batteries. Almost all programmable (and many electronic) thermostats meet this requirement.

Old style bimetallic thermostats have a 3 terminal connection and use 230V power from the programmer to run a tiny heater that gives much more accurate control to the bimetallic strip. This heater will not function as intended on a low voltage supply and the temperature stability will be all over the place.

Christian.

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so I will have to up the budget by £13 and buy a battery powered digital stat /if/ the boiler is compatible with such a device ?

thanks for your continuing input, how about the cable run in relation to the regs ?

[Christian McArdle]

How is interlock achieved? You don't have a flow switch in this design. Explain how the burner never fires if the outside temp is, say, 30C.

As it stands, this system will cycle, as when the water drops below 71C, it will fire up again even if there is no call for heat.

Christian.

[Christian McArdle]

Indeed.

If you're thinking of the around 12V sort of mark, that is officially "extra low voltage".

and any future cost

Well if the cable run is simple, then there's nothing to worry about. I would only use an RF stat if a wired one was not possible. Remember that a mechanical stat can't work on a twin wire installation. It requires 3 wires including a 230V supply from the programmer, which may not available from a combi.

Well, if redecorating is not a problem, then you can't beat wires for reliability.

Christian.

[Christian McArdle]

The boiler will be compatible with a battery powered digital stat.

Either:

(a) claim you know nothing about it (b) get it signed off at great expense by an electrician (c) get it signed off at great expense by the local council (d) use an RF stat

With it buried in plaster, I doubt you'd be able to claim it wasn't fixed wiring, despite the fact that the whole lot sits off an FCU/plug+socket. You could always ask your local BCO for their opinion.

Christian.

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ah, of course.

right, now you're up to speed :-) can you see any problems with the cable run following the gas pipe in the diagram depicted in my original post, in relation to the various regulations ?

[Christian McArdle]

Well a quick look shows that it is non-viable, as you may not conceal wiring behind the skirting board unless mechanically protected or bury it more than

50mm deep.

You might get away with it ELV (although I'm not entirely sure on this), but this will require a relay at the boiler if the boiler isn't already ELV on its thermostat connection.

Christian.

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is one option I am seriously considering. when we toshed the house out about

10 years ago, several cables were installed which haven't yet been used. remote control for the ex immersion heater, extra HD shower circuit, etc. and I do have a large amount of pre harmonisation T&E

it would be difficult to prove that the cable wasn't installed 10 years ago.

thanks again for your input, I'll print this thread out, pass it to brother in law and make a descision off the back of /his/ signature, I think.