Showers Running Costs - Electric vs Mixer

[SadieLovatt]

Hi,

We currently have an 8.5Kw shower in our bathroom and with my 'cost reducing' hat on (note not my 'energy saving' hat), I would like to know whether I would gain much by changing to a mixer shower that would run off our new combi (24Kw Vaillant).

I'm assuming that the flow rate of the combi driven system would be far higher than the electric shower but am confused as to whether I'd actually save any money in changing and would ask for someone to correct me if I'm wrong on the following: -

If a shower is on for an hour (as an example) then the electric shower would have used up 8.5KWh and using a typical rate of 11.5p/KWh would cost 98p. A combi system however, would have used up 24KWh and using a typical rate of 4.25p/KWh would result in a cost of 102p.

Is this right as I always thought that electric showers were supposed to cost more to run?

Sadie.

 

[mogget]

If you're using the full 24kw that your combi can provide, you'll have much more water coming out of the shower head than with the electric - nearly three times as much, which would make for a very nice shower indeed!

If you were to somehow restrict the flow from the combi, with a low-flow shower head for example, the boiler would modulate its heat output down to compensate. In this instance less than 24kw would pass into the water, with a corresponding reduction in the amount of gas used.

 

[SadieLovatt]

If you were to somehow restrict the flow from the combi, with a low-flow shower head for example, the boiler would modulate its heat output down to compensate.

To be honest I did think of this but don't boilers only modulate in CH mode? I do recall Worcester-Bosch made a big deal about their CDi range being boilers that modulates in DHW mode (as well as CH mode) and that this set them apart from other boiler manufacturers.

Sadie.

 

[mogget]

Every combi boiler I've ever seen modulates in hot water mode - a new Vaillant most certainly will.

Not all boilers will condense in hot water mode though, but since the hot water is a relatively small demand compared to the heating, it's not something I'd worry about.

 

[Tipper]

For a given water temperature rise and set flow rate a combi, or indeed any locally mounted boiler, will be cheaper than electric.

This is because the heat you need is the same but the electricity has been generated somewhere else and then used to heat your water. Electricuty in the UK is generated at an efficiency of somewhere around 40% (varies depending on what fuel is being used) whereas a new condensing boiler is supposed to be 90+%. Therefore cheaper to heat the water locally by gas. (of course this assumes similar gas prices.)

 

[muggles]

Depends on what you're looking at from a cost point of view - bear in mind that if you're buying a new mixer valve, you'll need to also pay for pipework alterations (or at least the materials to do it if you're intending to do it yourself). Electric showers are more expensive to run but it's going to take you a loooooooooong time to recoup the cost of making the switch. Remember also that mixer valves tend to be mounted lower down the wall than electric showers, so you'll either have to fill in the old pipework hole with grout, or re-tile the wall. Mind you, if it's a solid wall you'll have to rip the tiles off anyway to bury the pipe in for the new mixer.

Mira Combiforce is one of the best out there for combi boilers http://www.taps4less.com/PP/MIRA-FORCEV1C.html?ad=GoogleBase

 

[SadieLovatt]

Ok, now I am really confused.

I've just received information from Gainsborough showers who claim that our shower has a flow rate of 8l/minute, a figure that is not far off our Vaillant boiler (ecoTEC 824) which is 9.4l/minute.

So it's 8.5Kw of electricity to produce 8l/min vs 24Kw of gas to produce 9.5l/minute. With the difference in flow rates being quite small then the gas option wouldn't have much scope of having its flow rate restricted to be the same as the electric equivalent and the running costs still would show very little difference betwen the two.

...or am I still missing something?

Sadie.

 

[mogget]

...or am I still missing something?

Yes, and it's quite subtle.

The electric shower will be quoting the flow rate for water delivered from the shower head.

The flow rate for the boiler will be quoted for hot water only, which is then mixed in the shower valve with cold water, which brings it down to the required temperature.

Let's say, for the sake of this example, that for your required shower temperature, with a mixer valve, you need 50% hot water and 50% cold.

So, for an 8-litre-per-minute shower, this would be, for an electric shower, 8 litres per minute flow.

The same 8-litre-per-minute shower with a mixer valve (in this example) would need 4 litres per minute of hot, and 4 litres per minute cold.

Since the cold doesn't need to be heated, your boiler would be supplying (and you would be paying the gas bill for) 4 litres per minute.

With the electric shower (in this example) you would be heating (and paying the electric bill for) 8 litres per minute.