New Induction Hob - CU Change Needed?

[John Rumm]

I have noticed that it seems the option of having a combi *and* a hot water tank is one that never seems to occur to many people, yet there are good arguments for it. The tank can be heated from the CH side of the boiler in the normal way - so you get all the tank advantages. The boilers DHW side however can then be used to provide mains pressure showers, and potable hot water to the kitchen etc.

[Max Demian]

That's basically a 'system' boiler. Which can deliver DHW at mains pressure from the tank if desired.

[John Rumm]

A system boiler and combi are indeed similar - in that the system boiler is typically based on the same platform, and just lacks a diversion valve and plate heat exchanger.

The main difference in overall system however is that to get mains pressure showers etc from the system boiler requires an unvented cylinder or heat bank, which is a fairly substantial additional expense.

Replacing a heating only boiler with a combi and keeping an existing hot water cylinder can be somewhat more cost effective, and still give some of the same benefits at the expense of a bit of extra piping.

[ARW]

You are very lucky. There appears to be an awful lot of spare cable inside your CU.

I would recommend (if you are prepared to spend a few hundred quid) a BG CU with all RCBOs. TLC are doing the compact RCBOs for £15.72 but I will have to search elsewhere for a suitable enclosure.

Is it possible to tell how the cables enter the CU? It looks that they enter from the rear. If it was possible to bring them down from above it would help you space them out a little better and move the meter tails across to the right.

[Steve Walker]

Partly. The water forms layers, so the hot sits on top, which is why the tanks have the cold feed to the bottom and the hot take-off at the top, but of course there is some mixing and some heat transfer. The point isn't that we can use all 40 gallons and have piping hot showers, it is that even into the second day the water temperature remains, if somewhat cool, at least bearable, whereas a faulty combi will be giving very cold water from the start.

SteveW

[Kevin H]

They all enter from the rear apart from one (connected to the loft) but from what I can see they all then go up so it should be possible to bring them in from above.

What about the BG 10 way enclosure?

Would give me:

1 Ring 2 Ring 3 Ring 4 Loft (goes to another CU in the loft conversion) 5 Cooker 6 Hob (new) 7 Lighting 1 & 2 (were on separate MCBs) 8 Lighting 3 and smoke (were separate) 9 Immersion heater (less said about this the better) 10 Spare

[ARW]

I have first dibs.

[ARW]

I honestly am not sure if that is 10 spare ways or 10 ways with two ways taken up by the main switch. I'll have a look at BGs web site.

[ARW]

If I was doing the CU swap I would try to bring them in from above. Once you get up to ceiling level you should be able to free them off and put them in a nice orderly layout.

If you go ahead with it I'll post a few more tips than the wiki article about real life CU swaps.

[Kevin H]

Thanks. BG's site

says 12 module 10 way with 100A switch for the CFUSW10 CU

[ARW]

Sound ideal. Don't forget to order the grommet strip with your order.

[adsense_mjn]

I'd be interested in that anyway if you have the time? Would it be worth adding to the article?

[ARW]

John or NT usually add anything worthwhile I post to the wiki.

So here are a couple of tips that would apply if I had more than 2 hours to swap a CU!

1. Charge up your cordless drills.

1. Start early and give yourself plenty of time.

2. Mark off the live cables with a sharpie pen well before you start, ideally the day before. I use a series of dashes to identify the circuit numbers. Write down a schedule of what dashes represent which circuit. Take a photo.

1. Prep up the new CU as early as you can. Forgot the knockouts that come with metal clad CUs and get out the jigsaw and make your own. Its much easier to have a slot instead of a series of holes. If possible do this the day before and superglue on some grommet strip.

2. Have a supply of spare cable and crimps ready to extend the cables. If you are running out of time use strip connector and finish off with crimps another day in your own time without rushing. Or use WAGO.

1. After removing the old CU you should, if required and it is possible, reorganise the layout of the cables and the tails to suit the layout of your new CU.

So if fitting a split load CU you would put the upstairs sockets next to the downstairs lights etc. Prepare your preferred layout to match the markings you have made on the cables the day before so you can write it down.

This does not apply to an all RCBO setup (other than the two cables of a ring might need to be placed next to each other).

In particular to the OPs setup I would not be surprised if all the cables drop down from the ceiling and could not all be freed up and organised in any layout he wanted.

1. After organising the layout of the cables tape the ends of each circuit together ie LNE of upstairs lights or if a ring tape them as a pair. Labelling the tape also helps as the tape will probably be over the cable markings.

This will help you get get the cables into the new CU and stop neutral mix ups

1. Think about the exact positioning of the new CU. If the OPs existing CU was mounted right up to the ceiling then the front cover would not fit. Hence it is 5mm below the ceiling. The same might apply to your new CU. BTDTGTT.

1. If going all RCBO take out an insurance policy when buying the CU. ie buy a 6A and 32A MCB when you buy the CU. Use them instead of RCBOs if you have a nuisance trip and it will give you time sort out the cause of the fault in your own time. If fitting a split load CU you have to leave a non RCD space available to do this (not all split load CUs have a non RCD side)

2. The wiki covers the second fix of the CU. However I usually put the tails, earth and bonding earths in first followed by all the circuits earths.

1. Go back a week later and recheck the tightness on all the terminals.

[tabbypurr]

And test your ciruits beforehand, so you don't end up with a partly nonfunctional system for a while.

NT

[Mathew Newton]

Great stuff - thanks Adam!

[John Rumm]

OK I have woven some extra stuff into the existing article based on these comments.

[ARW]

And there is one final tip

DON'T PANIC

If things go wrong have 15 minutes rest and start again

[ARW]

It's certainly worth checking what each MCB currently does. A couple of months ago I found an incorrectly wired immersion and ring. One of the legs of the ring went to the 16A immersion MCB and the other leg and the immersion went to the 32A upstairs sockets MCB.

This would have easily been identified by turning the MCBs off one at a time.

[ARW]

Anyone one else (including yourself) got any other tips?

I generally get a couple of hours to swap a CU (plus time to test). It usually takes me 90 minutes to prep it up and mount the board with the cables where I want them. The last 30 minutes is fitting the live, neutrals and MCBs or RCBOs.

[tabbypurr]

Wylex RCBOs don't like to grip 2x 2.5mm cables unless they're twisted together. That can catch people out.

NT