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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 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
RoomDesign temperature Design Air Changes per hour
Living room 211 to 2
Dining Room 211 to 2
Bed sit room 211 to 2
Bedrooms161 to 2
Study211 to 2
Kitchen182 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

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.


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Bedroom 2 Area (m2) Temperature difference U value w/m"/c Design heat loss (Watts)    
External wall 3.5 17 .92 86    
Window 2 17 5 170    
Party wall 10 6 2.1 126    
Internal wall (1) 7.5 none 1.7 none    
Internal wall(2) 10 -2 (heat gain from adjacent room) 1.7 -34 (heat gain)    
Floor 12 -5 (heat gain from below) 1.36 -82 (heat gain)    
Ceiling 12 17 0.34 69    
Ventilation Air Changes Room Volume Changes Temperature difference Heat capacity of air Design heat loss    
2 30 17 .33 337    
    Total Design heat loss   672 Radiator size required Actual radiator size
    add 10% for heating up     739  
  Choose Closest radiator available larger than the size required   770
Living room Area (m2) Temperature difference U value w/m"/c Design heat loss (Watts)    
External wall 8 22 .92 162    
Window 2 22 5 220    
Party wall 15 11 2.1 346    
Internal wall (1) 10 none 1.7 none    
Internal wall(2) 15 3 1.7 76    
Floor 24 22 0.36 190    
Ceiling 24 5 1.62 194    
Ventilation Air Changes Room Volume Temperature difference Heat capacity of air Design heat loss    
2 60 22 0.33 871    
    Total Design heat loss   2059 Radiator size required Actual radiator size
    add 10% for heating up     2265  
  Choose Closest radiator available larger than the size required   2266
Room Area DEsign heat loss add 10% for heating up Actual Radiator size  
Living Room 2059 2265 2266  
Dining Room 1100 1210 1240  
Kitchen 300 330 350  
Hall 1200 1320 1364  
Bedroom 1 900 990 992  
Bedroom 2 672 739 770  
Bedroom 3 300 330 350  
Bathroom 200 220 280  
DEsign heat loss for the house 6731      
Total of required radiator sizes   7404    
add 3KW for hot water   3000    
Choose the next larger boiler size available probably 12 KW