Electric showers

[blacklion1725]

Afternoon all – doing my bathroom at the moment and have completely gutted the old one. Have everything sorted except the shower. Its an upstairs bathroom, not the main one, and had a low power (1kW) electric shower.

My system is a “normal” boiler, not water tank (upstairs) and cold water tank in the loft.

Looking at the electric showers now, they o right up to 10.5kW. I’ve run in a 10mm cable to the consumer unit to cater for a higher powered shower (I know a sparks who will connect it up for me), but given that the water pressure upstairs ain’t great, is there any benefit in getting a high powered shower?

Instinct says it can’t magically suck more water out of the plumbing so I might as well go for a cheaper shower and accept that it will take a bit longer to get wet (main shower is downstairs and AOK)

Not really sure about the relationship between water pressure and the power (in kW) of the electric shower so it anyone on here knows better I’ be grateful – ta!

[Ed P]

Simple hydraulic engineering confirms your instinct. In your words if you try to suck out too much water it doesn’t work. In simple terms if you suck out at too high a rate the water will cavitate (water hammer). This is very bad for a pump and your connections. In an industrial situation, cavitation (low pressure voids) causes pump bearing failure. In a household situation it is the cause of a bad ‘ water hammer’ and will eventually cause compression joints to fail and water everywhere is the disasterous result.

I cannot translate your 10 Kw as I’m not sure if that number includes a heater, but if it just powers a pump then it is a socking big one! For reference, our pumped shower has a 4amp dual (hot+cold) Salamander and that gives a more than adequate flow rate (it is rated to supply two showers simultaneously) This pump is located close to the hot water tank (the closer the better to avoid hammer).

[blacklion1725]

Many thanks Ed – 10kW is a reasonably typical power rating for the electric shower only (not a pump, heater or anything else).

The shower I too out (1kW rated) “worked” but by the time you got wet you could have had a bath!

As it is the least used (for bath/shower – most used for night time tinkling) bathroom, I don’t want to spend a fortune on the shower….and don’t want to start adding pumps or the like.

I was just staggered that the one in there (pre-dates me) was 1kW and all the current ones are 8 – 10 that power rating. Downstairs (main) shower is gravity fed (non electric) and works a treat. I think I’ll take a punt on a modest electric shower from somewhere I can return it and see how it goes.

thanks again

[JayCeeDee]

Depending on how your 10mm cable is routed, the current carrying capacity varies. See HERE.

Back in the day ( Ohms Law still applied:) ) a 3kW fire was rated at 13amps. A 10.5kW shower will minimally consume 3 1/2 times that, ie 45amps. Make sure your cable routing matches up.

The majority of that 10.5kW will be the heater that takes cold water in and produces “instant” hot water out. A smaller percentage will be the pump.

Your sparks friend would do better with a quick site survey beforehand, to make sure any work already done won’t need undoing, and any planned will meet the regs, with a final visit to connect and certify the installation.

[Bob Williams]

No technical information from me atm, as Electrician No.2 gson is away at a Stationary Engine Show with dad.

Just that I have the opposite problem to yours BL: in the bungalow, we have a Wet Room.  I cannot get in and out of a bath due to my spinal problem, and I have a skin/lymph system problem which prevents me bathing that way anyway. I need to use Hibiscrub (pink stuff surgeons use to Scrub up) and cannot lie in bathwater, have to rinse off as I wash from top down doing the whole thing twice.

Our village lies below the Wolds and is only just above sea level. Anglian Water gets to us at a fearsome pressure and I had to turn the Restrictor down a bit or face losing layers of epidermis! As it is, I still get really good pressure for my shower, but every time I finish, I turn it down even more, or listen to a scream from SWMBO when she uses it! We cannot turn on taps anywhere in the house, more than halfway.

There is a huge Aquifer under our village, an Anglian Water Station about 150 yards down the road, and an old village pump by the Church, which I am told could still be made to work. But as Bob Dylan wrote and sang, “the pumps don’t work cause the vandals took the handles” –(‘Subterranean Homesick Blues’). The vandals being the old Waterboard.

When the Thought Police arrive at your door, think -
I'm out.

[blacklion1725]

Cheers all

The cable run is very conventional and short and I believe the 10mm cable will take up to 65A – it will be going in to a 50A breaker on the consumer unit, and I think rating wise all my ducks are in a row (cable rated higher than shower and breaker)

Main concern is whether power (and so money) is worth it if the water pressure aint great

[Ed P]

Actually like all motors the starting current for the pump bit will be MUCH higher than the operating load. Typically 4 to 6 times the normal motor load, but you would need to get pump and heater ratings split out. Better to ask your electrician friend if in his experience you have a fat enough cable for that service.

[Wheels-Of-Fire]

My Mother has a similar setup with a mains fed electric shower on the second floor. She has very good water presure and the shower works fine until someone on a lower floor starts using water and when this happens the shower either cuts out or tries to boil you !

The water compay say the problem is the diamiter of the pipe from the street and I can believe that since it was installed in 1926 but they have quoted £5000 to replace it.

I was wondering if a ground level pump would help or would she just get this cavitation problem ?

[JayCeeDee]

@blacklion1725 – as per that table, you’re correct, it will take 64A, only if that cable run is “Clipped direct, or sheathed cables embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials)”.

If it’s routed in a ” In a stud wall with thermal insulation with the cable NOT touching the inner wall surface.” –  then it’s only 32A.

 

 

 

[Ed P]

Wheels-Of-Fire wrote:

…I was wondering if a ground level pump would help or would she just get this cavitation problem ?

Pump cavitation comes when the water pressure at the suction of the pump is too low. In simple terms this happens when the ‘mains’ pressure is very low (either because the suction is too high off the ground, or the pump is trying to ‘suck’ too much.)

In your mother’s circumstance mounting the pump at ground level gives no cavitation worries, however your real concern is water supply to the suction of the pump. A cold water header tank supplying the shower or flow limiters on the other users  would be better solutions.

[blacklion1725]

Thank you all. Sparks will be here this week. Part of the cable is behind a stud wall, but easy enough to move or adjust since on the inside of the bathroom there is new waterboard up (screwed on) – not tiled yet – which can be removed very easily. I’m inclined to just go for a lower rated shower. There is a few at 7.2KW which fits the 32A “worst case” criteria for the 10mm cable – but I’ll see what he says when he has had a look. I also hadn’t realised (till he told me) that standard mains is 230V now not 240V, which annoyingly puts the more common 7.5kW showers just over 32A. As I mentioned the one it is replacing was 1kW – so it can’t be any worse!

[The Duke]

Out of interest how old is the 1kw shower? I’ve never seen one before. Or never noticed one in my life. My last one was a 7.5 I’ve not long swapped for a 9. As it packed in. User error.

[blacklion1725]

Its older than my youngest who is 22 – not sure by how much but it was OLD.

[The Duke]

Thought it must be. Just think it’s going to cost you 9x more for every shower. ☺️

I have no idea how that works. It was just a joke, Before someone jumps in and explains my wrongs.

[blacklion1725]

I think your maths are right anyway – except the old one probably took 9 times as long to get wet so ended up about the same!

 

[Richard]

The sort of shower it appears you are proposing is a flash heater sort which often uses only the cold water supply. If the flow on that is already poor the results of a shower pack will also be poor to worse.  You should not normally use a boost pump (combined hot and cold feed) type of shower with a combie boiler type set up.

Most shower pumps are only suitable for use on a system with header tanks and need something like a 6 foot head supplying them, (see their detailed specifications). Ideally the cold water supply should come as a direct feed from the cold header tank and the hot water pipe should not normally feed other devices in other rooms. Otherwise you will get weak flow at the best and all sorts of problems at the worst, e.g. dangerous performance with the risk of scalding or chilling thermal shock, or both.

In most locations fitting a boost pump directly to the incoming water supply is a no-no as it can result in reduced pressure, or negative pressure on the mains which will allow crap, i.e. contaminated water to be drawn in. Get caught and you will be big trouble.

Normally the incoming supply should feed a low level tank with ball valves and suitable water standards approved fittings and the boosted supply is then pumped from the low level tank to the high level one. This needs suitable electrical installation, it is fairly straight forward but must comply with the water and electrical regulations for the area. Water regulations my stipulate a direct cold water feed for drinking water and possible for one WC; non return valves may also be needed, e.g. on garden taps, water softeners, and so on.

The theory of the header tank is that it can fill slowly when water is available and then meet the pulse of your demand when needed. It can then fill up more slowly once more as the level drops.

Watch out for specific rules regarding the use of booster as they might be banned in some locations because they can affect overall water demand.

Like Bob we have a very high mains pressure, about 5 times the rated pressure and sometimes higher for some devices so we needed a pressure reducer valve to stop items blowing leaks.

[Wheels-Of-Fire]

I thought a pump on the mains would be a no no.

My mothers set up is a contentional boiler with a cold water header tank on the second floor and a hot water tank on the first floor.

The hot water tank is mildly presurised to reach the hot tap in the second floor bathroom.

The second floor shower is a 3kw flash heater connected to the incomming mains feed only.

The problem seems to be total flow capacity like the water company said

Although you will have a job to stop the water by putting your finger over the tap there is only just enough flow to run 1 garden sprinkler.

To put it in electrical terms, plenty of voltage but low current ?

[Richard]

Wheels-Of-Fire wrote:

To put it in electrical terms, plenty of voltage but low current

One company had a graphical demonstration of that, with nothing turned on the pressure was impressive, use a tap or water cistern and the pressure dropped as near as dam-it to zero, yet the flow at that ground level was still impressive.

[Ed P]

Another way to look at it is that roughly 10psi (0/.7 bar) of water pressure will be lost between the ground floor and a typical attic cold water tank. 10psi will give you a pretty good jet of water from a hose.

There are ways of boosting the cold water supply pressure, but as Richard points out there are lots of rules and regs that must be followed.

[blacklion1725]

Reading up on Jaycee’s regs table is interesting. My drop inside a stud wall has no insulation (inside the cavity) but I can’t find one of the reference type that fits that case (only insulation where the T&E is either touching the plasterboard or not). In my case it is a hollow cavity so I don’t know. Won’t know till Thursday now either.

The “direct clipping” means the surface it is clipped to will conduct heat away, whereas running down the middle of fibre glass lagging will make it hotter. Also there is length of cable run (voltage drop, which I think I can ignore as it is only around 6 yards.

Be easier to have a bath!

[Author]

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