Heat water with gas or electricity...

TerFar wrote: »

...If electricity is more than double the cost of your gas, then gas is cheaper...

...so it was a no brainer to switch to gas.

But it is more complicated than that. The efficiency of the boiler is only one factor. Another, perhaps more important one, is the way in which you use hot water.

Most of my hot water is used early in the morning, sometimes during E7 hours. I then use small amounts of hot water regularly throughout the day. If I had a combi and the boiler fired up every time I turned on a hot tap (and then ran until the end of its cycle) I would probably consume a lot more energy than I do to store my hot water in a well-insulated tank located very close to the taps I use most frequently. Even the decision whether to have the combi boiler in 'comfort' or 'economy' mode is not that simple.

Exactly how much more or less energy I would use is difficult to determine, and would require monitoring of my energy consumption (taking care to exclude other consumption such as lighting and space heating) in order to find out exactly which is best.

Then my neighbour's use of hot water is entirely different to mine.

So altogether a long way from being a 'no brainer'.

SuiDreams wrote: »

I believe the OP is discussing heating Hot water Tank with either Gas boiler or immersion heater, not use of Combi boiler compared to immersion heater.

Yes, I got that, but the same issues apply. Will you need to heat your hot water tank at the same times you need space heating, where is the boiler compared to the HWC, do you use space heating all year round, what levels of insulation do you have?

I was using my circumstances as an example, but each of us is different and it cannot be reduced down to a 'no brainer'.

SuiDreams wrote: »

Tank Size, Tank Insulation and required timings would be the same for using the Immersion or Gas boiler so would not make a difference.

Not necessarily. Tank size and insulation have an impact on the length of time water heated using E7 will remain hot and in sufficient quantities for use towards the end of the day. Gas then has the advantage that a boost will be at the same cost per unit regardless of time of day.

Likewise, the time of day hot water is required also has an impact. Someone consuming most of their hot water in the evening is likely to have a more efficient (overall) system with gas, whereas early morning use might favour electric.

Insulation also is important on the circulation pipework between boiler and tank, but not in all circumstances.

SuiDreams wrote: »

Not sure if the space heating is really and issue as boilers can be set to do either separately or both together. I believe it wouldn't make a significant difference.

It may be a factor - depending again on circumstances. If there is no need for space heating then the boiler has to be fired just for hot water and needs to heat itself and the HW part of the circulation system before the hot water will be heated. If space heating is needed as well (at around the same time), then this initial energy consumption is spread between both needs, and some heat loss from the circulatory system (depending on insulation) will contribute to meeting the space heating need.

So the overall system efficiency (not just the boiler's) will depend on the use to which the system is put, as well as the specifics of how the system is constructed and operated.

For example, heating just enough water to do the washing up after dinner using a gas boiler and hot water cylinder will be highly inefficient compared to boiling a kettle, or a short boost from a top of tank immersion.

Conversely, heating a bathfull of water while the central heating is also running would be a far better option than running an immersion during non-E7 hours.

So whilst the OP may be correct about their circumstances, to offer the following as general advice is potentially misleading:

So check the cost of your electricity and gas (in kWh). If electricity is more than double the cost of your gas, then gas is cheaper.