Immersion Heater (Gas or Electric) or Electric Shower

chinamonkey

Hello,
We are redoing our bathroom and dont have a combi boiler. Currently we heat our immersion heater up prior to showers/baths etc. We can do this via our GCH or via Electricity.
We are considering an electric shower to save on the time/energy of continually switching our immersion on. It takes 10-15 minutes to heat the tank up.

WE want to know really what is the cheapest most efficient way of getting our hot water?

Is leaving it on permanently cheaper? it is well insulated and after being on we will have very warm water for the next day or so..(not hot enough to shower in though...)
Or would it be better to get an electric shower (my concern is that we wouldnt have warm water in the taps for face washes/hand washes etc..)

PS not experts in hot water systems but does just having the heating on for the house heat any water in the water tank?

Thanks for any help you can give!

ChumpusRex

Using your gas boiler to heat the water tank is the most cost effective way of doing this.

A well insulated tank will "leak" about 10-15p worth of heat per day into the house. In Winter, this will slightly offset the cost of heating the house.

You can slightly reduce the "leakage" by setting the Hot water timer on the boiler to only heat the tank at times when you want a shower - so, first thing in the morning and when you get home from work. In between times, the tank will stay warm, but won't be recharged.

You can also use the electric immersion, but this will triple the cost of heating, compared to gas.

Same with electric showers - you'll be paying 3x as much to heat the water. Electric showers are also quite feeble, especially in Winter, as unless you go for a 10 kW one, they just don't have enough heating power.

You save nothing in complexity, becauase you still have to keep your existing hot water system for bath, sinks, etc. I can think of no good reason to install an electric shower if you have a gas boiler.

stator

An electric shower would be more efficient than an immersion heater, I'd get one of those fitted. Leave your other taps connected to the immersion heater, set a timer so it only comes on in the time before you usually need hot water. (or if you have economy 7 turn it on only at night)
Gas heated water is always cheaper, but the cost of installing a combi boiler is probably prohibitive unless you are replacing it anyway.

Robwiz

The answers to you most of your questions could begin with 'It depends..' because there are compromises in all the options.

For example, if your boiler flow temperature isn't set high enough compared to your hot cylinder thermostat then you will use more gas because the stat will continually call for heat which the boiler can't deliver.

Also, if you have (or plan to install) solar PV panels, in summer you can divert your excess generation to heating your hot water which reduces your energy bills further.

It's generally advised that we should heat our hot water to 60º C to prevent Legionella bacteria growing. To get the water up to 60º C in the cylinder, the flow temperature has to be 70º C. But if the heating system's return temperature back at the boiler is above 55º C then the boiler won't operate in condensing mode, so will be less efficient than advertised.

If you heat your water using an immersion heater on Economy 7 night rate, the cost is probably comparable with gas (as the electric element is 100% efficient at 5.5 p per unit compared to the gas boiler operating at 80% at 4.5 p per unit). You would need to have it wired through a timer switch to take advantage of off peak electricity.

My suggestion is to try the electricity vs gas for yourself, taking both meter readings each day for a few days to see how they compare.

I did an experiment last year running the gas boiler to heat the hot water cylinder and compared it with the immersion heater and I was horrified how much gas we used. The explanation was that the heating coil from the gas boiler heats the whole tank whereas the immersion element only heats the top two thirds (which was enough for our daily needs), so we were wasting a lot of gas heating the bottom third.

It won't be cheaper leaving your immersion heater on all the time. That's because even a well-insulated cylinder loses heat to its surroundings. The losses are proportional to the temperature difference so it will use more electricity keeping your cylinder at your set temperature than if you allow it to cool a few degrees then heat it up once a day before you need the hot water.

Nebulous2

Robwiz wrote: »

It's generally advised that we should heat our hot water to 60º C to prevent Legionella bacteria growing. To get the water up to 60º C in the cylinder, the flow temperature has to be 70º C. But if the heating system's return temperature back at the boiler is above 55º C then the boiler won't operate in condensing mode, so will be less efficient than advertised.

How would I know the return temperature, or if it is working in condensing mode?

I have a new boiler fitted by British Gas and the flow temperature is set at 70C. This is the first condensing boiler we have had. Hot water is currently timed for a couple of hours in the morning and slightly less at night. Heating is on all the time.

Robwiz

Nebulous2 wrote: »

How would I know the return temperature, or if it is working in condensing mode?

I have a new boiler fitted by British Gas and the flow temperature is set at 70C. This is the first condensing boiler we have had. Hot water is currently timed for a couple of hours in the morning and slightly less at night. Heating is on all the time.

You'd have to measure the return temperature using a thermometer of some sort. Lidl sells an infra red probe thermometer in its DIY specials weeks. If the return temperature is above 55º then your boiler won't be condensing efficiently (i.e. extracting extra heat from the flue exhaust gas).

Cardew

Nebulous2 wrote: »

How would I know the return temperature, or if it is working in condensing mode?

Good question - I have raised this issue several times and I don't believe there is a satisfactory 'one size fits all' answer.


http://en.wikipedia.org/wiki/Condensing_boiler


Obviously the principle is to have the boiler water temperature as low as possible commensurate with providing adequate heating in the property.


However the heating load on the boiler can vary tremendously; from, say,(1) just a heated towel rail - with all other radiators switched off by their TRV, (2)to every radiator in the house 'demanding' heat(i.e. TRV opened)


Even though a boiler can modulate down, a typical minimum output is 8kW which is far too high for case (1) and the water will return to the boiler at almost the same temperature as it left!


I have also found it impossible to quantify the difference between a boiler in condensing mode and the same boiler in non-condensing mode. However there seems to be a consensus of opinion that the 90% efficiency figures claimed for modern boilers are simply not achievable outside of a laboratory; rather like claims that a V8 engined car will achieve 60mpg at 30mph!