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Hot Water For Annexe In Bedfordshire


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#21 Crofter

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Posted 29 March 2016 - 10:09 PM

I spent quite a while looking into Steibel Eltron for my own house, but did find that there were potential issues having one upstream of a thermostatic shower. I suppose i coukd have a simpler shower that just accepts the temperature coming out of the heater, but had onoy just installed the thermostatic one and didn't want to rip it out again.

For my little house, I do like the idea of a multipoint heater. There will only ever be two people in the house anyway so reduced likelihood of conflicting water usage.
Does an accumulator get away without the annual inspection costs that a UVC has?

#22 Nickfromwales

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Posted 29 March 2016 - 10:15 PM

An accumulator is a metal can with a rubber balloon inside it ;). Nothing more.
As it is full of cold water there is no G3 / other requirement, but periodically discharging the stored water and checking the air back pressure value is certainly a recommended maintenance requirement.
This is very easy to do btw, with a £5 'car tyre' type pressure checker, and prob all done start to finish in 30 mins or so.
Regards, nick.

#23 jsharris

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Posted 30 March 2016 - 08:10 AM

As Nick says, anyone can fit and maintain an accumulator. We have two 300 litre ones plus one 100 litre one (so 700 litres worth of accumulators in all, that store around 280 litres of water under just over 4 bar pressure) and there was no requirement for any inspection or properly qualified person to fit them. I also have two rigid GRP pressure vessels in the system, one holds about 50 litres, with a 20 litre air pocket above, the other holds about 30 litres with around 70 litres of filter sand and oxidising catalyst. All told we probably have around 350 litres of water sitting at just over 4 bar most of the time. Makes for a nice stable water supply though.

#24 SteamyTea

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Posted 30 March 2016 - 08:13 AM

As these accumulators are just pressurised bladders of water (I watched Guy Martin last night) does there need to be a non-return valve (NRV) on the mains side just incase the mains pressure drops?

#25 Crofter

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Posted 30 March 2016 - 08:58 AM

On a new build you need one at the property boundary anyway.

Continuing my slight hijacking of this thread, I'm wondering what wouod be the best way for me to add a IHW heater to my current house, whilst retaining the option to use the old tank for the very occasional baths that SWMBO likes.

#26 Nickfromwales

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Posted 30 March 2016 - 09:06 AM

View PostSteamyTea, on 30 March 2016 - 08:13 AM, said:

As these accumulators are just pressurised bladders of water (I watched Guy Martin last night) does there need to be a non-return valve (NRV) on the mains side just incase the mains pressure drops?
Yup. Fitted just after the cold mains stopcock and followed by a pressure reducing valve to cap the max static pressure that the install will see.
;)

#27 Nickfromwales

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Posted 30 March 2016 - 09:07 AM

View PostCrofter, on 30 March 2016 - 08:58 AM, said:

On a new build you need one at the property boundary anyway.

Continuing my slight hijacking of this thread, I'm wondering what wouod be the best way for me to add a IHW heater to my current house, whilst retaining the option to use the old tank for the very occasional baths that SWMBO likes.
A big ( ~15kw ) instant will fill a bath so you may get away with just that if your happy to wait for it to fill.
Regards. Nick.

#28 Crofter

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Posted 30 March 2016 - 12:58 PM

I don't thnk my power supply would handle that!

#29 gravelld

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Posted 30 March 2016 - 01:25 PM

I am interested - how do you decide how you limit how many of these heaters you have? Are you "permitted" to oversize and have a number of them per property? Actually, I'm not even sure what the maximum power draw is from single phase.

#30 SteamyTea

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Posted 30 March 2016 - 01:34 PM

It is set by the main fuse and the diversity factor as far as I know.

#31 Nickfromwales

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Posted 30 March 2016 - 02:10 PM

I re plumbed a big house in the sticks which had 1x 8.5kw shower in the master, a 10.5kw in the ensuite, an 8.5kw one into the garage / annex flat, an induction hob, electric oven, dishwasher, t/drier and a 60a incoming mains supply on PME. !!! Oh, and a 3kw immersion.
When my jaw hit the deck they said they've not had a problem in 15 years. On paper it was a fire waiting to happen but no sign of overheating anywhere.
That's how much diversity you can allow and still live to tell the tale. Oh and their teenagers used the showers in unison too, prob whilst mum was cooking so the mind boggles.
Regards, Nick.

#32 Crofter

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Posted 30 March 2016 - 07:30 PM

How big a danger is that? I thought the 60A fuse would pop- that's what it's there for, surely?

#33 SteamyTea

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Posted 30 March 2016 - 08:31 PM

A 60A main fuse does not blow once 60A is exceeded. It can run for quite a while when it goes over current.
There is a chart somewhere that shows the times (probably a 17th Edition appendix).

#34 jsharris

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Posted 30 March 2016 - 08:58 PM

Diversity is a wonderful example of uncertain statistics being applied to a real system that could well fail if reality doesn't match the statistical assumptions made.

The general diversity principle is that you can connect several loads to a circuit, that if used at the same time would cause the circuit protection device to operate (fuse or circuit breaker). The regs (currently 17th Edition) allow circuits to be deliberately overloaded, and make some statistical assumptions on the probability of this happening in a simplified set of diversity "rules".

A simple example is a ring main. It wouldn't be at all uncommon for a ring main to have 20 or more 13A rated outlets. However, it would normally be protected by a 32A fuse or circuit breaker. Clearly you could not draw 13A from more than 3 outlets without risking a fuse or circuit breaker trip. The diversity tables assume that three 13A loads on the same ring main would be unlikely, so a much larger number of outlets is allowed, on the assumption that, at any one time, the total load on the circuit is unlikely to exceed the 32A breaker rating for long enough to cause it to trip and protect the circuit.

The system seems to work, and despite being a bit ad hoc, the diversity rules generally seem to allow circuits to apparently be rated to carry much higher currents than they do in reality.

As it happens, I had an example of a diversity rule failure this morning. There is a 13A outlet connected to my car charge point. The assumption is that I won't be charging the car (at 15A) and running anything from the included 13A outlet, as the spur is protected by a 16A RCBO. This morning my car was charging and the landscaping chap plugged in a Kango to break up some over-spill concrete. Unsurprisingly, the RCBO tripped, as it didn't like the combination of a constant 15A load plus another handful of amps from the Kango. I could probably increase the size of this RCBO, as the cable is only about 5m of 2.5mm2 SWA, which is fine for a lot more than 16A, but when I installed the circuit I erred on the side of caution and fitted the lowest rated RCBO I could and still allow the 15A charge point to work OK.

#35 Natattef

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Posted 30 March 2016 - 08:59 PM

Great, I will have to sit and absorb this all and discuss it with the electrician (and probably plumber).

Many thanks, this is absolutely great.

Cheers,
Nathalie

#36 SteamyTea

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Posted 30 March 2016 - 09:15 PM

View Postjsharris, on 30 March 2016 - 08:58 PM, said:

Diversity is a wonderful example of uncertain statistics being applied to a real system that could well fail if reality doesn't match the statistical assumptions made.
I would claim that it is a case of statistics describing the real word incredibly well, if not you would expect a circuit to automatically disconnect somewhere between 1 and 5% of the time ;)

#37 Nickfromwales

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Posted 30 March 2016 - 10:11 PM

View PostCrofter, on 30 March 2016 - 07:30 PM, said:

How big a danger is that? I thought the 60A fuse would pop
Apparently, it wasn't a danger at all. Worked perfectly well for over 15 years, just DONT ask me how :huh: !
Jeremy's explanation of the humble ring main is a pretty good example, but missing one more piece of info. The ring main allows each 13a outlet ( single socket, or a double socket , or a 13a fused spur ) to be fed by two cables, one to bring juice in and one to continue onto the next outlet, thus creating the ring. The ring starts at the 32a MCB and goes to each outlet in turn, and then returns to the exact same terminals it started from eg two lives together in the MCB, two neutrals together in the N terminal, and same for earth.
2.5mm2 twin and earth cable is the weapon of choice for a generic ring main. 2.5 is rated around 26-28 amps current carrying capacity, so each socket has the ability to draw 52-56 amps without exceeding the max capacity of the cable run.
The 32a MCB will probably allow 36-38amps for a while before the bi-metallic strip inside it distorts enough to cause it to trip out, and may possibly give as much as 40amps peak for very short bursts ( known as 'shunt resistance' in sparky terms ;) ), but these will never be enough to cause harm to the cable as the two together exceed the max shunt that would ever occur.
Lower and longer 'overloading' would probably do the most harm, with the outlets themselves ( rated at 13a ) being the weakest link in the chain and often seen 'browned' around the plug pin holes where things like electric fires are left plugged in. The 13a fuses in a plug top will allow prob as high as 15-16 amps before actually blowing the fuse, where ,prior to blowing, the fuse runs 'molten'.
Imo, common 13 amp plugs and sockets should have been beefed up slightly as they're clearly the parts with the least amount of headroom.
Another major issue i see a lot of is with electric showers where the pull switch and shower electrical terminals ( with the 6.0mm2 or 10mm2 cable ) where the screw terminals were never punched down tight enough from day one and they go into meltdown. Same with a lot of 45amp cooker switches, especially where the hob has been upgraded but the sparky/ installer has just assumed that the switch is fine. :unsure:

View PostCrofter, on 30 March 2016 - 07:30 PM, said:

that's what it's there for, surely?
And don't call me Shirley :lol:
:ph34r:

#38 jsharris

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Posted 31 March 2016 - 08:55 AM

As an aside, 13A plugs are actually only rated for 10A continuous load. Despite having worked with electrics for years, teaching the subject to apprentices and holding a ticket years ago (15th Ed, so that tells you how long ago!) I only found this out a couple of years ago. My car came with a low power charging lead, rated at 10A, with the appropriate control circuit to ensure that the car cannot ever draw more than 10A when this lead is used. I wondered why, as it had a 13A plug, it didn't allow charging at the full 13A, as it used to be quite common to have electric heaters that could draw around 12A or so (I still have a 3kW electric convector heater that must be around 30 years old now).

A bit of digging around uncovered that the humble BS 1363 13A plug is only rated at 13A for short duration use and has a continuous current rating of 10A. This explains why you often see signs of overheating on sockets used to run old electric heaters, I would guess.

#39 SteamyTea

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Posted 31 March 2016 - 09:09 AM

I seem to remember being told that a fuse also has a fixed working life for any given capacity.
So a 5A fuse will eventually blow even if the load has only ever been 3A. The justification of this was that it forces a periodic check on the equipment.
Not sure how true it is, but makes sense in a funny sort of way.

Can't be arsed to read though this at the moment, but it may have something in it.

https://electronics....less-of-voltage

Edited by SteamyTea, 31 March 2016 - 09:12 AM.