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Smart hot water cylinders, our low carbon future


David White

Development Director


David evaluates the evolving role of the humble hot water cylinder to help us save money and decarbonise our homes.

How can households reduce energy waste and switch to low carbon energy sources easily? Step in, a new generation of smart hot water cylinder.

What if the humble hot water cylinder offered you a way to reduce energy waste and decarbonise your energy use without making any other changes to your home or heating system? What if your hot water cylinder were as smart as your smart heating control and could adapt to your household needs, whatever they are, so that you never ran out of hot water but never heated more than you needed either?

David White of Mixergy takes a look at the evolution of the hot water cylinder and the role it can play for all households in our low carbon future.


  1. The humble hot water cylinder
  2. Hot water basics
  3. Sizing a hot water cylinder
  4. Problems with incorrect sizing
  5. Smart cylinders, a long overdue evolution
  6. Smaller cylinders, through greater efficiency
  7. Charging on cheap electric – a path to our low carbon future 8. Combing with solar PV

1) The humble hot water cylinder

Your hot water cylinder, if you have one, is probably not an appliance you have given much thought to providing it is supplying you with enough hot water. Often found in the airing cupboard, surrounded by pipes and pumps, its operation is something of a mystery. It works away giving you no indication of how much water is available, how long it takes to heat up or how efficiently it is working. It often looks dated, complicated and can be prone to running out of hot water, so the trend has been to take it away and fit a combi boiler.

Combi boilers are not the answer for larger households with multiple bathrooms however and for those extending their homes and adding bathrooms, an existing combi is unlikely to provide the power you need to run two showers together. The hot water cylinder will always have a role for larger homes, as it will for homes that transition to air source heat pumps.

Space is a factor of course and the appeal of the combi is perhaps gaining the airing cupboard back. A boiler and cylinder cannot compete with that on space, but it can do better. Moreover, a hot water cylinder can still provide vital back up hot water when the boiler is out of action. What we really need are small, highly efficient hot water cylinders that meet our need for plenty of hot water but still gain us some space.

2) Hot water basics

Cylinders come into two forms: vented and unvented. Vented cylinders tend to be made out of copper, covered in foam insulation and with a header tank above them. They are ‘gravity-fed’ which means they use the height of the tank to increase water pressure to your taps. Unvented cylinders are a more recent technology that rely on good water pressure from your incoming mains. They are made out of steel and have a small expansion vessel close by.

Both tend to use a coil inside the cylinder to heat up the water which is powered by the boiler. Both can include an ‘immersion’ (an electrical element similar to a kettle) that will step in if the boiler is not working. Both can be timed to come on for short period or can be topped up continuously. Either way, the whole cylinder is heated irrespective of how much water you need.

3) Sizing the cylinder

The biggest challenge with hot water cylinders is sizing them for your household needs. Get it wrong and you risk running out of hot water. This will happen because the cylinder is too small for the household or too small to meet the peak demand for the household, for example if everyone showers in the morning.

According to the Hot Water Association, as a rule of thumb, a cylinder should be sized to provide 35-45 litres for every occupant. A four-person household will in theory need a 180-litre cylinder.

The problem is that household hot water needs vary enormously and your daily demand for hot water will depend not only on the number of occupants, but also on 1) how many showers need to run at the same time (peak demand) and 2) whether you are a low-consumption or high-consumption household. A low consumption household might only need 20-30 litres per person whilst a high consumption household might use 50-70 litres per person.

The type of shower you have also plays a big part. A powerful shower providing 18 litres per minute of ‘blended’ water (i.e. 60 degC water from the cylinder mixed with cold water), will use around 12 litres per minute of hot water from the cylinder. A 5-minute shower at this flow rate will consume 60 litres of hot water from the cylinder. A four-person household that uses powerful showers in the morning in close succession will therefore need a 240-litre cylinder. A four-person low-consumption household might only need a 120-litre cylinder or less.

4) Problems with incorrect sizing

To size the cylinder for the worse case scenario, even for a larger household, means that the cylinder is very large. As household demands change, for example children grow up and leave home, the cylinder is huge for the remaining occupants; same problem if a smaller family occupies the house in future. Conversely a cylinder sized for a low-consumption household will not cope for new occupants who decide to upgrade the shower.

In summary, there is no flexibility in how much water is heated to allow for varying demands over time and this either leads to inefficiency or a shortage of hot water when you need it.

5) Smart cylinders, a long overdue evolution

It was with this head-scratcher that the founders of Mixergy set about developing a product that answers these questions: how can a hot water cylinder work out how much hot water you need and, assuming it can just heat a portion of the tank, how does it prevent the portion of heated water mixing with the unheated water below? Their solution uses three pieces of technology that, combined, create the first smart cylinder:

  1. Using a smart control, or AI, to learn what you use daily, show how much you have and predict how much to heat the tank.
  2. Harnessing a process called ‘thermal stratification’* to heat the water from the top of the cylinder and down as far as you need to meet your hot water needs.
  3. Using a ‘inlet diffuser’ to stopping the heated water from mixing with the unheated water when only a portion of the tank is heated.

This built-in flexibility reduces energy waste without running out of hot water when you need it (for example when the children are home or guests are staying). Tests have found that the ‘heat what you need’ approach can save 12% in gas consumption for hot water production and 20% in electric when using just the immersion.

6) Smaller cylinders, through greater efficiency

But the technology has another upside. By removing the coil from the cylinder and using an external heat exchanger to heat the water, combined with faster re-heat times by minimising hot and cold-water mixing, a smaller cylinder can be fitted.

For example, a 210 Mixergy cylinder instead of a 300 litre standard cylinder, or a 120 litre Mixergy cylinder instead of a 150 litre standard cylinder. The difference is important for 1-bathroom households where hot water demand is relatively low, as they can opt for a 90-120 litre cylinder and keep the top half if the airing cupboard free.

7) Charging on cheap electric – a path to our low carbon future

With the common problems around hot water cylinders dealt with to make them viable for all households, their untapped potential for decarbonising our energy use is now an excellent bonus feature. 22.5 million homes are on mains gas in the UK and around 4 million are off the gas grid (the majority of which use an oil boiler).

Fossil fuels will need to be phased out by the mid-2030s (probably earlier for oil households) and electric powered heating systems including air source heat pumps used in their place. Electricity has the unique advantage that it can be 100% renewably generated. Whilst hydrogen (a green-gas) may be rolled out to some households, this technology at present seems less of a certainty.

Households will be required to better insulate their homes to reduce our energy use and switch to renewable heat sources when possible. But this is a long road and what we do along the way counts, even if those changes are a small contribution.

All households could use 100% renewable electricity from their supplier to heat their hot water, rather than using the gas or oil boiler, providing they retain or fit a hot water cylinder. This can be achieved with any hot water cylinder providing it has an immersion. On a 100% renewable electricity tariff, we can begin to decarbonise our energy use now with very little adjustment. The downside is that electricity costs 18p per kWh compared to mains gas at 3p per kWh. Old-style Economy 7 electricity packages are still available that allow the cylinder to charge overnight at a lower cost per kWh.

But electricity providers have become more sophisticated in their tariff structure so that night-time tariffs, when demand is very low and electricity generated by windfarms cannot be stored, can often be cheaper than mains gas. This is where the Mixergy cylinder gets really smart. The smart controls can interface with electricity providers such as Octopus, who release their tariffs every 24 hours, and look ahead to when the cost per kWh is at its lowest. The controls will then set the cylinder to charge at this time. In this way households can effectively decarbonise without increasing energy costs.

8) Combing with solar PV

Solar PV panels are an accessible option for many households as most of us have a roof. A small solar PV array can provide most of the electricity needed to run the home and any surplus can be re-directed to hot water generation. This can be achieved with any hot water cylinder with an immersion.

With the option for an inbuilt solar PV diverter on the Mixergy, the smart controls monitor when the hot water is needed and utilise the cheapest source of electricity, which maybe the surplus solar PV energy. So instead of sending it back to the grid, it can be used to heat hot water. Government funded trials have found that this set up can reduce imported electricity from the grid by upto 35%.

*Hot water will sit on top of cold water and not mix unless something causes it to mix

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