Self Build Advice Please - consumer units, sockets & switches...

[janner43]

Back to a sensible question - you made a statement about the property being "sealed" or similar - how do you get rid of unwanted moisture?

and what happens when all the air has been used up, and he's breathing in CO2?

https://en.wikipedia.org/wiki/Heat_recovery_ventilation

Heat Recovery Ventilation the best way to remove and replace stale, moist and unwanted air from your home

:waving:

 

[telectrix]

I think people need to look towards these new ideas rather than ridiculing them and sticking to how things have always been done. If we didn't progress like this then we'd still be living in houses with no cavities and no loft insulation. And we'd still have houses full of 60W and 10W lamps. Daz

best thing since the hindenburg. a bit of savings through heat loss. £3000 to replace corroded wall ties and repoint half the house.

 

[janner43]

best thing since the hindenburg. a bit of savings through heat loss. £3000 to replace corroded wall ties and repoint half the house.

You got that right. On south / western UK exposures, ie West Scotland, West Wales, Cornwall and parts of Devon no-one should install cavity insulation - especially if they have exposed brick.

In other parts of the U.K. it should be alright, but I could take you to two houses I personally know of where the houses are colder after the cavity insulation. One has failing internal plaster because of penetrating damp. The cavity insulation gets wet, the house gets cold.

Not good

 

[sean_ork]

Yet builders are still throwing up shoeboxes faced with brick, just enough FG to satisfy the building inspector, dense block inner, and glueing PB to it.

That's so 1960s

 

[Leesparkykent]

Yet builders are still throwing up shoeboxes faced with brick, just enough FG to satisfy the building inspector, dense block inner, and glueing PB to it.

That's so 1960s

More often your find they are now mostly timber framed. As they are manufactured off site.

 

[sean_ork]

More often your find they are now mostly timber framed. As they are manufactured off site.

Most of them are up here, the kit goes up in two days, but even then there's still the daft preference for a brick outer, sticking to what they know.

 

[davesparks]

Yet builders are still throwing up shoeboxes faced with brick, just enough FG to satisfy the building inspector, dense block inner, and glueing PB to it.

That's so 1960s

That's because the overriding decider in mass built houses is cost, the less it costs to build the more money the developer makes.

Houses built for an individual customer can be built far better if the customer is willing to pay for it, but again people don't like to spend money on sensible things.
It's the same situation we come across as electricians, people begrudge spending a few hundred on improving electrical safety but will happily spend thousands on fancy gadgets to plug in to the ancient sockets etc.

 

[davesparks]

Most of them are up here, the kit goes up in two days, but even then there's still the daft preference for a brick outer, sticking to what they know.

But a brick skin has worked for us for hundreds of years, so we kind of like it.
It does beg the question of what alternatives are readily available?

 

[sean_ork]

That's because the overriding decider in mass built houses is cost, the less it costs to build the more money the developer makes.

Houses built for an individual customer can be built far better if the customer is willing to pay for it, but again people don't like to spend money on sensible things.
It's the same situation we come across as electricians, people begrudge spending a few hundred on improving electrical safety but will happily spend thousands on fancy gadgets to plug in to the ancient sockets etc.

Yep, developers don't care what the ongoing costs are to keep it warm, as long as it takes longer than 10 years to fall apart they are alright jack.

 

[Octopus]

That's because the overriding decider in mass built houses is cost, the less it costs to build the more money the developer makes.

Houses built for an individual customer can be built far better if the customer is willing to pay for it, but again people don't like to spend money on sensible things.
It's the same situation we come across as electricians, people begrudge spending a few hundred on improving electrical safety but will happily spend tens of thousands on a fancy new kitchen

corrected that for you!

 

[Andy78]

Yep, developers don't care what the ongoing costs are to keep it warm, as long as it takes longer than 10 years to fall apart they are alright jack.

And even if there are problems within the 10 years they still don't give a .....
Every new build house less than 10 years old that I have been round to in the last 4 years have been in some sort of years long dispute with the developer.

One of the electrically related ones was a couple who had requested chrome accessories throughout as an upgrade to the standard white plastic. The guy who fitted them was apparently a site sweeper upper. Yeah you guessed it, he chopped all the CPCs off behind all the switches.

 

[Midwest]

Lived in a new house since the late eighties, standard brick construction, cavity (now full of insulation). Never had any issues with it, apart from it wasn't big enough. Gas central heating, and hot water. Not a log cabin in the forest by a lake. Wouldn't float Kevin McCloud's boat, but I couldn't afford his prices.

 

[Midwest]

I'm all for new building designs & systems. My eldest has a heat recovery ventilation system in his flat, not very impressed, albeit probable bodged in. Noisy main box, fan boost for kitchen, bathroom is pants, ensuite is always full of moisture after the shower is used. And I'm used to having the bedroom window open at night, not just to get rid of the fumes!

 

[M-Ty]

I have been abusing a 9.5-kW Redring Powerstream as my whole-house water heater (including bath, takes 20-25 minutes to fill) for 20 years.
A single 12-kW one would be even better. It is fine for washing-up. There would be no need for a separate electric shower. Position it nearest the most frequently-used outlet to minimise waste of cold "dead" water and plumb it to everything else in well-insulated microbore for the same reason would be my recommendation.

 

[davesparks]

Yep, developers don't care what the ongoing costs are to keep it warm, as long as it takes longer than 10 years to fall apart they are alright jack.

Actually it only needs to stand for 7 years before the Nancy warranty runs out [emoji12]

 

[janner43]

I have been abusing a 9.5-kW Redring Powerstream as my whole-house water heater (including bath, takes 20-25 minutes to fill) for 20 years.
A single 12-kW one would be even better. It is fine for washing-up. There would be no need for a separate electric shower. Position it nearest the most frequently-used outlet to minimise waste of cold "dead" water and plumb it to everything else in well-insulated microbore for the same reason would be my recommendation.

Now that is very interesting indeed. Thank you for posting.

I have decided that 3 x 12kW heaters is most definitely overkill & an unnecessary design "risk" and have arranged for them to be returned. Fortunately I am still within the window to receive a full refund on them.

I'm keeping the Gröhe shower, mind you

I'm minded to get 2 x 9.5kW Redring RP1s one for the two basins in the bathroom & one for the utility room & cloakroom.

The kitchen is another story...

Currently I'm thinking this way...
1) Hand clothes washing in the utility will only happen once a week at most. The cloakroom might get used once or twice a day.
2) The basins in the bathroom, certainly for 6 months of the year get used during "non-daylight" hours exclusively and for the shorter days, only once a day when it is light. IE 75% of their use pa will be in the non-daylight hours.

As a result, none of that hot water usage is going to make much use of the Solar PV

3) The kitchen, however, is likely to see 90% of its use during daylight hours, and "she who must be obeyed" is quite concerned that she has hot (50° - 60°) water for pre washing greasy dishes if required. In view of that, we might end up with a 5litre unvented 2/3kW heater for the kitchen. Stiebel-Eltron do one as do Hyco. In both cases, they say they take 12 minutes to heat from cold and then use 500 watts in 24hours to keep hot. That sort of usage is going to make full use of the Solar PV while being negligible as far as design impact is concerned.

 

[1Justin]

Hi Janner..If you are still here.
I got to this thread late. I've been fitting out all the electrics in a Passivhaus this year. I have also installed MVHR. i completely (almost) understand where you are.
I have failed though, reading through things, why you see the need for instant water heating? -The future for our energy grid will only be possible with storage for smoothing. That may be batteries, but it will most certainly require hot water tanks since heating loads are so large. (Obviously crap for all those households which only have the space for stupid combi gas boilers, but they'll feel the pain one day) - So storage also means hot water tanks.

Plus you have PV, - Only a financial consideration, but why not make use of that with a diverter, rather than exporting it at 4p a unit? Solar thermal is also very good indeed. It's not often installed but is about three or four times as efficient as current technology PV (area for area on your roof)

 

[sparksburnout]

Blimey, i have just spent far too long reading this thread. I don't like the sound of this sealed up clinical environment too much, i bet all you can hear most of the time is "shut that bloody door". All this rubbish about not using gas appliances in 20 years time, what a load of crap, what do some people think the average person lives in in the country? Me, I like sitting in front of my roaring open fire, with plenty of stored hot water and a few draughts here and there to keep all the moisture and damp out. The way some people talk you might as well live in a space station. I wouldn't want to rely on me and the missus generating enough heat to keep warm some nights, well most nights, all this smacks of some grand designs none sense to me, designer crap for boring folks with nothing else to do.

 

[janner43]

Hi Janner..If you are still here.
I got to this thread late. I've been fitting out all the electrics in a Passivhaus this year. I have also installed MVHR. i completely (almost) understand where you are.
I have failed though, reading through things, why you see the need for instant water heating? -The future for our energy grid will only be possible with storage for smoothing. That may be batteries, but it will most certainly require hot water tanks since heating loads are so large. (Obviously crap for all those households which only have the space for stupid combi gas boilers, but they'll feel the pain one day) - So storage also means hot water tanks.

Plus you have PV, - Only a financial consideration, but why not make use of that with a diverter, rather than exporting it at 4p a unit? Solar thermal is also very good indeed. It's not often installed but is about three or four times as efficient as current technology PV (area for area on your roof)

Hi Justin,

Glad someone else actually understands what I'm on about - clearly not everyone does or even understands the remotest concept of not wasting the resources we have...

I'd opt for battery storage if it wasn't so expensive. When it comes to immersion heaters I loath them. They seem to me to be a daft way of heating lots of water that we'll never use. As I have said previously, clothes washing happens in a washing machine, dishes in a dishwasher. The amount of hot water we will use in a day isn't likely to be more than 30 litres on a "bad" day. The smallest type of tank is normally 4 times that size!!

No, the water heating is an experiment, I'll grant you, but I believe that the concept is sound. When it comes to export quantity, I am hoping to have a Smart Meter if I can get one, so the typical estimate of 50% for the export tariff shouldn't apply.

Our budget isn't endless and I have tried to only spec the tech that actually works with a cost-benefit analysis - which is why, even though we are in the most expensive area of the U.K. for water, we are not having rainwater reclamation for use in the house. The numbers are just not worth it - nowhere near.

Cheers

 

[Leesparkykent]

Hi Justin,

Glad someone else actually understands what I'm on about - clearly not everyone does or even understands the remotest concept of not wasting the resources we have...

I'd opt for battery storage if it wasn't so expensive. When it comes to immersion heaters I loath them. They seem to me to be a daft way of heating lots of water that we'll never use. As I have said previously, clothes washing happens in a washing machine, dishes in a dishwasher. The amount of hot water we will use in a day isn't likely to be more than 30 litres on a "bad" day. The smallest type of tank is normally 4 times that size!!

No, the water heating is an experiment, I'll grant you, but I believe that the concept is sound. When it comes to export quantity, I am hoping to have a Smart Meter if I can get one, so the typical estimate of 50% for the export tariff shouldn't apply.

Our budget isn't endless and I have tried to only spec the tech that actually works with a cost-benefit analysis - which is why, even though we are in the most expensive area of the U.K. for water, we are not having rainwater reclamation for use in the house. The numbers are just not worth it - nowhere near.

Cheers

30liters on a bad day....do you an your partner not shower at all then?

 

[Midwest]

30liters on a bad day....do you an your partner not shower at all then?

Op is gonna have electric shower.

 

[1Justin]

Hi Justin,
They seem to me to be a daft way of heating lots of water that we'll never use. As I have said previously, clothes washing happens in a washing machine, dishes in a dishwasher. The amount of hot water we will use in a day isn't likely to be more than 30 litres on a "bad" day. The smallest type of tank is normally 4 times that size!!

Cheers

Well, as others have already pointed out, that heating load will need some very careful thought, and probably some sort of "hard" interlock to prevent the all operating at once. It may be unfortunate for someone having a shower if the kitchen is using the load though.

Personally I think hot water tanks are very good, and the heat loss from a well insulated tank at moderate temperature (say 55C) is pretty good. Assuming PIR foam 100mm thick, (U 0.2), for a 2M x 700mm tankful of water at 55C, that comes to about 36 watts. - Still too much to be loosing, give the convenience?

3 x 12KW + 9.5KW ! You are out on a real journey with this one! I have no solution other than some sort of TBD interlock. Let us know how you get on.

 

[davesparks]

Well, as others have already pointed out, that heating load will need some very careful thought, and probably some sort of "hard" interlock to prevent the all operating at once. It may be unfortunate for someone having a shower if the kitchen is using the load though.

Personally I think hot water tanks are very good, and the heat loss from a well insulated tank at moderate temperature (say 55C) is pretty good. Assuming PIR foam 100mm thick, (U 0.2), for a 2M x 700mm tankful of water at 55C, that comes to about 36 watts. - Still too much to be loosing, give the convenience?

3 x 12KW + 9.5KW ! You are out on a real journey with this one! I have no solution other than some sort of TBD interlock. Let us know how you get on.

Why interlocks? Two people living in the house aren't likely to go around and use all 4 heaters at once.

I can see his point with this, and a hot water storage doesn't fit his requirements. You don't just have the loss through the tank to think about, but how much water gets heated but not used and the amount of hot water which remains in the pipework and cools down rapidly. If there is a hot water circulation pump in use to satisfy the need for hot water to be delivered quickly then the losses go up again

 

[janner43]

Don't know if Justin missed my post about abandoning the 3 x 12kw heaters?

Anyway we have now decided on...
2 x Redring RP1s @ 9.5kW each (rated at 240v, at 230v only 9kW/40amp)
1 x Stiebel Eltron SHU 5 (2kW, 5 litre unvented) - it uses .31kW in 24 hours to maintain the water temperature once heated.

I feel much happier with the expected usage from an electrical safety point of view, and I have the comments made in this thread to thank for that. I guess maybe a handful of times in a year 2 of the 9kw items might be used at the same time for a moment and that still only gives 80 amps for maybe 30 seconds, allowing 20amps 4.5kW for other things if they are on at the same time.

 

[davesparks]

Don't know if Justin missed my post about abandoning the 3 x 12kw heaters?

Anyway we have now decided on...
2 x Redring RP1s @ 9.5kW each (rated at 240v, at 230v only 9kW/40amp)
1 x Stiebel Eltron SHU 5 (2kW, 5 litre unvented) - it uses .31kW in 24 hours to maintain the water temperature once heated.

I feel much happier with the expected usage from an electrical safety point of view, and I have the comments made in this thread to thank for that. I guess maybe a handful of times in a year 2 of the 9kw items might be used at the same time for a moment and that still only gives 80 amps for maybe 30 seconds, allowing 20amps 4.5kW for other things if they are on at the same time.

It would be interesting to know exactly how much current they actually use when they are running, they may very well have multiple elements totalling 9.5kW but not always use that much. I've found some 9kW showers actually include a thermal switch so that only one of the elements runs if it gets hot enough on its own (4.5kW)

 

[janner43]

It would be interesting to know exactly how much current they actually use when they are running, they may very well have multiple elements totalling 9.5kW but not always use that much. I've found some 9kW showers actually include a thermal switch so that only one of the elements runs if it gets hot enough on its own (4.5kW)

At the risk of embarrassing myself, I don't know of a way to measure the current draw - is there an easy way to do that?

 

[davesparks]

At the risk of embarrassing myself, I don't know of a way to measure the current draw - is there an easy way to do that?

Yes, we use a device called a clamp meter. This is a meter which has a set of jaws which encircle a conductor and measure the current flowing through it. When a current flows it produces a magnetic field around the conductor, by measuring this magnetic field you can calculate the magnitude of the current flowing through it, the meter does the calculation for you and displays the actual current flow in amps.
(Apologies if teaching you to suck eggs with any of this)

 

[newtothis]

Take your point it’s not an investment.

That’s good, because with respect you may be creating something, were no one would want to lend on, or buy it, without wanting to knock you for cash to conventionalise it. What’s the point of investing in a significant asset that can’t easily be liquidised ?

Tweaking a concept or standard downwards until it doesn’t achieve the criteria of the concept or standard is counterproductive to any USP attached to the concept.

Just random thoughts, please feel free to ignore.

 

[Vitesse]

It would be interesting to know exactly how much current they actually use when they are running, they may very well have multiple elements totalling 9.5kW but not always use that much. I've found some 9kW showers actually include a thermal switch so that only one of the elements runs if it gets hot enough on its own (4.5kW)

A bit off topic but along the same lines. What is the score Dave with a domestic property proposing to run 2 x 9kW electric showers from a 100A main fuse that don't have the ability to switch off one of the elements? My understanding is that it is a obvious no no due to the current draw, unless you have some sort of relay that will switch off one, when both are switched on at the same time.

I assume you could employ the same sort of idea in this case with the 9.5kW heaters?

I was sizing up a job today in a pub that needs a rewire in the living accommodation upstairs and it needs another electric shower, fortunately it has 2 x single phase 100 amp supplies, the existing electric shower it is on one supply that was drawing 55 amp on my clamp, the other drawing just 16 amp which will supply the upstairs and new shower, so not a problem.

Just I am not familiar with this relay switching if anyone could enlighten me?

 

[Richard Burns]

A bit off topic but along the same lines. What is the score Dave with a domestic property proposing to run 2 x 9kW electric showers from a 100A main fuse that don't have the ability to switch off one of the elements? My understanding is that it is a obvious no no due to the current draw, unless you have some sort of relay that will switch off one, when both are switched on at the same time.

I assume you could employ the same sort of idea in this case with the 9.5kW heaters?

I was sizing up a job today in a pub that needs a rewire in the living accommodation upstairs and it needs another electric shower, fortunately it has 2 x single phase 100 amp supplies, the existing electric shower it is on one supply that was drawing 55 amp on my clamp, the other drawing just 16 amp which will supply the upstairs and new shower, so not a problem.

Just I am not familiar with this relay switching if anyone could enlighten me?

Two 9kW demands would total 80A or so and so long as the anticipated demand from the rest of the property was below 20A then there would not be a problem. However each situation would have to be assessed on the anticipated maximum demand in reality.
It is not likely that someone would turn on the oven and all hob rings and go and have a shower, but it might be more likely that a family of four would be running a bath whilst someone else is doing the washing up and so using two heavy demands simultaneously.

Shower priority units do not appear to be common in Great Britain, I am not sure why, but they are in use in other areas on a fairly regular basis.
They would be something like this. Two options whichever is on last comes on (the other goes off), or whichever is designated "main" stays on or comes on in preference to the other load. I have seen triple systems I think before but I cannot remember where.

 

[Midwest]

OP, I remember someone pointing out to check the rating of your supply with your DNO, have you done that yet?

 

[Midwest]

Yes, we use a device called a clamp meter.

OP your electrician should have one. Normally clamp around the tails for this measurement, but best left to him. But measuring will be like locking the stable door, after the horse has bolted. You need to consider the design first, as davesparks mentioned earlier.

 

[janner43]

OP, I remember someone pointing out to check the rating of your supply with your DNO, have you done that yet?

I'll double check with Western Power, but having spoken with them about the maximum size single phase domestic connection & them saying it is 100amp I can have, I will be pretty annoyed if it is the old 65amp service. Thanks for the reminder.

 

[marc8]

hi Janner43

be a little careful with MVHR unit to do the most of the heating I know of a heating engineer that was working at some new build places & the developer did not put in any heating just MVHR units & it was cold in the winter & too late for them when it was finished.
on a course for MVHR units few years ago with a guy who was working on a passive house with very good insulation values, they were even putting 20mm conduit through walls for the outside lights when first fixings so no losses through cavities & then silicone the tube when wires installed & then again when light fitted trying to keep air tight.
sockets bagging around the back sides creating a seal as such to the outside.
used some MVHR units with wet rads inside them, they have temperature sensor mounted inside the unit which then feeds a stat that opens heating valve & heats the air coming into the property making them even more efficient.
another thing to keep in mind that manufactures do not tell you is what to do with unit should there be a fire I rang one company about this they said down to installer no guidance.
we installed heat detector in loft with unit & on a relay base N/C should fire detectors go off we stopped the unit. did not want to give the fire one of the elements it needed.
hope it helps

 

[Midwest]

As I mentioned before, the only experience I've had with MVHR, is my son's new build flat. It's pants. A couple of guys on here with experience said the ducting was probably poorly installed. The main unit makes a hell of a racket (installed on anti-vibration mountings), and for obvious reasons is constantly on. Perhaps a decent system properly installed, it might be different. You get what you pay for I suppose.

 

[janner43]

Take your point it’s not an investment.

That’s good, because with respect you may be creating something, were no one would want to lend on, or buy it, without wanting to knock you for cash to conventionalise it. What’s the point of investing in a significant asset that can’t easily be liquidised ?

Tweaking a concept or standard downwards until it doesn’t achieve the criteria of the concept or standard is counterproductive to any USP attached to the concept.

Just random thoughts, please feel free to ignore.

I take your point - but I actually think the reverse will apply here. At least by the time it comes for the property to be sold. At the moment, there is little understanding in the public eye of the value of high insulation, 21st century build techniques, a fully networked and AV enabled property and no need for radiators everywhere or for an expensive gas boiler that needs regular servicing. On top of that, as a two bedroom detached bungalow, on a generous plot with a floor plan and access specifically designed to meet the needs of a couple as they get older to reside in their own home for all of their life, it will command further benefits in the eyes of the likely purchasers.

Our previous home sold for more than any comparable house in the street in August 2015. It had no gas central heating and no cavity insulation and a poorly calculated EPC. I say "poorly calculated" because despite 200mm of insulation under the rafters and compact fluorescent lighting throughout, and utility bills that were 2/3rds of the national average it still only received a "D" simply because it didn't have gas central heating!!

In 30 years - when the new property is likely to be on the market - I think it is very, very likely that older, largely uninsulated properties with poor connectivity and old fashioned design will be harder to sell, while the majority of homes will have been retro fitted to different levels of success.

A custom built home that still looks normal but is, in reality only costing 25% of the normal running cost of an older property, and which has excellent connectivity and which is cheap to service is likely to carry a premium in terms of value and should be snapped up on the market.

But who can tell...

 

[andyb]

I don't think this design will catch on, indeed I'd go so far as to say that if it became common it would be banned.
All the discussion regarding how many people would use the shower at any one time is fine for one house, but what about your neighbour and the rest of the street? Then the rest of the town? Then the rest of the country? It's not sustainable for the existing grid.

An eco house is a very good idea, I worked on one a few years back and they pay next to nothing for energy. Proberly nothing.
But it depends on storing heat that has been harvested for free from the thermal panels on the roof.

 

[marc8]

janner43

also MVHR unit running through loft space good easy install just remember to insulate pipes all, as for previous post with MVHR constantly running & noisy sounds like a bad install.
keep duct runs as short as you can no sharp bends, should run nice & slow, slower the better less energy to be used then when boosted will still run slower than flat out. just done a block of flats with it in works great, also used the foam input & output pipes to & from unit to manifold & to atmosphere cost a bit but again no noise & better flow rate than flexi duct.

 

[janner43]

hi Janner43

be a little careful with MVHR unit to do the most of the heating I know of a heating engineer that was working at some new build places & the developer did not put in any heating just MVHR units & it was cold in the winter & too late for them when it was finished.
on a course for MVHR units few years ago with a guy who was working on a passive house with very good insulation values, they were even putting 20mm conduit through walls for the outside lights when first fixings so no losses through cavities & then silicone the tube when wires installed & then again when light fitted trying to keep air tight.
sockets bagging around the back sides creating a seal as such to the outside.
used some MVHR units with wet rads inside them, they have temperature sensor mounted inside the unit which then feeds a stat that opens heating valve & heats the air coming into the property making them even more efficient.
another thing to keep in mind that manufactures do not tell you is what to do with unit should there be a fire I rang one company about this they said down to installer no guidance.
we installed heat detector in loft with unit & on a relay base N/C should fire detectors go off we stopped the unit. did not want to give the fire one of the elements it needed.
hope it helps

Thanks for the idea about fire detection in the loft. An excellent idea and one I will include.

Our home is slightly different in concept in that it doesn't rely on tapes or sealants to be airtight. It has an air tightness value of 0.8 as built. We will not under any circumstances penetrate the walls. All cabling for outside lights etc will run up into the loft though conduits that are built into the walls. They will then drop through the soffit.

Here is a shared Dropbox folder containing some photos and a PDF of the tech spec of the build type if you (or anyone else) are interested.

https://goo.gl/ID2Ofk

 

[davesparks]

A bit off topic but along the same lines. What is the score Dave with a domestic property proposing to run 2 x 9kW electric showers from a 100A main fuse that don't have the ability to switch off one of the elements? My understanding is that it is a obvious no no due to the current draw, unless you have some sort of relay that will switch off one, when both are switched on at the same time.

I assume you could employ the same sort of idea in this case with the 9.5kW heaters?

I was sizing up a job today in a pub that needs a rewire in the living accommodation upstairs and it needs another electric shower, fortunately it has 2 x single phase 100 amp supplies, the existing electric shower it is on one supply that was drawing 55 amp on my clamp, the other drawing just 16 amp which will supply the upstairs and new shower, so not a problem.

Just I am not familiar with this relay switching if anyone could enlighten me?

I've never put any sort of relays in, but the most we fitted was 3 showers on one 100A supply in a house which had no problem.
Don't forget the chances of people all showering at the same time in a single house are pretty slim, especially since the water pressure would drop away to a dribble!