Refurbishment: Gas heating systems

• Refurbishment provides the opportunity to install a new or replace an existing heating system with a new more efficient type.
• There are a number of factors which need to be taken into consideration when choosing a new system:

Fuel type: In terms of CO₂ emissions the best choice will be between gas (where available) and wood fuel. If choosing a wood-based system consideration should be given to availability of fuel supply.

Boiler size: The size of the boiler needs to be carefully calculated based on projected hot water and space heating demand – particularly where a building has undergone significant insulation and air-tightness improvements.

User needs: The pattern of use might determine the type of boiler selected. If, for example, a large family uses hot water at the same time, a system including hot water storage would be preferable on the other hand, a combination boiler is likely to be perfectly adequate for a small flat; Another consideration might be a boiler’s capacity to heat a space in reasonable time.

Space: The amount of available space can also determine the boiler type. Where space is at a premium, or if locating a feed tank and its associate pipework in a loft is undesirable, then a form of combination boiler is possible choice whereas if hot water storage is important for example if the system is to be integrated with solar heating or a heat pump, then a system that includes a storage system will likely be preferred.

Renewables: The inclusion of renewable sources of heating energy, particularly solar thermal, should be a consideration. The selection of a gas heating system will be determined in large part depending on whether ancillary sources of heating are to be accommodated.

NB

• The heating system examples illustrated below are for general guidance only. When specifying, confirm with the product manufacturer their installation methods and expected performance. For more detailed information, consult a heating engineer

Boiler types

• Boiler technology is in a state of technological development. This is particularly true in the case of developing systems that produce both heat and power (CHP). However, though it is important to understand the potential of these developments, the system specifier is currently much more likely to consider the selection of a more tried and tested traditional system.

• Choosing between a regular or combi/CPSU boiler depends on the heating and hot-water requirements of the household and the positions in which the boiler can be located. This must take into account requirements for flueing and ventilation. If the water pressure and pipe size are adequate, avoid using the cold loft space for water tanks and pipes, due to the risk of freezing. If this cannot be avoided, ensure that pipes are adequately insulated.

• If ‘renewable’ hot water provision (eg solar hot water or heat pump) is to be included, the choice of a normal combination boiler is precluded because the water source is from the mains. This can be overcome by the addition of a ‘thermal store’ that will provide pre-heated water to the combination boiler at mains pressure. With this combination of storage and boiler, it is essential to ensure that boiler and storage are compatible. Typically, the boiler will be of a ‘modulating’ type that is able to monitor the water temperature in the boiler and to adapt the flame with the solar water heating system demands

‘Regular’ or ‘Heat only’ boiler

• A regular boiler is one which provides space heating directly, but stores hot water in a separate storage system.
• Along with the boiler unit, it requires a feed tank, an expansion cistern, controls and a hot water cylinder.
• Regular boilers are more efficient than ‘Combi’ boilers, but require more space.

Application:
• Suitable for larger homes, where there may be demands for hot water simultaneously.
• Where ample space is available for installation. (eg loft space)

Provides a high hot water flow rate.
More efficient than combination boilers.
Straight-forward design ensures easy maintenance.
Can be used with low mains pressure.
Space requirement for feed tanks, hot water cylinder and pipework.
Heat loss associated with hot water storage and pipework.
Low pressure hot water.
Poor flow rate for showers

‘System boiler’

A system boiler is a sub-type of regular boiler but designed for use in conjunction with a ‘Sealed system’ (see below). Components associated with the system, such as pumps and expansion vessels, are included within the boiler casing – so removing the necessity to locate the components elsewhere.

Application:
• Suitable for larger homes, where hot water is required simultaneously, and where cold water tanks are undesirable.

More efficient and responsive than ‘regular’ boilers.
They remove the need for an expansion cistern, feed and water tanks.
There are less separate components means that it is easy to install and maintain.
Can be used with low mains pressure.
Space is required for hot water storage cylinder.
Limited to use with a ‘sealed’ system only.
Heat loss associated with hot water storage and pipework.

‘Combination’ or ‘Combi’ boiler

• A combination boiler is one which provides space heating and hot water directly. In its simplest form (an ‘instantaneous combi’) water is provided ‘on-demand’ rather than via storage.
• The boiler is fed directly from the mains supply, obviating the need for cold water tank and associate piping.
• Combination boilers are available that include hot water storage within the appliance (see below)
• Showers must be of a ‘mains pressure’ type.

Application:
• Suitable for smaller homes where simultaneous hot water demand is unlikely.

Instantaneous hot water.
High pressure hot water.
Space efficiency.
Good flow rate for showers
Cheap to run
Drinking water available at all taps
Unsuitable for anything other than small homes.
Unsuitable for low pressure supplies.
Ancillary water heating (eg solar) requires extra storage and compatibility.
Poor flow rate for baths.
No back-up water supply if mains supply fails.

‘Combined Primary Storage Unit’ (CPSU)

• This type of combination boiler incorporates very large hot water storage, 70 litres or more.
• This large water storage capacity creates a high hot water flow rate to taps and showers and heats up radiators quickly, making them good for larger homes. They also provide the ability to run baths and showers simultaneously with minimal pressure drop.

Application
• Suitable for larger homes where the hot water demand is often simultaneous.

Combines the advantages of regular and combination boilers.
Enables rapid space heating.
Less efficient than a normal ‘combi’.
Requires more boiler unit space than a normal ‘comb.
Few models on the market.
Usually expensive