Heating water - Gas vs Electricity

[DrFfybes]

Somone asked on here a year or so back which was the cheaper way of heating water, and I cannot find it now but there are a whole host of factor.s.. boiler efficiency, length of pipe run to the tank, unit cost, whether the heating is on anyway, etc.

Having now got the gas smart readings reappeared, I thought I'd have a stab.

From 'my' Solar and PV thread..

FWIW I have been monitoring my water heating this summer whilst the gas heating has been switched off. [...]
I consistently use 3kw hours of gas to heat the water. Sometimes once per day, sometimes twice per day. So given today's gas tariff is around 7p per unit, a tank of hot water is costing me roughly 22p.

I have a four hour off peak electricity rate that I could use to heat the water. It's about 10p a unit. I conclude it's not worth me paying an electrician to install a timer on my electric immersion heater. I'm continuing to use gas for water heating.

Our boiler comes on 7:30 - 8am and heats the water. It pretty much consistently seems to use 4Wh of gas. A couple of times it is a tad more, sometimes a bit less, and sometimes the towel radiator comes on as it is on the same circuit as the cylinder and the TRV is set to 18C.

The 20 min afternoon top-up is about 3kWh, however I've stopped that now we have Solar and use the Immersion heater in the afternoon as it has been mainly sunny.

These are summer figures, gas is about 7p per kWh, electricity 30p, however if we generate it then it costs 17p as that would be the export rate.

Measuring the electric required is not simple, as with the battery you only get net import/export, and other things are running, so what I did was set the Immersion to come on for 30 min, and watch the 5 min consumption graph. I concluded that an afternoon top-up takes 15-20 min of power, so let's say 1kWh.

I'd guestimated previously that we used 4x as much gas energy as electricity to heat the water, looks like that might be nearer 3x.

Either way, for us, with our 15 metres of pipe work from the boiler to the cylinder, it is probably marginally cheaper to heat the water with gas, than electric, unless I use the 3 hour cheap 16p slot overnight, or excess solar power.

However, it isn't that simple On the couple of days it was cloudy and I didn't do an afternoon topup, the following morning the gas was only 1kWh extra. Perhaps the initial loss heating up the boiler and pipework is most of the use, it isn't clear. Would a tank twice the size use 2x, or 1.2x as much gas?

And then there's the amount of hot water. The boiler heats using a coil in the bottom of the cylinder, the immersion element length is not know, but I bet it doesn't go all the way to the bottom and therefore doesn't heat the whole tank, additionally the thermostats for the 2 systems are at different heights.

So, what has all this observation, monitoring, measuring and thought revealed?

Bu66er all I suspect, but as I pulled my back picking rasperries, Mrs has gone to her sister for the weekend, and I have managed to lock myself out of my Govt Gateway account I need to do something to fill my day.

Paul

[BullDog]

Thanks for the feedback DrF. It's interesting.

Our hot water cylinder is almost directly above the boiler. Piping runs about 2m or less, I should think. The boiler is high on the downstairs wall. The cylinder is stood on the upstairs floor.

It seems to me that if my daily hot water gas use costs 22p some days and a maximum of 44p other days, it's not worth worrying about too much. I did fit a thick insulation jacket on top of the cylinder spray foam. It's definitely made a difference because the airing cupboard is a lot less warm than it was. Not a quantifiable difference really given my 22p a day gas bill. But certainly one of the best and easiest energy improvements I've made as it was really cheap and easy.

I do recommend a functioning hot water cylinder thermostat. It makes sure we always have a full cylinder of hot water every morning and I don't have to worry about the "off" timing of the boiler. The cylinder thermostat is surprisingly effective in that respect and very cheap.

If anyone has a hot water cylinder heated from their heating system without one, I would say fitting one should be #1 priority. Along with an additional insulation blanket.

[DrFfybes]

It seems to me that if my daily hot water gas use costs 22p some days and a maximum of 44p other days, it's not worth worrying about too much. I did fit a thick insulation jacket on top of the cylinder spray foam. It's definitely made a difference because the airing cupboard is a lot less warm than it was.
[..]
I do recommend a functioning hot water cylinder thermostat. It makes sure we always have a full cylinder of hot water every morning and I don't have to worry about the "off" timing of the boiler. The cylinder thermostat is surprisingly effective in that respect and very cheap.

If I measure the temp between the jacket and the spray insulation the tamk it is 35-40C which suggests a lot more heat retention. With the jacket we can get away with one dose of water heating per day in Summer with just enough left in the evening for washing up if we have quick showers. Not so in winter, probably due to colder water coming into the tank as much as extra heat loss.

Our tank stat for the CH had died, about 10C of 'play' in it and not very accurate. The new one is better, but the immersion has a seperate 'stat' in there somewhere.

Paul

[Mike4]

Whatever the fuel, it takes the same amount of energy to heat the same volume of water through the same temperature rise. There is no getting away from this root fact.

Gas is less than 100% efficient as there are perhaps 10-15% losses in the boiler (heat lost from the flue) and some more losses from the pipes there and back. Also electricity is needed to run the boiler and in particular, the circulator.

Leccy however as you identified, rarely heats up the same volume of water as the gas does, or to the same temperature all the way up. So a fair bit less leccy energy is likely to be used to get to the same temp at the hot tank outlet simply because less water is being heated and to a lower average temperature.

Moving onto cost, gas it WAY cheaper per kWh than electricity. The market is rigged by the government in such a way that electricity costs on average 3.5 times that of gas per kWr. Electricity tariffs get impossibly complicated compared to gas especially once exporting and surge pricing tariffs are concerned so I'd suggest broadly, people with solar installations are probably better off using the leccy to heat their water, and people without are far better off using gas.

[BullDog]

It seems to me that if my daily hot water gas use costs 22p some days and a maximum of 44p other days, it's not worth worrying about too much. I did fit a thick insulation jacket on top of the cylinder spray foam. It's definitely made a difference because the airing cupboard is a lot less warm than it was.
[..]
I do recommend a functioning hot water cylinder thermostat. It makes sure we always have a full cylinder of hot water every morning and I don't have to worry about the "off" timing of the boiler. The cylinder thermostat is surprisingly effective in that respect and very cheap.

If I measure the temp between the jacket and the spray insulation the tamk it is 35-40C which suggests a lot more heat retention. With the jacket we can get away with one dose of water heating per day in Summer with just enough left in the evening for washing up if we have quick showers. Not so in winter, probably due to colder water coming into the tank as much as extra heat loss.

Our tank stat for the CH had died, about 10C of 'play' in it and not very accurate. The new one is better, but the immersion has a seperate 'stat' in there somewhere.

Paul

Indeed, the immersion heater will have a thermostat but inside it's cover which obviously has to be removed (after switching power off!) to access it. I have in the past known the contacts to fuse together leading to the water being boiled inside the cylinder.

I was surprised that after nearly 30 years our cylinder thermostat is working so well. I was going to fit a new one when I fitted the extra insulation jacket. But when I tested it, to my surprise it worked perfectly.

[Mike4]

If I measure the temp between the jacket and the spray insulation the tamk it is 35-40C which suggests a lot more heat retention. With the jacket we can get away with one dose of water heating per day in Summer with just enough left in the evening for washing up if we have quick showers. Not so in winter, probably due to colder water coming into the tank as much as extra heat loss.

Our tank stat for the CH had died, about 10C of 'play' in it and not very accurate. The new one is better, but the immersion has a seperate 'stat' in there somewhere.

Paul

Indeed, the immersion heater will have a thermostat but inside it's cover which obviously has to be removed (after switching power off!) to access it. I have in the past known the contacts to fuse together leading to the water being boiled inside the cylinder.

I was surprised that after nearly 30 years our cylinder thermostat is working so well. I was going to fit a new one when I fitted the extra insulation jacket. But when I tested it, to my surprise it worked perfectly.

I suggest replacing it regardless, as new ones will (should!) also contain an overheat protection thermostat for when the contacts fuse together just as you describe. A regulation requiring this was introduced about 20 years ago, following the death of a small child caused by the tank in the loft being induced to boil when the thermostat contacts fused. The plastic tank then collapsed and 30 gallons of boiling water brought the ceiling down, drenching the child and killing them. A truly dreadful incident we must avoid happening again.

[88V8]

.....the thermostats for the 2 systems are at different heights.

Which reminds me.... when I installed our gfch 41 years ago, I put two stats on the HW cylinder, one a third down from the top and one a third up from the bottom.
In the kitchen there was a brass changeover switch to select one or the other, seen here below the programmer.



With two of us in the house it was sufficient to heat just the top of the tank. When a bath was wanted, one switched to the lower stat to heat the whole tank.
I have no idea how much this saved, but in these cost-concious days, as a simple means of saving money I imagine it has some merit.

V8

[Mike4]

I see you still have the same radio too!



Lovely door.

Oh, and good idea about the two stats, and a very elegant switch.

[DrFfybes]

I see you still have the same radio too!


Oh, and good idea about the two stats, and a very elegant switch.

I think the techincal term is "wireless".

As for the switch, not too long ago I saw an Economy 7 flat that had 2 immersions at different heights in the tank, the 'normal' one and the upper one for 'boost' if you just needed to wash up or whatever. The old dear living there had no idea about how they were supposed to work, so just left both on 24/7.

[yorkshirelad1]

This debate has been on my mind in the last week or so as I had someone in to look at installing an EV charging point, and he was trying to do a sales pitch on replacing my boiler and bathroom and electric shower.

I checked my most recent month's gas & electric bills (Octopus), and the overall cost per kwh for gas was higher than for electricity, all things considered. This is mainly due to the low usage of gas units in the summer and the daily standing charge. It's a different matter in the winter when gas usage goes up and the standing charge is defrayed across more gas units consumed.

Just shows how many factors you have to throw into the mix and there's no simple answer.

And I quite like having the facility of having an electric shower if the gas (water) heating goes on the blink, or vice versa.

[DrFfybes]

This debate has been on my mind in the last week or so as I had someone in to look at installing an EV charging point, and he was trying to do a sales pitch on replacing my boiler and bathroom and electric shower.

I checked my most recent month's gas & electric bills (Octopus), and the overall cost per kwh for gas was higher than for electricity, all things considered. This is mainly due to the low usage of gas units in the summer and the daily standing charge.

Unless you are considering disconnecting the gas this is a spurious thought process. The standing charge is there whether you use any fuel or not, like paying council tax but not needing social care or calling the police or setting your house alight or filling your bins.

There used to be tariffs where you paid more per unit for the first portion of your gas, which effectively covered the standing charge spread over a few hundred units, but Martin Lewis amongst other campaigned against them as penalising low gas users to subsidise high users so they stopped them. Which cost low users a fortune in summer and left them no better off in winter.

[ayshfm1]

Assuming a reasonable boiler I think gas is cheaper.

The energy input unit is the same kwh, gas is circa 7p and electricity circa 30p.

Electricity is 100% efficient so what you pump in all turns into water heating, gas less, which where the boiler comes in todays boilers are 90% efficient so it takes circa 10% more kwh to achieve the same effect, but is a not going to be enough to make the gas water heating more expensive.

So gas is cheaper if you are paying for the electricity.

What happens if you are not paying. It then depends on your export tariff.

Then you have to compare what you would be paid for the electricity and you'd get 15p from Octopus, so it's better to export. That would get to look even better with agile I suspect. Even on a poor export tariff (less than 7p) spending money on something expensive like an eddi looks to have a very long payback tending towards never,

Currently I'm not getting paid for my export, but decided even so the cost of fitting an eddi did not add up, in the interim I bolted in SONOFF POW Elite 20A Smart Power Meter Switch £25 on Amazon. I don't use it anymore, there are a number of problems specific with such a solution.

1) My immersion is in the top half of the tank so it only heats that
2) It's all or nothing, the immersion draws 3.5kw when on, which is a lot, I need good sunshine to generate that much and if the generation dips it draws from the grid and the advantage is lost.
3) The thermostat in my immersion seems unreliable and it kept heating when it should have turned off - this is dangerous!

To be fair I could mitigate some of this using home assistant, I have wifi thermostats on my tank and my generation is captured in real time so I can use triggers to turn the thermostat on and off according to these input. In the end I concluded the small savings were not worth it.

I am waiting on my battery, but when I get it and am getting paid, I suspect I will get Octopus Agile, charge the battery to the max on the cheaper rate and export as much as I can during the solar day, I'll need to do the math.

Some unrelated rambling.

The outgoing tariffs change the game. I'm wondering if the battery is worth it (financially). Use what you can and be paid 15p per kwh for everything you can't. Given how expensive battery storage is I suspect this is the most financially sensible way to go.

However having watched my solar generation I'm also wondering if a non-battery install would do more harm than good. If the sun comes out it jumps to 5kw, if a cloud passes over it, it drops (a lot, less than 1Kwh sometimes), it starts producing a little when the sun comes up and gets into it's stride at about 11:30, then drops off to nothing as the sun sets. Multiply this by all the solar installations out there and it looks to me like the grid will have to an inordinate of real time juggling. Even with a battery I suspect my charge to the max and the export everything plan will be only slightly less anti-social than the current very variable export profile.

[DrFfybes]

Assuming a reasonable boiler I think gas is cheaper.

The energy input unit is the same kwh, gas is circa 7p and electricity circa 30p.

Electricity is 100% efficient so what you pump in all turns into water heating, gas less, which where the boiler comes in todays boilers are 90% efficient so it takes circa 10% more kwh to achieve the same effect, but is a not going to be enough to make the gas water heating more expensive.

So gas is cheaper if you are paying for the electricity.

Kind of. The 90% figure for the boiler is probably not accurate, the flow temp will need to be quite high to get the water hot, and with a small coil the return will also be quite high, so even though the boiler modulates, it probably doesn't run in condensing mode properly. This means a lower efficiency. Couple this with heat loss in the boiler and pipework itself and the efficiency drops further.

My 30 min gas morning heat up used an average of 0.43m3 of gas over the last 5 days, which is 5kWh.

From my PV generation/consumption graph, the immersion takes under 30 min before it cuts off for a similar top up. So about 1kWh. But for probably less water.

Paul

[DrFfybes]

Over the last few nights I've set the immersion timer to come on for 30 min at 4am - using the Octopus cheaper leccy. However I forgot to turn the hot water timer off on the boiler controller so it still came on for 30 min before we get up.

To my surprise, gas use in the 30 min 'hot water' period was pretty much unchanged, in fact it was slightly higher than some mornings.

Now I'm even more confused.

Paul

[Mike4]

Over the last few nights I've set the immersion timer to come on for 30 min at 4am - using the Octopus cheaper leccy. However I forgot to turn the hot water timer off on the boiler controller so it still came on for 30 min before we get up.

To my surprise, gas use in the 30 min 'hot water' period was pretty much unchanged, in fact it was slightly higher than some mornings.

Now I'm even more confused.

Paul

Probably heating the lower half of your tank, unlike the immersion heater.