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Hot water efficiency tips


Jo Alsop

Heating Hero

The Heating Hub

Producing hot water for the home is unavoidably inefficient, but it can be made worse. Jo looks at the extra causes of hot water inefficiency and how to avoid them.


Help to reduce inefficiency from your hot water systems

For safety reasons producing hot water is unavoidably inefficient, but it can be made even less efficient by  losing heat through the supply pipework, creating more hot water than is needed and, by far the worst, making the heating system equally inefficient. In this blog we look at the extra causes of hot water inefficiency and how to avoid them. We'd like to thank all the contributors to this blog via our social media sites who have listed other causes of hot water inefficiency. If you have other examples please contact us


  1. Combi boilers and hot water pre-heats
  2. Boilers that work with a hot water cylinder
  3. Cylinders in cold spaces
  4. Cylinder sizes
  5. Cylinder temperature
  6. Uninsulated hot water pipes
  7. Single lever mixer taps
  8. Leaking hot taps and showers

1) Combi boilers and hot water pre-heats

One of the biggest complaints from combi owners is the length of time it takes for the hot water to arrive at the tap. Hot water cylinders are often on the first floor, near the bathrooms and over the kitchen. All very central with quick draw off times. Combi boilers can be tucked away in a corner of the house, on an outside wall, meaning that many minutes and litres of water are wasted waiting for the hot water to arrive.

Some boiler manufacturers introduced a ‘pre-heat’ function that means the boiler fires up regularly to keep the hot water in heat exchanger hot. Whilst more convenient, with less water wasted, it is hugely inefficient, particularly in homes where the occupants are out at work all day.

Based on a boiler firing every 90 minutes, using 0.16kWh of gas each time, it uses 934kWh pa, however there will be some overlap during the heating season. Based on some metered examples, our best estimates are around 500-750kWh for an untimed preheat depending on how much you're at home. You can watch our Boiler Tweaks To Tackle The Cost Of Living Crisis video which covers how to turn off hot water pre-heats for common boiler models (7.52 minutes onwards). We also give guidance on how to lower the flow temperature for heating to save more gas.

Depending on occupancy, we recommend you switch off the pre-heat and fit a 'Combi Save' instead. A combi save is a piece of pipework that will restrict the amount of water going through the boiler until the water has reached temperature, at which point it will open the valve fully. This reduces water waste by around 80% and reduces the amount of gas used as it is heating less water during the start-up period.

2) Boilers that work with a hot water cylinder

By far the biggest cause for avoidable inefficiency is a heating system that is running too hot because the boiler can only supply one temperature and that has to be a high temperature for safe hot water storage.

Hot water cylinders need to be kept at 60degC (HSE) to kill off legionella bacteria, which can be fatal for humans. Just to be clear, we do not actually use 60degC hot water, it is far too hot, so it must be blended down again to a lower temperature at the shower or mixer tap.

For gas and oil boilers the conventional set up is to supply hot water to the cylinder at 70degC to allow for any losses in the pipes on the way to the cylinder. Where boilers are left at their factory settings the hot water is produced at 80degC.

However 70/80degC is too high for modern boilers to operate efficiently for heating. For condensing boilers, a lower flow temperature is needed in order for the boiler to recycle heat that was previously lost through the flue. This has the potential to improve the boiler efficiency from around 78% efficient (non-condensing boilers) to 92-97% efficient for a condensing boiler. In order to reach 92% efficiency however, the flow temperature for heating needs to 60degC or less, but this is too low to maintain a safe cylinder temperature.

What is the solution? Ideally the system needs to be set up to supply two temperatures, one for heating and one for hot water. I.e. the boiler operates at 70degC for hot water and 50-60degC for heating, or less if possible. 

As heating accounts for around 80-95% of fuel consumption, so there are big savings available for the 9 million homes with a hot water cylinder if the heating temperature can be separated out from the hot water temperature. The good news is this can be achieved in most homes. The bad news is it is not practiced or understood by 99% of installers.

As it stands, more than 99% of homes with a hot water cylinder will have a boiler that supplies a single flow temperature for both heating and hot water, i.e. 70degC or higher, which means the boiler can never reach its efficiency potential. A sign that your boiler is NOT condensing is lots of white plume exiting the flue very fast.

It is always possible to separate the heating and hot water temperatures, although the set ups will vary depending on the boiler you have and compatible heating controls. We provide bespoke advice on this based on your boiler, along with other system improvements. We do charge £120 and there will be a small investment in the system to bring about the changes, but this money that will be made back quickly with fuel savings, particularly as gas and oil prices continue to rise. Go to How We Work for more.

3) Cylinders in cold spaces

Modern cylinders must lose no more than 2kWh in per day (i.e. 24 hours) and some models lose as little as 1kWh per day. Spray foam cylinders can lose 2-3kWh per day and copper cylinder with loose red jackets up to 4kWh per day. The loss will depend on the insulation thickness and the size of the cylinder – the bigger the cylinder the more it will lose - but it can soon add up to over 1,000kWh per year.

Losses can be increased further if the cylinder is located in a cold space, such as loft or garage. An internally located cylinder lose less heat because the air around is warmer and it can usefully transfer any losses to the room or airing cupboard during the heating season. Whilst moving your hot water cylinder out of the house does free up space, it is not efficient. If you can accommodate it in a utility room, bathroom or airing cupboard it will use less energy and cost less to run.

4) Cylinder sizes

This is such a ‘how long is a piece of string’ discussion. How do you size a hot water cylinder for a large house with only two people living there, or a small house with two adults and two teenagers?

The Hot Water Association state that the average person uses 35-45 litres of hot water per day per. As hot water is heated to 60degC, so a 120-litre cylinder of 60degC hot water will provide 180 litres of useable hot water at 40degC. In theory, enough to supply a 4-person household.

So much comes down to occupancy however. Households can be big hot water users (50-70 litres per day) or small hot water users (20-30 litres per day). Some people have quick showers using only about 18 litres at a time. Others long baths using 100 litres of 40degC water at a time.

The problem with catering for a large hot water household is that larger cylinders mean greater heat losses and also producing more hot water that is regularly used. Larger cylinders are also more likely to be relocated to a cold space. Our advice is to go for a modest hot water cylinder proportional to the number of bedrooms in the home and stagger hot water use, i.e. do not create a demand for 4-5 power showers every morning. Smart cylinders, that learn patterns of use overtime, can also reduce how much hot water is heated.

5) Cylinder temperature

As set out under section 1, we are advised by the HSE to maintain a cylinder temperature of 60degC even when it too hot for us to use. Air source heat pumps maintain a cylinder temperature of 50-55degC and comply with the HSE advice by periodically running a 'cleansing cycle' which raises the temperature of the cylinder to 60degC at least once a week. There are lessons here for gas and oil systems but they have to be set up correctly and by a competent person.

6) Uninsulated hot water supply pipes

By far one of the biggest heat losses is the too-often-over-looked hot water supply pipes. An Energy Savings Trust study found an average 2% efficiency difference between a combi boiler and boiler with a hot water cylinder due to the losses from pipework and cylinders. In reality the loss is likely to come from the cylinder because combi boilers have the same amount of hot water pipework and the same potential losses particularly if a pre-heat function is in operation (see below).

It is so important to properly insulate hot water supply pipework. With pipework often routed over long distances through cold spaces, such as beneath the floor or in the loft/garage, it has the potential for big heat losses if not properly insulated (aka ‘lagged’). Foil coated foam insulation, taped at the joints will go a long way to reducing losses.

The same pipework insulation advice applies to combi boilers as it does to hot water cylinders. Whenever you turn on a hot tap the boiler will start to generate hot water and continue to generate it until you turn off the tap. A full pipe-length of hot water will sit in the pipe losing heat until it is called on again. By that time it has often cooled.

7) Single lever mixer taps 

Where your tap has a single lever turning it on for cold water can accidently trigger the boiler to fire to produce hot water. Fit taps that have separate hot and cold water levers or handles.

8) Leaking taps and showers

If a hot tap or shower is 'letting by' (aka leaking) and it is enough to bring the boiler on you will be wasting an enormous amount of energy. All of the losses through the pipes and continuous firing of the boiler will increase your fuel bills very quickly. Get a plumber and get it fixed asap.


Jo & Caroline - Heating Heroes

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