Central heating design guide

A central heating system should be able to match the heat loss from the dwelling at the design conditions, with an allowance for intermittent operation and hot water usage.

The principal design considerations are:-

• The required internal temperatures of each room. (see table 2)
• the design outside temperature (Usually -1C but -3 or -5 in exposed locations)
• The Ventilation rate of the room(s).(see table 2)

The calculations will include provisions to ensure that the heating up time of the entire system is not excessive. Possibly including a factor to allow for a sheltered or exposed site. The usual process is to calculate the heat loss rate and hence the radiator sizes, for each room in turn and then to add these losses together to arrive at a total. Then add on a factor for the hot water and thus obtain the boiler size required.

There are two sources of heat loss from a building.

Fabric heat losses and Ventilation heat losses.

Fabric heat losses are losses directly through the walls, windows,doors, floors and Ceiling of the room. For ease of calculation, it is assumed that these losses are at a uniform rate through each surface. The heat loss rates are obtained by multiplying the area of each individual surface by the design temperature difference and the heat transfer co-efficient Called the 'U' Value.

Thus:- Design heat loss = Surface area * Temperature difference * 'U' Value.

U values have been calculated for most construction types and are given in the table below. For those who really want to know, a method for calculating 'U' values is given here.

Ventilation heat losses are caused by the air flowing through a dwelling. Ventilation rates can be quite surprising, and are usually quoted in air changes per hour. Defined as the volume of the air flowing through the room in one hour divided by the actual volume of the room itself.

This air clearly needs to be heated by the central heating system. and the heat required is calculated by multiplying the volume of the room by the air change rate ,the temperature difference between outside and inside and by the heat capacity of air.

The air change rate is a design consideration which is extremely variable and can be minimised by draft proofing of the home. However, minimum ventilation rates are recommended, to prevent such things as condensation, stuffiness and smells, these design air changes are also given in table 2 In a room which has an open flue (Coal or gas fire) it is common to allow 5 air changes per hour as the operation of the flue entrains excessive air into the room. So opening up a fireplace to put a fire in can often be counter productive.

Table 2. Typical design temperature and ventilation rates

Room	Design temperature	Design Air Changes per hour
Living room	21	1 to 2
Dining Room	21	1 to 2
Bed sit room	21	1 to 2
Hall	18	2
Bedrooms	16	1 to 2
Study	21	1 to 2
Bathroom	22	2
Toilet	18	2
Kitchen	18	2 to 3

Design temperatures

Internal design temperatures should be chosen to ensure satisfactory comfort conditions . For normal circumstances recommended temperatures are given in table 2 The external design temperature should allow for all but the most extreme conditions. a figure of -1 is normally chosen, but in Further north ie. North of England and Scotland, - 3 or even -5 is chosen.

Allowance for heating up

Additional heating capacity is needed both in the radiators and in the boiler,to allow for the demands of intermittent heating and minimise the heating up period. This allowance is usually made as an addition to the calculated room heat loss. A figure of 10% gives acceptable results and is is normally used. Only in exceptional circumstances, such as where the customer requires a shorter heat up period than usual, should a larger addition be considered. up to 30% may be appropriate in such cases.

Allowance for hot water

Where the boiler supplies both the hot water and central heating simultaneously, an additional 2Kw is usually required in calculating the boiler output.

Allowance for exposure

Where a dwelling is situated at a very exposed site, allowance should be made for possibly higher heat losses than those calculated using standard u values, which assume average exposure. This could take the form of an arbitrary addition to the calculated heat losses (possibly 10%) but should be based on local conditions and experience.

Calculation method

An example of the overall calculation to establish the correct boiler size and radiator heat output is illustrated in figure 4. The steps are as follows

• For a particular room,
• Identify all the surfaces (walls Cielings floors) etc
• For each surface,
identify the design temperature at the other side of the surface.
• Identify the “U” value of this surface.
• calculate the surface area of each surface and multiply by the u value and temperature difference to get the heat loss/gain
• add up all of the losses/ gains, to get a total fabric loss.
• calculate the volume of the room and identify the air change rate
• calculate the ventilation heat loss by multiplying the volume by the air change rate by the SHC of air
• add together the design fabric and ventilation heat losses. Add the allowance for heating up and then select a radiator with an output as close as possible to, but not less than this value.
• Repeat this process for each room to be heated
• Add up the calculated heat requirement, to give the total space heating requirement
• Add the appropriate allowance for domestic hot water to give the total heat requirement from the boiler.
• A Boiler with an output as closed as possible to, but not less than this total heat requirement should then be selected.

It should be noted, that no further margins (eg. for pipe losses) should be added as oversizing the boiler unnecessarily reduces its overall efficiency and therefore wastes energy.