Cavity Wall Insulation- Should I Bother?

[Archie]

There is currently a promotion in my area for free or subsidised Cavity Wall Insulation. I can get it installed for 90 quid. It should be a no brainer but one problem I have is that the electric shower cable runs up the wall cavity and insulating the cavity would mean the cable needs to be de-rated. The shower is 9.5kW and the cable is 6mm which is just enough with an uninsulated cavity. I hope to get away with replacing the

6mm cable with 10mm. I used the cable calculator at tlc direct but it is flawed. e.g. for a given set of parameters and using the default 230V it gives a higher current answer than if 240V is used in their calculator. A shower will consume a higher current at 240v than at 230v.

When I installed the shower cable about 20 years ago I also replaced the CU so the shower is fed from a 40A MCB on the RCD protected side of the CU. The cable runs up the cavity into the loft, along the gable end then over the loft insulation down a stud partition to the shower. Re-routing the cable to not use the cavity is not something I relish. Any comments?

Archie

[Roger]

I wouldn't bother changing the cable, it will be OK. That is how mine was done!

5 years later = no problem.

[Dave Osborne]

Archie wrote: I used the cable calculator at tlc direct but it is

Which is correct, because the TLC calculator is based on the rated voltage of the appliance, not the voltage of your supply.

Correct also providing you're talking about the supply voltage not the rated voltage.

This is what TLC calculator does: -

Shower rated 9.5kW @ 240V gives current demand of 9500/240 = 39.58A

Shower rated 9.5kW @ 230V gives current demand of 9500/230 = 41.30A

But this is what you're thinking of: -

Shower rated 9.5kW @ 230V run on a 240V supply gives current demand of

41.03A x 240/230 = 42.81A

Shower rated 9.5kW @ 240V run on a 230V supply gives current demand of

39.58A x 230/240 = 37.93A

[PeterC]

With a shower the period of use is usually[1] a few minutes, so any rise in temperature should be limited by that.

[1] Unusual/more fun useage might invalidate this point.

[NT]

Its worth it. The cost of rerouting the cable is small compared to typical savings.

NT

[John Rumm]

The shower will probably only give 9.5kW at 240V, the output will fall to 8.7kW ish on 230V

The de-rating is not as severe is one side of the cable is in contact with the masonry, rather than surrounded by the insulation. Not sure how you would enforce this though.

[NT]

usually cavity cables hang free, and theyre not likely to kink sharply on entry, so it would be buried all round.

If you wanted to reduce shower power slightly, it's doable, whether you'd like what it entails I'm not sure.

at inductors and perhaps transformers

NT

[fred]

Fair comment.

40A inductor? 8kW mains transformer? Not really practical or necessarily desirable.

For the o/p, assuming a 5m length in the cavity then there is only a 1V drop on that length which at 40A gives a dissipation of 40W (4W/m). This is next to f'ck all and given the expected duty cycle of operation of an electric shower I would have no problem in replacing the existing 6mm run with 10mm and insulating the cavity.

I've assumed that poly beads are out of fashion these days as they would leech the plasticiser out of the cable with direct contact, better with blown fibre or foam.

[js.b1]

If the house is a semi with neighbour having has polybead & no brush in the cavity, guess what you must have? :-) You could use some form of timed interlock to limit shower run.

However cable goes thro 'ole, up cavity, thro 'ole into shower? Can you drill thro the wall & run the cable up the outside?

If you can that is a quick solution.

- 6mm FTE clipped direct outside - not ideal, better than cooking

- 6491X singles in 20mm conduit - doesn't look bad in black

- SWA CW glanded off to galv besa box w/ back-outlet or bushed

Sleeve the cable thro the wall in 20mm conduit. I use a flexible conduit which unusually slips tightly inside female conduit bush, the convolutions prevent water tracking from outer to inner leaf, it is easily removeable and handles "imprecise alignment" re inner/outer hole since drills tend to wander if your bricks vary in hardness from sand to diamond.

The CWI installers may spot your cable and refuse to do the install. Alternatively they may drill right through it - they drill holes spaced quite closely to stick the hoses in.

Is CWI worth having? Definately because 1) it reduces your heating bills 2) you gain the thermal mass of the inner leaf so the house temperature is more consistent avoiding the "cold air plunge, hot air rise draughts" particularly in hallways 3) you get less suffocating heat in summer particularly if south facing and you have the loft insulated (without doing CWI you just created a greenhouse re solar gain of walls).

PS. Do it in 10mm so you can later upgrade the shower :-) You can buy that in cut length 6491X on TLC, although I think you'd need 25mm not

20mm conduit to route it (might be ok with 10mm LN & 4mm CPC 6491X).

[NT]

To drop 20v on an 8kW load requires a 400w transformer. The link given explains that. And that's not a matho, its duty cycle.

but the OP does

NT

[fred]

ITYM 700VA transformer (35A x 20V), quite a lump of iron.

It's mince anyway as the diss in the cable (10mm2) only drops from 4W/m to 3.5W/m which seems a bit pointless.

[John Rumm]

And drop the performance of your shower from 9.5 to 8kW in the process.

One needs to take a bit of care regarding duty cycles on showers. Lots of people seem to assume that you won't spend long in one, but 20 mins or more is not going to be uncommon I expect.

[NT]

T

I see you missed ' And that's not a matho, its duty cycle.'

The diss can be dropped to whatever brings it within spec. The 20v drop was an example.

[fred]

Are you saying it's ok to put a transformer on 100% overload for any length of time?

and give the o/p a cold shower?

It's probably good to remember that the worst case distance through insulation in this situation is 25mm, ie if the cable runs in the middle of the cavity so I think John's comments about being close to 2 big lumps of thermal mass are vaild, and combined with the light overload and short duty cycle of operation I'd be happy to use 10mm at 40A in this situation.

[NT]

I've designed equipment using transformers at well over 100% 'overload' as you put it. Big ones especially have a lot of thermal capacity. Microwave ovens typically use a transformer at around 1.5kW output that would struggle to manage 500w continuous. Running for example a 400w continuous transformer at 700w for 10 minutes is very practical.

8kW isn't my idea of a cold shower.

Indeed.

one has to work to regs though.

NT

[fred]

So, buy a transformer and run it at 100% overload for 10mins or run the cable at 25% overload for 10mins, hmmmn.

Btw, what happens to the magnetic circuit in a transformer when it is overloaded? Saturation? Loss of magnetism? Effective short circuit at the primary?

I'd be pissed off if someone sold me a 9.5kW shower and it only kicked out 8kW, wouldn't you?

The light cable overload with full shower power works for me every time.

[fred]

In article , js.b1 writes

Good point :-)

How 'bout flex conduit in the cavity?

"In conduit in an insulated wall" says 10.5kW max, sounds good.

Might be a bugger to run but take a brick out (inside, temporarily) and it would run a lot easier.

[js.b1]

Flex conduit is fine. Buy a bit more than you need so you can pull out the first mangled bit.

Nylon is the most tough as you probably know, many electrical factors do it cheaply in "nylon contractor packs" of about =A310-12 for 10m (with male glands etc. What you don't use re length or glands you can dump on Ebay to knock a bit off.

It would, you can round the back hole of the brick whilst it is out (chisel, core drill bit, screwdriver & mallet if it comes to it). It might be worth buying 25mm, because that would be ideal in allowing say 10mm FTE in the future and might fill the cavity sufficiently to discount any insulation effect to "insignificant". Cavity might be

25mm, 30mm, or 35mm+.

[NT]

T

Its not an overload at all, transformer power ratings are normally continuous ratings. Since you wont be running it continuous, you get a large margin to play with.

It keeps working just the same. The only difference is more copper loss, and thus faster heating and poorer regulation.

that isn't the situation though. In years long past I've occasionally reduced the power of appliances to enable them to run on a supply that wasnt fit to provide them the full power rating, and it never pissed me off at all.

OK if you don't need to meet regs.

NT

[Archie]

I will go ahead and replace the 6mm with 10mm.

Re-routing the cable is not an option just now. I can't take it outside as there is an attached flat roofed garage/workshop and I would need to take the cable up through the flat roof. SWMBO won't let me lift carpets to get under the upstairs floorboards. She says I never put them back down properly - she's probably right. Maybe I will re-route the next the I decorate. I do have a particular dislike of surface mounted cable both inside and outside and always ensure cables are hidden.

I am happy that although 16mm would be the correct cable to fit, 10mm will be ok for two showers a day. Screwfix don't do 16mm anyway.

Thanks for all the comments guys. They have helped.

Archie