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Selecting the emitters for water central heating

The are a number of different types of emitters available:

'Skirting' heaters.

skirting heaterLow level convector heaters which are positioned against the walls at floor level. These tend to have relatively low output per unit length but do even out the heat input around the room. .

To select the appropriate model for a given room, the heat requirement needs to be established (as per the example house) and the length of wall (or walls) to fit the heater needs to be calculated. Then simply by dividing the heat requirement by the length of wall(s), the required heat output per unit length can be established. Reference to the manufactures data sheet will allow the appropriate model to be selected.

Although promoted a few decades ago, these skirting radiators never really caught on and are hard to find in the 21st century.


Fan assisted convectors.

fan convecterVery similar in basic principle to traditional radiators, the heat exchange unit is housed within a casing (usually wall mounted) and a low volume fan blows unheated air across the exchanger and into the room. In principle, because of the forced movement of air, more heat can be extracted from the circulating water and the room heated quicker. The fan is normally controlled by a thermostat which will stop the fan when the set temperature is achieved. One draw back is that by encasing the heat exchanger and fan, the units tend to be bigger than traditional radiators. Another drawback is that each heater requires a mains electricity supply to operate the fan.

To select the appropriate heater for a room, the heat requirement needs to be established (as per the example house). Reference to the manufactures data sheet will allow the appropriate model to be selected. It is worth remembering that any room can have more than a single heater, with rooms greater than 6 metres (18 ft) in any one direction, it is worth considering distributing the heat inputs to minimise the thermal gradient within the room.

Radiators.

radiatorsRadiators, as the term is normally used, are simple heat exchangers which distribute the heat by natural air circulation (hot air rises, so the heated air next to the surface of a radiator rises pulling cooler air up from the floor level). They are simple (very little can go wrong), easy to install and operate.

It is worth remembering that any room can have more than a single radiator, with rooms greater than 6 metres (18 ft) in any one direction, it is worth considering distributing a number of radiators to minimise the thermal gradient within the room.

Normally the manufacturer's data sheet will quote the output for when there is a temperature difference of 56 °C (100 °F) between the water in the radiator and the air in the room. Where the temperature difference is not 56 °C, the following correcting factors are necessary to determine the actual anticipated output from the radiator.

design temperature difference
correcting
factor
°C
°F
 
50
90
0.87
53
95
0.94
56
100
1.00
58
105
1.07
61
110
1.13
64
115
1.20
67
120
1.27

Where the outputs quoted on the manufacturers data sheet are based on temperature differences other than 56 °C (100 °F), the figure used should also be quoted on the data sheet as should the appropriate corrective factors. Always check these figures and adjust the quoted output figures before selecting a radiator.

To select the appropriate radiator for a room, the heat requirement needs to be established (as per the example house). Reference to the manufactures data sheet with any appropriate adjustment, will enable the appropriate size of radiator to be selected, however, it is very unlikely that any radiator will match the exact heat required, so select the first size of radiator above the heat requirement. A number of radiators will probably be suitable, each of different dimensions or with single or double panels, the final choice may depend the space available to mount the radiator.

Different manufacturers will quote slightly different outputs for seemingly similar sized radiators, so where possible always use the figures from the manufacturers data sheet However, as a guide, the following table shows the approximate outputs for various sizes of panel radiators - where a manufacturers data sheet is not available, these figures can be used without much risk.

size (mm)
output
   
single convector
double convector
height
width
watt
BTU
watt
BTU
400
500
396
1350
759
2590
600
475
1620
911
3110
700
554
1890
1064
3630
800
633
2160
1216
4150
900
712
2430
1369
4671
1000
791
2700
1521
5190
1200
950
3240
1824
6225
500
500
479
1635
925
3155
600
576
1965
1109
3785
700
671
2290
1294
4415
800
768
2620
1480
5050
900
863
2945
1665
5680
1000
960
3275
1849
6310
1200
1152
3930
2220
7575
1400
1344
4585
2589
8835
1600
1536
5240
2960
10100
600
400
450
1535
865
2950
500
563
1920
1081
3690
600
674
2300
1297
4425
700
787
2685
1514
5165
800
900
3070
1731
5905
900
1013
3455
1946
6640
1000
1125
3840
2163
7380
1200
1351
4610
2595
8855
1400
1575
5375
2997
10225
1600
1801
6145
3461
11810
700
400
514
1755
982
3350
600
771
2630
1474
5030
800
1029
3510
1965
6705
1000
1287
4390
2456
8380


 

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