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Gardening Guide

Installing central heating

One of the key requirements of any home is to have an efficient heating system. If there is a gas supply available to the premises then this will probably the most efficient running cost.

Lets first talk through some of the key components of a heating system before we get in to too much detail.


First the heart of the system is the boiler. There are various types of boilers and some of the main types are indicated below;

Conventional flue boiler is where the flue is connected to a chimney and discharged usually at roof level. The boiler takes its combustion air from the room so it is important that there is a permanent opening to the out environment (there are special regulations dictating the area of the openings).

Room sealed boilers are ventilated by ducting fresh air direct from outside to the boiler combustion chamber which is sealed from the room. This sort of boiler is best suited in a kitchen where there are kitchen extract fans operating and there is no risk of sucking in fumes through the flue.

Other types of boiler can be provided in the form of back boilers an situated in a fireplace in the lounge. The boiler unit is ventilated using a conventional flue arrangement. The boiler heats water for the heating and hot water use. In the front of the boiler a separate gas fire is provided which heats the room.

One of the latest types of boilers having a high efficiency is a condensing boiler. With this type of boiler additional heat is extracted from the warm flue gases which condense to a vapour. Efficiencies can be up to 20% greater than with other type of boilers. The down side on these boilers is that they are more expensive and often there is a plume of steam discharging at the flue terminal.


Boilers can be supplied in various combinations and can be used just to heat water which is then used to heat the building through radiators and then indirectly heat hot water for the basins and baths. The alternative is called a combination boiler which contains package control systems, pumps and facilities to provide water for domestic hot water use. This is often easier to install but can be more expensive to maintain especially if the boiler is no longer manufactured.


Radiators are heat emitters which supply the heat to the various rooms. Radiator technology has changed over the years from cast iron heavy radiators to pressed steel. These incorporate fins to increase the surface area a give a higher output of heat. Aluminum radiators have also been popular giving the opportunity to create special effects in a modern environment.

Radiator heat output is expressed in Btu/hr or kWh

Example of a pressed steel radiator with fins

The radiator selected for the room must been sufficient capacity to heat the room when it is cold outside at freezing temperatures. The heat required for each room will vary depending on its size, thermal properties and the temperature you require in the room. Buy Radiators online


Heat loss calculation

The heat loss calculation indicated below is a simple example of how the heat requirement is calculated for a room. Our chart to how hot you should heat each room in your home will help you calculate the required radiator size and output [here]

First the air volume must be heated – Measure the room length height and width in metres and multiply by 0.33 and then by the number of air changes an hour you expect that room to have. Generally this will vary from 1 to 2 depending on its location and use.

The next step is to calculate the heat loss through the windows. Measure the area of the windows in square metres and then multiply by the thermal properties of the window i.e. The ‘U’ value. For a single glazed window the ‘U’ value will be 5.8 w/m2/degC and 2.9 for double glazed windows. If you have very special glazing system the ‘U’ value may be different so check with the manufactures.

A quick ready recknoner is provided here to help you work out the output required from your radiator to heat your room [guide]

The heat loss through the external walls is calculated similar to the windows. Measure the area of the external wall and multiply by the ‘U’ value which could range from 1.5 for a conventional cavity brick wall to 0.3 if highly insulated. Again you will need to check with the manufactures. Heat losses through the ground floor and roof are also calculated in a similar way. When all the surfaces which are lose heat have been calculated add up the total and also include the air volume. This will give you a value fore the room expressed in watts/degC. Multiply this by the maximum temperature you wish to heat the room usually 21 degrees C. with an outside temperature of –1degreeC. This represent a temperature rise of 22 deg C. This value will now be the heat required in Watts to heat the room. If you want this value in kW the just divide by 1000.

Example calculation for heat loss


Air Changes
External Wall
Room temperature required 21 degree C when -1 outside = 1386.62w

Domestic hot water

The domestic hot water system is also provided in conjunction with the central heating system. Heat is supplied from the gas boiler and there are two main ways in which this can be achieved.

  1. By using a combination boiler or
  2. The more common way by providing an indirect hot water heating cylinder. Majority of systems use this method and it prove reliable and flexible it also has the added advantage that the cylinder can be positioned in the linen cupboard and the heat emitted by the cylinder help air the clothes.

Control Systems

Controls systems for central heating and hot water systems can be very sophisticated and you could write a book covering all the variations.

It is other best to consult the boiler manufacturers installation instructions as they other put forward some proven control solutions.

Generally there are two main types of configuration of a heating and hot water system.

  1. Pumping water from the boiler to the radiator heating system with a gravity circulation for the domestic hot water or,
  2. A fully pumped heating and hot water system.

There are advantages and disadvantages to each arrangement but possibly the pumped system may have the edge because pipe sizes are reduced to the hot water cylinder. A diverter control valve is used with this arrangement and this gives the option of running the heating or hot water systems together or separately.


Pipe sizing

Pipes have to be sized to ensure that they allow sufficient water to flow through the pipes to heat the radiators and domestic hot water cylinder.

Below is outlined a very simple approach to pipe sizing for a domestic heating system. The chart indicates approximately how much heat a pumped copper pipe will convey. As you plan your system each radiator will be feed with a flow and return pipe this all connect back in parallel and as each radiator is added to the circuit you need to total the heat required to determine the size of the pipe.

Size of copper pipe Heat conveyed based on 20degc temperature drop.
15mm 6000 watts
22mm 14000 watts
28mm 28000 watts
Installing the heating system

Legally there are parts of the installation you cannot undertake and these must be carried out by a CORGI registered Contractor. You can do some of the work yourself such as positioning radiators and install the heating pipes (not gas pipes). If you are competent in stalling electrical systems you may be able to complete the controls systems. However at the end you must have a CORGI contractor to connect the gas supply and generally check the installation for safe working and commissioning of the system.
Installing the radiators

Start with installing the radiators the normal position is under the windows unless you have full height curtains then you may wish to position the radiators on an adjacent wall. The radiators need to be about 100 –150mm from the finished floor to allow cleaning underneath and space for valves. On the flow connection install a wheel valve (for general isolation) and on the return a lock shield valve (for regulating the water at commissioning).

Pipe work

Plan your pipes to run on the surface or under the floorboards. Remember to make sure that all pipes vent to the radiators otherwise air locks will be created and the system will not work. If you do run pipes under the floor mark the area they are installed in so nails do not damage them. The fittings can be compression joints or capillary joints. These are soldered using a blow-torch. Remember if you do use a blow-torch protect flammable materials and check that the solder has properly run around the joint otherwise you will have a leak.

Allow room for the pipes to expand and contract. Also don’t make a too permanent feature of concealing the pipes because one day you may have to maintain them.

Route the pipes back to the boiler and also ensure that the system has a cold feed and an open vent pipe connecting to the feed and expansion tank which is usually positioned in the roof space.

Installation of the boiler

The boiler must be installed and commissioned strictly in accordance with the manufacturers instructions.

Commissioning the system

When you have completed all the installation make sure that all the air vents on the radiators are closed. Start filling the system with water and vent through each radiator. Check for leaks. Before turning the boiler on run the heating pump to ensure that as much air in the system is dislodged and then re-vent. Never vent the system while the pump is running. Then turn on the boiler and as the water warms up balance the water flow to each radiator using the lock shield valves.

  • Check all the controls and safety systems to ensure the system is working correctly.
  • Allow the system to run for a few days then check for any leaks and ensure the system is fully vented
Future Maintenance

The boiler will need regular maintenance from an approved Maintenance Contractor usually about every 12 months.


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