Background

Heating and hot water represent the greatest proportion of domestic CO2 emissions and also consume far more energy than household appliances. The average household with gas central heating consumes around 26,000 kWh of energy per year, of which around 83% is for heating and hot water. Replacing old, inefficient boilers can significantly cut fuel costs and CO2 emissions.

Electric hot water systems

• Similar in appearance to a hot water tank.
• Use an immersion heater to heat water.
• Two elements are required if you use large amounts of hot water in a short period of time.
• One heater at the bottom of the tank will take a long time for all the water to be heated to the desired temperature and the thermostat to turn off.
• If there is a smaller heater positioned nearer the top of the tank it enables you to have a smaller amount of hot water available in a shorter time – although this alone is insufficient if you require large amounts of hot water.
• Electric immersion heaters are most commonly found if you are not on the gas network and often run on economy 7 fuel pricing.

Combination (combi) boiler

• The water is heated in a similar way to a standard hot water tank – but this is contained with the heat exchanger found inside the actual boiler.
• A combi boiler can be programmed to heat water at a selected time- although stores a much smaller amount than a standard tank.
• Water is drawn directly from the mains, so a cold water storage tank is not necessary.
• When a hot tap is turned on the water is drawn from the mains into the heat exchanger where it is warmed up and piped to the tap.
• Storage combis are available. The size of the hot water store can vary greatly and can improve the efficiency of the hot water heating. Once the store of hot water is used the boiler will begin to act as an instantaneous hot water heater again.

Standard hot water tank

• If there is no combination boiler then hot water will be heated up in a hot water tank/cylinder which is separate, and often larger than the boiler.
• The hot water tank contains a coil which is filled with water heated from the boiler. Surrounding this coil is the water that will be delivered to your hot taps – separated from the water in the coil.
• The tank is filled up via a cold water storage tank, usually found in a loft and fed from the mains water supply.
• The hot water is then pumped directly from the hot water tank to the tap when it is needed.

Combined primary storage units (CPSUs):

• Include a very large store of water (minimum 70 litres) to reduce frequent boiler cycling.
• Allow radiators to warm up very quickly and provide a high flow rate of hot water to taps and showers.

Solar hot water

• Solar thermal systems require a hot water tank. A boiler fed or electric hot water supply will be required to top up the hot water on cold days.
• Twin coil tank (a tank with a coil fed by the solar thermal panel and one fed by the boiler) or a separate tank can be used.

Controls

Hot water storage systems should have time and temperature controls.

Electrically heated systems:

• Normally have thermostat incorporated into the immersion heater – check factory settings adjusted to 60°C.
• A timer might be used in conjunction with off peak metering for the lower immersion heater, with the top element either used with a separate timer or boost switch.
• Gas and oil fired systems indirectly heated from a boiler.
• Require a cylinder thermostat – interlocked so that if neither room nor tank thermostat is calling for heat the boiler is off – set to 60°C.Time control normally via the central heating programmer.

Insulation

It is important that your hot water tank has adequate insulation.

• Most cylinders fitted in the last 20 years will have been factory insulated usually with sprayed polyurethane foam characterised an ‘orange peel’ type surface finish.
• If the cylinder is factory insulated and marked as compliant with the relevant British Standard (BS1566, BS 699 or BS 3198) then its heat loss is probably acceptable.
• If space permits then an additional separate insulation jacket can be cost effective, particularly on older cylinders.
• If the cylinder is either un-insulated or has a badly fitting old lagging jacket then a new lagging jacket is always the first priority.
• Any hot water jacket less than 80mm thick should be upgraded.

Insulating pipe work will minimise heat loss when hot water is travelling to the desired location.

If you have any further questions, why not try our Advice Library.

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