Heat up hot water by Gas or Electricity?

markin

The immersion could be set to 80c so use more, check the temperature of the water.

armith

On a slightly related theme - is it better/cheaper/worse/horribly-expensive to boil a kettle using hot water or cold? It takes about 3 minutes to boil the kettle from the cold tap - but only a minute to boil it from the hot, with the water having run from the tap for about 30 seconds. This is with a combi boiler - I wouldn't do it from an immersion heater tank.

macman

Using hot water, because the bulk of the heating is done by (cheaper) gas. However, you will be using more water, if metered, so factor that in. 
Given that the cost of boiling a 3kW kettle for 3 mins is about 5p, the saving is going to be trivial, probably less than 2p.

[Deleted User]

matt_drummer said:

Mstty said:

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

It's difficult to know but I would say that 98% efficiency is extremely optimistic.

I used 90% when I estimated my heat loss.

I agree based on 12 months of heating by electric compared to gas. I could reheat a 210L unvented cylinder with about 6kWh of gas. Electricity only required 3kWh. 

Boilers tend not to condense when just heating a HW cylinder so efficiency is unlikely to be in the high 90%. Secondly, heat is pulled from the cylinder as the coil heats up. Thirdly, unless the pipes to/from the boiler to the cylinder are lagged then the copper pipes act as mini radiators.

Raxiel

As well as the immersion being more efficient as others have said, all other things being equal its greener too, if you care about such things.

Grid electricity will always have a proportion of low carbon energy even on a still night, but it's frequently 50% or more.

Unless someone comes up with a way of synthesising 'green gas' using renewable energy, or at least blending it in, the boiler will always be 100% fossil fuel of course.

armith said:

On a slightly related theme - is it better/cheaper/worse/horribly-expensive to boil a kettle using hot water or cold? It takes about 3 minutes to boil the kettle from the cold tap - but only a minute to boil it from the hot, with the water having run from the tap for about 30 seconds. This is with a combi boiler - I wouldn't do it from an immersion heater tank.

If the water in the pipe is already hot from doing something else like running a basin, filling the kettle with hot and then boiling it may well be cheaper, but running it from cold means pouring away some water, then heating up the boilers heat exchanger, the pipe work, and the water that remains in the pipe between the boiler and the tap after you turn it off and is unlikely to be cheaper in terms of raw cost.

Of course, if you put a price on your time too, you might consider saving a couple of minutes to be worth it, since heating a cups worth of water from 4-5°C to 100°C will cost a trivial amount whether you split the job at ~50°c between gas and electric or not.

Belenus

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Not necessarily. It depends on how the electricity was generated.

It is a complex subject.

If the electricity is generated by wind turbines or hydro systems then maybe it is greener.

However, a lot of UK electricity is generated using natural gas so (ignoring differences in efficiency and transmission losses) using that electricity to heat domestic water is no greener than using natural gas in a domestic boiler to heat the water.

If it is generated using coal or oil then it is perhaps less green.

born_again

armith said:

On a slightly related theme - is it better/cheaper/worse/horribly-expensive to boil a kettle using hot water or cold? It takes about 3 minutes to boil the kettle from the cold tap - but only a minute to boil it from the hot, with the water having run from the tap for about 30 seconds. This is with a combi boiler - I wouldn't do it from an immersion heater tank.

How long does it take to get hot water to the tap to make any difference.
In our kitchen, the tap which is about 3 meter's worth of piping from boiler, will have filled the kettle with cold water before any hot gets through. 
So in effect all you would do is heat water that is just going to cool down again in the pipe. (waste of gas) Unless you are going to wait till hot comes through. But then if you have a water meter. You have to factor in the increase of water usage. 🤷‍♀️

Belenus

Alnat1 said:

You could always try heating with gas for only 90 minutes and see if you have enough hot water. That's one way to save a bit.

It is likely that the OP's 30kW gas boiler is running for less than 90 minutes anyway as the thermostat will cut off the water heating once it reaches the set temperature.

Our 26kw gas boiler heats up a 150 litre water tank from about 35C to about 60C in under 30 minutes.

I don't know the capacity of the OPs Megaflo but let us assume that it is 150 litres. If anyone feels like doing the maths on calculating the energy required to heat 150 litres of water from 35C to 60C I am sure it will be a lot less than the 45 kws used in 90 minutes by a 30kw boiler.

I would guess less than 15 kws would be required but I may be wrong.

I have made some assumptions and the calculation will vary depending on the size of the tank, the high temperature required and the temperature of the incoming water etc.

Tgrewal

thanks everyone for all your comments. I think some of you have already mentioned this but even though I have my boiler programmed to heat up the hot water for 2 hours (in fact, I've just checked and it's programmed for 1.5 hours Monday - Friday), the boiler heats up the water pretty quickly so the boiler is not buring gas continuously for that period.

Ignoring the 'keeping greener' piece which I know is important, I think from a pure cost point of view, keeping to gas is probably best

Raxiel

macman said:

Using hot water, because the bulk of the heating is done by (cheaper) gas. However, you will be using more water, if metered, so factor that in. 
Given that the cost of boiling a 3kW kettle for 3 mins is about 5p, the saving is going to be trivial, probably less than 2p.

This thread has piqued my curiosity enough that I've finally decided to figure out exactly what it takes to run a kettle.

1 Cup of water is about 0.25 litres, and it takes 250 calories to raise that much water by 1°C

The temperature of incoming water varies throughout the year, but lets give it a pretty conservative lower bound of 4°C, so that's ▲96°C to raise it to boiling (in reality the bulk of the water in a kettle is only close to boiling but we'll ignore that for this exercise).

So thats 250 x 96 = 24,000 Calories, or 24kCal (not the food type)

24kCal = 100,416 Joules or 100,416 Watt Seconds, 28 Watt Hours or 0.028kWh

At the most expensive April 23 EPG single rate of 33.11p/kWh that means a perfectly insulated kettle would cost 0.9p to boil one cup of very cold water.

A typical cheap plastic jug kettle isn't that well insulated, but I doubt the losses in the time it takes to boil (a 3kW jug should be about 40s per cup) would extend the heating time to push it over 1p/cup.