Green Gas or Green Electric Eco7 best for heating hot water cylinder?

  • 25 September 2019
  • 10 replies

I'm in the process of switching from a gas combi boiler to an unvented hot water cylinder for our DHW (domestic hot water) needs. I'm an OVO customer using both Green Gas and Green Electricity (Economy 7), and I'd like to minimise the environmental impact of my energy use.

Unvented cylinders come in two flavours - direct, which are heated directly by heating elements, powered by mains electricity; and indirect, which are heated via hot water coming from a gas-powered boiler. (There are other options, but these are the two that I"m looking at).

On the face of it, a direct cylinder should be better for the environment than an indirect one, because electricity comes from nice things like solar panels and wind turbines, but gas is a fossil fuel and evil. But:

  • Green Gas uses (some) biogas, so isn't all evil
  • If using electricity, I'd be heating the water between midnight and 6am, so presumably none of that electricity is going to come from solar panels
  • Gas is allegedly "more efficient" but that seems to be more about £ per Kw than anything else.
  • Electricity production isn't perfect, with greenhouse gases used in high voltage equipment, etc.
(for what it's worth, whichever option we go for, I'm planning on adding solar thermal panels, to minimise the amount of gas/electricity that we do draw on).

In general, it seems hard to find a direct comparison between the two options, so I'd welcome any thoughts/facts that people can add.

At the moment I'm still erring towards using electricity to heat the water, but I'd welcome thoughts on which is "better" environmentally. I know that electricity is going to push up my costs a bit, from something like £30 pcm to £45pcm but I'm prepared to pay for something that's better for the planet... if it is better!


10 replies

Userlevel 7
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Excellent question, @dante.

So - two initial observations on what you've just written.

A. If your unvented cylinder is mainly for DHW, then it can only be installed and worked on by a qualified heating engineer (Building Regs; Part-L). If it's mainly for "space heating", then you can install and modify it yourself.

I don't understand why there is this distinction, but it was pointed out to me by a qualified Building Control Surveyor who was persuading me to install a thermal store.

B. If you're considering adding solar thermal panels then you'll need to have an indirect cylinder. The liquid in the loop that runs to/fro the external panel will require between 20-40% glycol to act as a freeze-inhibitor, so this requires a separate heating-coil at the base of your cylinder.

Here's a photo of my 300-litre thermal store as it's currently configured:

Because the "return" to my gas System-boiler is at the bottom of the cylinder, it is always less than 45ºC. Thus the boiler always runs in condensing mode. You will understand that is greatly more efficient than the combi boiler which you are moving away from.

Let me stop there and allow you to comment or clarify what you want to know next.
Userlevel 6
Great to question here, @dante!

Welcome to the forum, it's great to have another member on board that is proactively trying to be more environmentally aware.

I'm going to take your question away and get back to your with a well informed answer, in the mean time... I'm looking at the other members of our forum to step in if they have any thoughts to share.

@Transparent, @Phil_H , @Leo Moran, @ITGeek123, @So much for subtlety, @PeterR1947 any insight would be ace! 😁


**edit @Transparent, I hadn't refreshed my feed before posting, great work!**
Userlevel 6
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Interesting topic!

The other thing to take into account with an indirect cylinder heated from a Combi boiler is the length of (usually) copper piping from boiler to water tank. These pipes in my 30+ year old house are under floorboards and uninsulated so make a good radiator. I wonder what the efficiency is, my guess is only about 50% but I'd like to hear other's opinions.

The main issue with using immersion heating is the placement of the elements; most domestic tanks have single immersion heater in the top, the problem being that as they heat up, the hot water rises and cuts of the thermostat prematurely so the bottom of the tank stays relitively cool. Best to have a tank with two heaters, one top and one bottom.
Userlevel 5
Hello Dante,

I've looked into a few options regarding solar water panels and thinking as I was a plumber in a previous lifetime and used to fit heating systems it would be possible to plumb in myself....but gave that idea up because so much has changed in the 45 years since I fitted a system commercially.....hence the need for a qualified engineer to install (odd that you have to be CORGI registered in order to instal an electrical system but hey ho).

In the end I thought as I only live basically in three rooms in the house and there's only me, rather than put on a 120,000 BTU boiler and heat the whole house with the gas boiler I'd look at three or four oil filled electric radiators. I have plenty of wall to mount them on and they can run off a 13 amp socket (though I'd probably put a separate circuit in for each one, belt and bracers). I can also put wi fi and timed controllers on each radiator and set them so I only need the back lounge on an evening, the bedroom last thing at night and first thing in the morning, the kitchen at meal times and the bathroom at morning/ night times.. I already have thermostatic controlled valves, (discounted from OVO) which have cut my bills down greatly. Basically it gives me the option of running the the full heating if I have visitors or just a couple of electric radiators if I'm in by if you're on Economy 7 then you should save quite a bit.

You can probably run the domestic hot water off the boiler just as normal or if you manage to get a cylinder set up, run just the hot water off the solar panels. I did research them but decided not to get them due to a few problems.

Sorry to be so negative and what suits me may not suit everyone but I hope I've been able to give you another thought.

Hello everyone,

Thanks for all of your replies, you've given me a lot to think about. I've also done more digging, and thinking, and I think I'm beginning to get a clearer idea of the options I have. Here's what I've found out so far:

Q1) Is night-time electricity greener than daytime electricity?

Yes, this certainly seems the case. I had a good poke around the OVO website, and found statements like the Vehicle-to-Grid charger "will charge your vehicle battery with electricity from the grid when electricity is cheap and more likely to be of a lower carbon intensity", and the Smart Charger which will "reduce your charging emissions by up to 22%" ... "Savings based on Average Grid Carbon Intensity (gCO2/kWh) and by moving three-hour charge window from 5pm to 11pm."

But the big find for me is the Carbon Intensity website which maps out the carbon intensity of electricity generation in the UK: where you can clearly see the CO2 impact dropping on a nightly basis throughout the UK.

I can't find any suggestions that UK electricity is more environmentally friendly in the daytime than at night, so I think that one is answered.

Q2) is night-time electricity more or less green than a combi boiler, if used to heat an unvented cylinder for DHW?

According to that Carbon Intensity website, emissions from electricity generation drops to ~100 grams of CO2 per KWh at night, although there seems to be a massive regional variation, with parts of Scotland generally up the top with 15 and parts of Wales generally bottom with 350.

To compare this with domestic boilers, I found a 2016 Carbon Footprint of Heat Generation paper published by the UK Parliamentary Office of Science and Technology. This says that "New boilers are required to have ... a carbon footprint of 210-230". (I'd quite like to find an exact figure for my boiler, but can't. I may email the manufacturer).

Using these figures, Scottish electricity is 14 times "greener" than my combi boiler, but Welsh electricity is 50% worse. So if I were in Scotland, it would appear to be a no-brainer, but as my region is towards the bottom of the list, it's not so clear cut.

I also haven't taken into account some other factors, which could push it one way or another in a close competition, like:
  • I've assumed that both forms of heating are as efficient as each other. However, as @PeterR1947 points out, some of the heating from the boiler will be lost as the water makes its way to the cylinder, whereas the electrical losses should be near zero.
  • Some of the gas will "green" biogas, so presumably those emissions don't count, but I don't know if biogas is part of the mix of gas that is piped to my property, or if like the Scottish/Welsh electricity divide, it's other people elsewhere that have access to that energy source.
Overall, I haven't yet reached a conclusion here. I also feel that, if I'm consuming electricity on a green plan, then I'm driving demand for green electricity, which should encourage suppliers to invest in renewables. If I'm consuming gas, then I'm not helping shift the demand from electricity to gas (a bit like if I fly but carbon offset my travel, I'm still paying a company that's going to make a profit and use that profit to fly more planes and expand more airports and promote air travel, none of which my offsetting will offset). But, for energy supply, I don't know how valid this argument is; perhaps OVO will use the profits from my gas supply to fund renewable electricity? But it seems unlikely that the wholesale gas supplier will.

If anyone has any other thoughts or opinions then please do share them. Rather than make this reply any longer, I think I'll reply separately to individual comments above, but again thanks you for all of them, they are all very helpful.
Ah, yes, there was another aspect of the problem that I wanted to describe. I've had a closer look at the Solar Thermal costs, and compared them to the cost savings they should give me.

I haven't had a full quote, but an initial estimate suggests that upgrading the cylinder to a solar cylinder, purchasing the panels pump etc, and having them fitted, will cost at least £5k.

In comparison, my summer gas consumption (which is only hot water as we don't run the heating) is somewhere in the region of £15 per month (once I've factored in things like the standing charge, which I'll still have to pay as I'm not doing away with gas entirely).

This gives a payoff for the solar panels of 40 years if they supply 70% of my heating, which seems to be the figure that most solar thermal panel manufacturers tout. Again, I'm ignoring a few other factors, like the fact that the solar panels will require additional servicing and maintenance, and that I'm assuming that they won't fail within those 40 years.

I suspect that in order to make it worthwhile, solar thermal really needs a proper thermal store like @Transparent shared, which can then also service things like my underfloor heating, but unfortunately I'm not in a position to change any of that right now (and suspect that if I were, a heat pump of some description would be a better option. But I digress, as I'm only changing the hot water!).

As I said in my original post, I'm not adverse to paying more in order to be more green, but I think that in this instance, the £5k would be better spent making other efficiencies around the home / in our life / invested in some renewable enterprise. But, again, I'm happy for thoughts and opinions on this if anyone has any.
@Transparent yes, you're absolutely right about the solar cylinder needing an indirect coil for the solar water. My "direct" and "indirect" terminology comes from the Heatrae Sadia brochures, where an "indirect solar" cylinder has two indirect coils, one for solar and one for gas, and a "direct solar" cylinder has a coil for solar and an immersion heater for electric. I imagine that different manufacturers call them different things, apologies for any confusion caused.

@PeterR1947 yes, there's definitely a difference in the placement of the immersion heaters in different products, and so an impact in how much water they actually heat. For example, the 300L Heatrae Sadia cylinder that I've looked at has the immersion heater above the solar coil, so whilst you get 300L of hot water in the summer, you'll only get 200L of hot water in winter. It's another reason which going solar probably isn't right for us, but the 40 year payoff is the main one.

@Leo Moran yes, I had wondered about electric heating. One thing we are doing is swapping the towel radiators in both bathrooms over to electric; it doesn't make sense to run them at the same time of the day as the main heating, plus we want dry towels in the summer but not a hot house. It probably won't make much difference but at least I'll feel better about it!

Thanks all of you for taking the time to reply.
Userlevel 5

It's been a cold night here in Hartlepool and it's presently 5c. I've just popped out to the garage and there's a bitter Northerly wind and I can hear the waves breaking on the shore 3 miles away. Just popped in the garage and it's as warm as toast with an air temperature of 15C....

One of the side effects of charging and discharging the car for 16 hours on Vehicle to Grid is the garage is kept lovely and warm over the car returns 4.5 miles per kW compared to the normal 3.5 miles per kW in the if I can only fathom a way to get the air from the garage to the house I could save a bit of money on the heating bills.....and that's not as daft as it sounds.

One of my previous houses was a Barretts House which had ducted air heating. So, if I can connect an air duct from the ceiling in the garage to the first floor of the house I could use that heat in my bedroom and bathroom for first thing in the winter mornings.....just a thought.

Userlevel 7
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@dante - I'd like to comment further on your observations of Carbon Intensity and your quest to find the "greenest electricity".

As I don't know where you live (please complete your Forum Profile!), I'm gong to use an illustration from my area of the country rather than yours.

The Carbon Intensity website you refer to uses National Grid data. This may be quite different from the Energy Mix which you are offered by your regional Distribution Network Operator.

I live in the SW Region where the DNO is Western Power. There is a high level of renewable generation from both solar and wind, together with smaller amounts from local hydro- and anaerobic-digesters.

The National Grid lines into the SW Peninsula do not have sufficient capacity to take the amount of renewable generation which is being offered. Therefore almost all the Bulk Supply Points and Primary Distribution transformers are under "constraints" because the amount of renewable electricity being offered is at their limit. They can't pass it back to the Grid Supply Points.

Here's a simplified diagram of these sub-station categories:

and here's the distribution network map for the SW Region showing all GSP, BSP and Primary stations:

Apart from the cluster around Plymouth where there is available capacity, almost all of these higher-level transformers on the Distribution Grid are beyond their maximum capability for feeding back renewable generation.

This means that we use a far higher proportion of regionally-produced "green" electricity than you'd suppose by looking at the energy-mix data for what we derive from the National Grid.

My own BSP is Ernesettle (Station Ref Number 33/0001), and I can visit Western Power's Carbon Tracer website to view the actual live statistics for each BSP.

Here's my Energy Mix histogram for today, and below it I've requested the actual data for the hour 11:00-Noon, when solar-generation was at its peak.

As I write this now, the sun is setting and solar generation is approaching zero.

Here's what National Grid are sending into my Region based on the Carbon Intensity website you mentioned:

contrasted with what I'm really obtaining from Ernesettle BSP on the Distribution Grid:

My actual carbon intensity is much higher because we've just fired up the local gas-generation plants in the South West.

In my area the greenest electricity is more likely to be in the middle of the day rather than overnight.
Userlevel 7
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And may I now respond to your next post about Solar Thermal, @dante ...?

The issue isn't so much about the costs. And the photo I sent of my thermal store looks somewhat more complex than most installations because it shows features I added to permit me to operate from a well. There's no mains water here!

Your main problem is that you have to operate within the laws of physics... and specifically the Laws of Thermodynamics. If you need a "refresher", I recommend listening to the exposition by Messrs Flanders and Swann, which contains the erudite truth that

In my thermal store, the solar input coil is at the bottom of the cylinder at about the same level as the outtake connection for my UFH. This is set at 46ºC.

I have a thermal collector with 30 evacuated tubes. On a typical sunny day, infra-red radiation will raise the temperature in the collector to somewhere between 50º - 65ºC. That provides enough of a temperature gradient to make it worthwhile the controller running the pump.

Allowing for losses in the two sets of (well insulated) pipework, that provides water to the UFH radiation plates at about 42º and yields a pleasant oak-floor surface-temperature of 26º.

But if you are wanting to heat only Domestic Hot Water, you will probably require the water in your cylinder to be hotter than my 46ºC. The greater the temperature difference between the water in the solar-coil and the main cylinder, the more efficient is the system.

To obtain a higher temperature from the external collector you require more vacuum tubes or the higher-efficiency triple-walled type.

Have a look at the website of Navitron to get an idea of what the component costs are likely to be. They are a highly reputable supplier with some good Guides which you can download.

Exactly the same rules apply to a heat pump... except that it takes more electricity input from the mains than the 80w which is required to run the controller and pump on my Solar-thermal system.