Conflicting info re electric shower - please help!

[Happydaze2]

I’ve been told by 2 electricians that if I replace my existing 10.5kw shower with a 7.5kw the “power” of the spray won’t be affected, it’ll just take a bit longer for the water to heat up to the desired temperature. But my own research seems to suggest otherwise - that the spray would be weaker. We’re thinking about replacing it cos we’ve discovered the existing cable can’t support 10.5kw which is why it’s tripping sometimes - and replacing the cable is a big, expensive job. But having a strong jet/spray while showering is important to us. Can anyone advise, please?

How is the water pushed out of the shower head? What relation (if any) does that mechanism have to the amount of energy used by the heating element in the shower? The shower head might - just as example - be powered by the pressure of the incoming water, and not need any electricity to make it work. Or it might rely on a high-power-requirement pump.

I honestly don't know the answer, but those are the sort of questions I'd want to be asking your electricians.

There are apparently shower heads (that I have only seen in ads) that can give a better flow. Might that be an option if you go with the 7.5?

Do you have to go as low as 7.5kw? Think ours is maybe 9kwh and we are at the borderline of what our cable will support.

When moving from a 7.5kW in one house to a 10.5kW in another, I honestly didn’t notice any real difference (neither had a separate shower pump).

Firstly, why is replacing the cable going to be so expensive? That would be my first port of call before replacing a shower I liked with an inferior, less powerful one.

I think what you've found from your research is right. I assume you're talking about a conventional electric shower here - one which doesn't rely on a separate hot water feed. It takes cold mains water in, and heats the water as it goes.

With this type of shower, the flow rate is dependent on the difference in temperature between the cold water coming in from the mains, and the heated water coming out of the shower head. The less powerful the shower, the more the flow rate will be restricted as you turn up the heat - as the less powerful heating element cannot heat the water up fast enough to maintain the flow.

The problem with a lower wattage shower is that it may (and I stress "may") not be able to maintain the temperature you prefer at the flow rate you want. You might have to sacrifice one to get the other. And really, because it's your personal preference, the only way you're going to know that is to try it.

Also, you may find it has good flow rate at your preferred temperature in the summer, but not in winter, when the mains water is colder to begin with (and also when you're most likely to appreciate a hotter shower!)

Changing things like the shower head or hose is not really going to help much with flow because that's not the limiting factor with this kind of shower.

If you want the strongest flow rate possible at high heat then you need the most powerful electric shower you can reasonably get.

You could potentially fit a completely different type of shower - e.g. a power shower, or a thermostatic bar mixer shower - which takes a hot water feed as well as cold, but I can't see how that wouldn't be more expensive (in the case of the power shower, vastly more so) than a new cable for your electric shower. And it may not even be feasible depending on the existing plumbing and hot water system in your home.

@MaybeSmaller
Cost wise, could be the distance from the electric shower to the consumer unit is expensive for the upgraded cable, then maybe the consumer unit is really old (plastic) and has been recommended for replacement and the circuits upgraded to latest standards. Finally the existing shower cable might be buried in walls and plaster with no easy alternative route for the upgraded cable.

The electricians sound like idiots, sorry. It's basic physics. An electric shower heats water on demand. If the shower is twice as powerful then it can heat twice as much water to a particular temperature every second, and twice as much water at that temperature can come out of the shower head.

Yes, there are shower heads you can use which can make a lower flow rate "feel" more powerful. But if you want more actual water flowing at the same temperature then you need a more powerful heater.

If you want a shower with copious hot water, a hot water cylinder will be better than an electric shower. Either unvented or pumped.

Next best is a combi boiler.

If cost is of interest, bear in mind that energy from electricity costs four or five times as much as energy from gas.

@MaybeSmaller yes - we currently have a 6mm cable but to replace with a 10mm is a big upheaval at a cost of £885 + vat and although a route has been found to do that (drilling through exterior wall at opposite side of the house to the shower and running it the length of the loft then dropping it down) we can’t afford to do that as well as replace the consumer unit - which is old and needs updating. Replacing the CU we’ve been quoted by 3 different electricians will cost us £750 + vat.
@PigletJohn it’s a sort of swings and roundabouts situation, with costs and amount of work involved in switching even more daunting!

How much did they quote you to fit the 7.5kW shower ?

If it is a low price, maybe less than £500 perhaps you could just change it and see whether or not it works, and if it doesn't then go for the upgraded power ?

All about whether it is worth spending say £500 on the off chance it may save you spending £2K. You might be able to get a discount if you use the same electrician as well.

www.plumbworld.co.uk/electric-showers-explained-infopage

I have no idea whether the above is correct, but it looks like you could get away with 9kW on the 6mm cable (?)

@GasPanic we can’t afford to do everything at once but weighing things up think it’s probably a better investment to replace the consumer unit then as and when the shower gives up the ghost replace it with a lower kw one. We haven’t asked for quotes for replacing the shower but think you’re right- it is likely to be the cheaper option.

If you lower the temperature enough, you can get the same flow!
It is noticeable on our (10.5kW) electric shower if I go in after DS and turn the temperature up, the flow drops.

@Happydaze2 Fair points and I hadn't considered some of the work that might be involved.
One thing, are you sure the CU definitely needs replacement (as opposed to just "recommended")?
@tanstaafl I didn't think the CU being a plastic one is reason enough in itself for replacement (mine is plastic and was installed in about 2013, so not old) but if it is really old and lacks basic safety features such as RCDs or RCBOs, then it could do.