Electric Shower vs Mains Fed

If fitting a new shower, I always opt for an electric shower of at least 9.5kw, as anything below this does not necessarily give sufficient hot water in winter (when the input water temperature is lower). Unfortunately, many people install showers of a lower power rating on a 6mm cable, but a 9.5kw shower has to have 10mm cable, so it is sometimes necessary do do some re-wiring of the shower circuit. In all circumstances you should use a qualified electrician (even if you could do the job better yourself) otherwise you are breaking the law and possibly invalidating your buildings insurance.

Having both an electric shower and boiler fed shower makes it easier for couples to shower together - and the indulgent singleton may also use both at once. This brings more wear and tear overall, but happier tenants....

I am in the process of buying a place with a 15 year old ordinary (non-combi) boiler (it has a hot water storage tank). There is no shower. I think I could use a shower with this boiler if I installed a water pump. How would this compare with a shower from a combi boiler?

Thanks

Hi there
You need to do some research on the internet regarding installation options.
Most electric showers have to be fed from the mains to get enough pressure.
However I faintly real that at least one electric shower has a pump built into it and can be fed from a cold water tank under gravity.
I suspect that if you take the same feed from a hot water tank which has pressure derived from a cold water tank above it, there would be a danger of the water being too hot for a shower. On the one hand the water in the boilers hot water tank would have to be carefully restricted to be beneath the temperature required from the shower. On the other hand if it were above this temperature there would be no means of cooling it down by adding cold water.
In my humble opinion your commendable thinking out of the box, is making the whole installation too complicated and potentially putting the shower user at risk.
I would have two separate showers. This would give resilience. It would thus also avoid the problem that I had last year when I rang seventeen plumbers before I could get one to come out to fix the boiler. The tale is painful but I lost over £350 and he failed to fix the boiler.

Hi Paul

Thanks for your reply. I don't understand why you say 'On the other hand if it were above this temperature there would be no means of cooling it down by adding cold water.' So I assuem I would connect the hot and cold to a standard thermostatic shower mixer. Would it not be the case that either a) the cold water would have enough pressure when coming from the header tank in the loft or b) I could also pump it (pumps commonly can serve more than one pipe) or c) I could run it off the mains.

I will have a place with a standard boiler and so need to understand how to fit a shower with it, regardless of whether I also fit an electric shower.

Cheers

Types of shower for domestic purposes:
You have 4 options:

Electric:

Fed from mains pressure cold water. But obviously requires an electric cable to the shower complete with the correct size cable and adequate electric circuit protection – RCB etc.

From a combi boiler:

This would also be from mains pressure – mains pressure hot and mains pressure cold water pipes to a thermostatic mixing shower.

From a storage tank (gravity fed with a header tank):

This can get complicated as there are numerous options to overcome this situation, but the key thing to remember is to have equal pressure to your thermostatic shower mixer valve from the hot and the cold water, and also non return valves if you have mains pressure cold and tank fed hot. There are hundreds of booster pumps on the market so it’s best to seek advice from an experienced engineer to decide what’s best as the combinations of hot water storage and header tanks can be endless.
Usually I would fit a twin hot and cold booster pump on the hot water pipe flowing from the cylinder and the cold water supply from the header tank. Again this could be different for every situation.

From a storage tank (mains pressure fed with an unvented kit):

This is similar to the combi boiler set up as you would just run a hot and cold to the thermostatic shower mixer and the shower would be mains pressure fed both hot and cold.
There is little need for a booster pump on this set up unless your water pressure is low.

On the applications where you use a thermostatic mixing valve, don’t worry about the storage temperatures in the hot water cylinder as the thermostatic valve will control this to an acceptable level for the shower user. And they usually fail safe, if the valve breaks or water pressure drops it will only put cold water through it so no risk of scalding.

Key things to decide what’s best to do in all shower situations:

What is the water pressure nearest the outlets where you want to put the shower?

What is the flow rate? A lot of things can affect flow rate like the size of pipes, number of bends and elbows etc?

The allowance of diversity – if there is more than 1 shower installed in the property what will happen when all of them run at once.

Any situation can be overcome usually very simply with either a whole house booster pump or a local one to the shower or whatever you want.

Hope this helps!!
Joe

An electric shower is ok it can get you about 5 litres per minute of flow rate which is ok however most people would be a lot happier with 10 liters per minute flow rate which would mean a 20KW shower but anything above 11KW is quite rare

However thinking this though why don't electric showers have mini water tanks. A 5 litre tank pre heated to allow 10 litres per minute flow rate for about 6 minutes. The user would need to hit a pre heat button and wait 3 mins before starting the shower but it solves the problem nice and easy.

Should be easy and simple to do it would not be much more complicated or costly than a kettle

Hello Robert,
You can install a 9.5kw electric shower on a 6mm cable and pass all regs, max length of cable run though is about 23 metres due to volt drop. Don't let the sparky's tell you it can't be done, phone the NICEIC and they will confirm it. If you have a 10mm cable then go for a 10.5Kw shower unit.
Always put both electric shower unit and thermostatic mixer valve off the boiler in a rental property as guarantee a boiler will break down. Tenants don't mind having no heating for a couple of days if boiler breaks but they get mad when they can't wash. Always keep a couple of PAT tested fan heaters for boiler breakdowns.
Ex electrician and design engineer.

Reply to the comment left by "Jim S" at "25/09/2015 - 23:02":

Hi Jim S

Three different unconnected electricians (all NICEIC accredited) within the past 3 years have told me that a 9.5kw shower has to have 10mm cable going to it. I'm sure they can't all be trying to scam me into paying extra for wiring I don't need, so they must have got this idea from somewhere in their training or in the regulations?

Reply to the comment left by "Robert Mellors" at "26/09/2015 - 10:57":

Hello Robert,
I hear what your saying but they are all wrong, it's difficult to get a design engineer electrician. You have to really work hard to find the very qualified electrician. Probably 95% of electricians will give you that answer as they do not know how to do design calculations, providing you have miniature circuit breakers for your protection ie commonly called trip switches then there will not be a problem with disconnection times, or current carrying capacity for a 6mm cable. The problem is volt drop and this is not an issue if the length of cable run is less than about 23 metres. I can't remember the exact length of run and I'm not about to do the calculation.
Phone up the NICEIC and ask for their technical helpline, they don't like speaking to you because you are not a registered electrician with them but if you explain then they usually help you anyway. The problem is that sometimes even the electrical inspectors at the NICEIC will not have the knowledge to answer correctly. I think they should be able to get that one though, as it is fairly easy. The telephone number used to be 08700 130 391 but I've not rang them for a few years.
Let me know how you get on and if you still get a duff answer I'll look into it.