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Heating and hot water


Most UK homes have central heating with radiators.

Most UK homes have central heating with radiators.

Central heating, fuelled by mains gas, oil or LPG is the most common type of heating in the UK. A boiler heats water, which is piped round the building to heat radiators. It also heats water for washing and bathing. The boiler accounts for around 60% of all the money you spend on energy bills according to the Energy Saving Trust.

Some people rely on electricity, generally using lower priced, night-time electricity to heat storage heaters. This is expensive, inefficient and carbon intensive.

An efficient heating system is one that uses the least amount of energy to achieve the desired level of warmth. Making your heating system as energy efficient as possible is likely to be a combination of changing the way you do things, and adapting or changing the controls, or the whole system. 

Before you spend lots on heating your building it’s worth making sure than all the heat is staying where you want it, and not escaping from the building: read about heat loss, windows and doors, and draft proofing.

 Image: kozumel

Replacing your boiler

replacing your boiler can save you energy and money.

replacing your boiler can save you energy and money.

If you have an old gas or oil boiler, replacing it with an A-rated condensing boiler could save you up to £300 per year in energy bills according to the Energy Saving Trust. Gas boilers installed since 2005 and oil boilers since 2007 are likely to be condensing boilers. If you are not sure whether or not yours is, look and see if your boiler has a plastic flue. If it does, it's probably a condensing boiler. If you are replacing an old boiler, you will probably be eligible for Green Deal funding.

If you are not on the gas grid, you may want to consider alternative forms of heating: biomass boilers are suitable for those who have space for storage of wood pellet or logs; heat pumps are an option in well-insulated buildings, and particularly for those currently heating with electricity.

A heating system’s efficiency strongly relates to the right meeting of boiler output power and radiator size and number. So having the correct boiler and output setting are crucial first steps to saving energy and money on heating.

There are 3 main types of boilers used in the UK. Combi (combination) boilers are the most common. With them you don’t need a separate hot water tank or cylinder, so they are a good space-efficient option. Combi boilers take cold water directly from the mains and heat it and give hot water when it’s needed for taps and central heating. They are efficient, because their flow rate acts as a restrictor on high use, but you may have less pressure if taking hot water from more than one tap at the same time, so they are less suitable for homes where more than one person showers at a time.

Conventional boilers (also known as regular or vented boilers) come with a hot water storage cylinder (which is typically in the airing cupboard) and a feed and expansion cistern in the loft. They are often fed by a cold water storage tank (also in the loft). They are suitable if your home needs to get hot water from multiple taps at the same time. The downside is that if the hot water runs out, you will have to wait to reheat it. Because of the cylinder and the tank it takes up more space than a combi boiler.

System boilers (also known as a sealed system), like conventional systems, work by storing hot water in a cylinder. However, the hot water is pumped through the heating system, and does not need a feed and expansion tank in the loft. The pros and cons are similar to a conventional boiler, but the installation is quicker and more efficient.

All of the above types of boilers can be condensing boilers. Condensing boilers save fuel by taking some of the heat that might escape the system, and reusing it. They are much more efficient. New boilers are required, unless in a rare exception, to be condensing. Note that for the boiler switch into condensing mode the flow temperature returning to the boiler should be less than 55 degrees, and the lower the temperature from 55, the more it will condense.

Image: Butterfields Plumbing and Heating

Types of heating system

Underfloor heating: one form of low-temperature heating

Underfloor heating: one form of low-temperature heating

All central heating systems have three components: a heat (or energy) source (for example mains gas), a distribution medium (for example the water that flows through the radiators), and a heat emitter (for example, the radiator). Heat sources include: gas (most common in UK), oil, LPG, electricity (from the grid or renewable sources), biomass, coal, combined heat and power, solar thermal and heat pumps. The most common distribution medium for heating in the UK is water (called a ‘wet’ system), which can be high or low temperature. Heat can also be distributed using air, steam, or electricity. The most common heat emitter in the UK is a radiator, but several others are discussed below.

Low or high temperature central heating?

Traditionally, radiators fed by central heating systems operate at higher temperatures (from 60 to 80 degrees C). A lower temperature flow (as low as 35 degrees) is becoming more common, and is desirable because it is more efficient. Low temperature heating is common in underfloor heating, which can operate at around 35 degrees C, but is also possible and increasingly common with a radiator system.

Whether a low temperature system is suitable for your home depends on:

  • Your building’s heat loss value: If your building suffers from significant heat loss that cannot be corrected, it may not be practical to switch to lower temperature heating.
  • How much space you have: low temperature system will require more surface area of radiator (or other heat emitter) to work as effectively. You may need to install new radiators.

With a heat pump, the efficiency increases as the temperature decreases. So a lower temperature system is preferred. These are commonly installed with underfloor heating, but can also be installed with radiators.

Generally speaking, for a lower temperature heating system, it is more efficient to leave the heating on continuously, and for a higher temperature system it is more efficient to turn it on around key demand periods. Heat loss from a building is proportional to the inside/outside temperature difference, but the more airtight and well-insulated your building is, the less this becomes important. This explains why in well-insulated buildings using low-temperature heating (for example many houses in Scandinavia) it becomes efficient to leave heating on 24 hours a day.

Underfloor heating

Underfloor heating works like having a big radiator under the floor: hot water is passed through coils under the floor, which rises to heat the room. Because underfloor heating usually operates between 35-45 degrees C, they are ideal for condensing boilers which require a lower return water temperature to condense. Underfloor heating is also suitable for heat pumps, which provide lower temperature water. Because underfloor heating is a form of low-temperature heating, it is most suitable to spaces that are well insulated and with less air exchange. It is most effective left on for extended periods rather than heating at key times, so is ideal in environments with continual occupancy.

Types of heat

All heat emitters give off heat in some combination of convection (movement of heat through the air) and radiant heat (heats objects directly - like the sun). Where there is a lot of air exchange, poor insulation or high ceilings, the use of higher radiant heat becomes more efficient, because the heat is not lost in draughty air. Fan convectors and natural convectors have a very high convection to radiant ratio, of around 80%, and they are most suitable for use in homes with low air exchange rates, low ceilings, and good insulation, or heating small closed spaces. Radiators have a convective/radiant ratio of 50-70%.

Passive solar heating

Passive solar design takes advantage of the sun’s warmth and stores and releases heat back into the home in the evening. This happens to some extent in every home, as sunlight passes through windows and is absorbed by walls. But there are steps you can take in design which will increase the passive solar gain.

Image: BSFinHull

Heating with electricity

There are several forms of electric heating.

There are several forms of electric heating.

85% of centrally heated homes are heated by gas, which is preferable because the cost is much cheaper per unit and it is cleaner than electricity. Eight per cent of centrally heated homes are heated with electricity. This is generally done using an electric storage heater. Electric storage heaters take advantage of cheaper energy tariffs at night, releasing heat into the space when the tariff is higher again during the day. They are relatively cheap and easy to install and maintain. But they are limited in their capacity to store and control heat output, and can lead to expensive recharging in the daytime, or can lose all of their heat during the day, leaving the space cool again in the evening.

Close to half of homes in the UK without central heating use electric heating.

Electric space heaters

Individual electric heaters are suitable for heating a space sporadically. They have high running costs compared with gas. Depending where your electricity comes from, they may have higher carbon emissions than central heating. A regular dilemma for many people is whether it’s more efficient to on the central heating, or just heat the room you’re using with an electric space heater.

With an efficient gas boiler, radiators with temperature controls and a well-insulated or small house, it tends to be more efficient to turn on the central heating rather than just heating one room. If not, it may be more efficient to get an electric or gas heater for the room where you spend a lot of time, but be careful if your thermostat is in the same room as the extra heating as it can affect the whole house background heat. Room heating may be more efficient if you have storage heating.

Types of electric heaters include:

  • Convection heaters: These are most suited for spaces with low air exchange, good insulation and low ceilings. They are silent and pose low fire risk.
  • Fan heaters: Like convection heaters these are suited for spaces with low air exchange, good insulation and low ceilings. They should not be placed near any flammable object.
  • Electric fires: Electric fires are a form of radiant heater, which means they directly warm people and objects in the room rather than air. They pose a fire risk.
  • Halogen heaters: They emit a high portion of radiant heat, which means they warm objects and people directly rather than heating air. They are suitable for warming quickly, and use in draughty and insulated spaces.
  • Oil-filled heaters: These retain heat better but come with a longer start-up time, so will not provide heat as quickly.
  • Infrared (or radiant) heat panels can be used either as a form of heating for one room or throughout the house in a programmable central heating system. They are commonly used as panels on walls or ceilings. They heat objects and peope rather than surrounding air. Click here to read more about heating with infrared panels. (link to the infrared panel blog)

The running cost of an electric heater relates directly to its capacity in watts. The higher the wattage, the higher the costs.

Heat pumps are a lower carbon way of using electricity. They work by taking heat from the ground air or water and using it to heat the home. They can also heat water. Heat pumps need some electricity to work, but less than direct electric heat. They work best in buildings that are well insulated. For more information, see YouGen’s heat pumps page.

Image: kyz

Heating controls

The room thermostat should be placed in the most used room.

The room thermostat should be placed in the most used room.

The controls are your tools for making your central heating system run as efficiently as possible. The more actively you manage them, the more efficient it will be. If you just programme in some times and leave them until it's warm enough to turn the heating off in spring, you will use more fuel, and have higher bills, than if you fine tune according to the time of year, and how you use your property.

Programmer: this is where you can set the on/off times for your heating and hot water. It's worth making sure that you can control the heating and hot water separately if you are thinking of installing solar water heating.

Room thermostat: turns the boiler (and thus the heating) on when the temperature falls below the required temperature and off when it rises above it. It should be placed in the most used room. Digital thermostats are much more accurate than the older dial type. It should be set to the lowest comfortable temperature (typical levels are between 18 and 21 degrees).

You can now get programmable room thermostats which allow you to set different temperatures for different times of day.

Thermostatic radiator valves (TRV): These are the most basic form of zone control. Fitted to individual radiators, they allowing you to control the heat in each room. The radiator in the room with the room thermostat should not have a TRV as well. To be effective they need a free flow of air to sense the temperature in the room, so shouldn’t be obstructed by furniture or curtains, and should not have a radiator cover.

TRVs with electronic temperature sensing are becoming available now, which allow individual radiators to be set to different temperatures when desired.

Cylinder thermostat: systems with a hot water cylinder, should have a thermostat on the cylinder to switch off the boiler once the water reaches the required temperature. To maximise efficiency it is recommended that this is set at 60 degrees. It should not be lower than this to prevent the risk of legionnaire’s disease.

Boiler thermostat: this should be set at 65 degrees, to enable it to deliver water of 60 degrees (see above). While radiators will get hot quicker if the boiler thermostat is set at a higher temperature, the boiler may not condense, which will reduce its efficiency by 10-20%.

Image: larjuh

Getting the settings right

Using your heating effectively will  help you save energy.

Using your heating effectively will help you save energy.

This is the key to having the right level of warmth for the minimum use of fuel, whether that be oil, gas or electricity. What's right for you will depend on your lifestyle and your family. It will be different for a single pensioner who spends most winter evenings in one room and a family with children who may need the heating on in more rooms.

As a rule of thumb it makes sense to have less heat in corridors and hallways, and to have bedrooms at a lower temperature than living rooms. Try edging the thermostat in each room down a little at a time until you find the room temperature you want.

In cold snaps it's tempting to turn the thermostat up. However, it's not the overall temperature of the system that's the problem. It's that it takes longer for the house to warm up because it's starting at a lower temperature. So the best thing to do is adjust the programmer to put the heating on a bit earlier.

Weather compensators calculate heat loss from a building and apply enough heat to make the desired inside temperature. They are smart enough to store and monitor heat loss data, so that they can engage the system to the most efficient heating levels by remembering the times and temperatures that caused a boiler to come on and off. They can make your heating system more accurate than using thermostats. Some boilers and heat pumps come with weather compensators, and they are commonly used in Germany.

You should always keep your boiler thermostat set higher than your cylinder thermostat.


Water heating

Heating water for taps and showers uses energy.

Heating water for taps and showers uses energy.

Which type of boiler system is most suitable for your home depends on factors including space available, and also what kind of hot water use it needs to be designed for. A family with a large home and several people who need to shower at the same time has different needs than a single person living in a small flat. Please see ‘Replacing your boiler’ for information about this.

What kind of shower?

A traditional shower (or mixer shower) mixes hot water from the boiler and cold water from the tap. Electric showers take cold water and heat it on-demand. Because they heat water as it is needed and tend to have a lower flow-rate, they can be more efficient. But if you are on mains gas, an electric shower may not be as economical, because gas is much cheaper than electricity. Electric showers should not be confused with power showers, which heighten the flow rate, making showers feel stronger but also using a great deal more water and energy. 

Eco shower heads are available, which reduce the water use by aerating it. This means that it still feels reasonably strong, but less water needs to be heated.

Immersion heaters

In a home without a boiler, water can be heated using one or more immersion heaters, often one at the top and one at the bottom of the cylinder. This applies to homes with electric storage heaters. Usually one of the immersion heaters will switch on during the night to take advantage of cheap electricity and heat the whole cylinder and the other will be used for topping up hot water as needed. Knowing how your immersion heater works and making sure that you do not leave the peak rate heater on continually will help keep your energy bills low.

Using renewables for water heating

Solar thermal panels are a renewable way of heating water. Between April and September, solar thermal panels should be able to provide much of your hot water. For more information about solar thermal, see YouGen’s solar heat and hot water information page.

If you have solar PV, one way to use excess electricity generated on-site is to power an immersion heater. You will probably still need your boiler to heat water, particularly in the winter months, but at times when your PV doesn’t heat your full tank it will still offer a pre-heat so your boiler has less work to do. For more information about ways to feed PV electricity to an immersion heater, see ‘how to use excess solar electricity for water heating’.

Biomass boilers burn logs, wood pellets or wood chips to provide heating and hot water. Biomass is a good option for a home off mains gas and with enough space for the larger boiler and fuel storage. Some people combine biomass and solar thermal for water heating, so that the boiler doesn’t have to be switched on in the summer.

What is a thermal store?

Thermal stores are commonly installed with renewable technologies as a way of storing renewable heat until it is needed or buffering against high peaks and troughs from input systems. A store is commonly used with renewable heat generators such as biomass and solar thermal. Different types of thermal stores are suited to different technologies, or to combining. They may use heat exchangers and in built immersion heaters.

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