Heat Emitters - Mitsubishi Electric Ecodan HUS210FTC4ST Installation Manual

Heat Emitters

13
Please note that heat pumps provide lower flow temperatures to radiators than a conventional gas or oil boiler, the
best performance is achieved with lower flow temperatures. Rather than the radiators turning on/off locally as with a
fossil fuel boiler, heat pumps provide a more consistent lower flow temperature which allows for more efficiency and
greater comfort.
Sizing of Radiators
All heat emitters should be selected in order to overcome the heat losses as calculated for the area to be served by
that emitter. In the case of radiators the most important factors which contribute to specific output are the desired
room temperature and the MWT or mean water temperature (flow temperature + return temperature / 2). All radiators
sold in the UK must have their output verified in accordance with BSEN442, a difference between room temperature
and MWT of 50ºC is typically used (bT). A heat pump system will work on a bT of much less than a fossil fuel system,
in the case of a flow temperature of 47.5ºC and a return of 42.5ºC for example a MWT of 45ºC will apply.
At these conditions radiators need to be significantly larger than those used for a conventional fossil fuel based
system. All radiator manufacturers provide adjustment factors so that the correct radiator may be selected when
designing a system to operate at lower flow temperatures, an example of how to use these is given below.
Sizing Example
First the room losses must be established by undertaking a heat loss
calculation to the required standard, in this case room losses are assumed.
Room losses = 1200 Watts (at -3°C outdoor and 21°C indoor)
In the case of a fossil fuel system a radiator may be selected directly from
the manufacturer's brochure.
K2 (double panel/double convector) 450mm x 900mm, output = 1268 Watts
For a heat pump system with a MWT of 45°C and a target room temperature
of 20°C a ∆T of 25°C will apply, the adjustment table from the relevant
manufacturer should be used as below:
In this example the appropriate adjustment factor for 25°C ∆T should be
used, in this case 0.406 is selected.
To work out the corrected output for an existing radiator, multiply quoted
radiator output by adjustment factor
1268 x 0.406 = 514.8 Watts
This radiator will now produce less than half what is required to adequately
heat the room.
To work out the required output as quoted in the brochure for a new radiator running
at a lower flow temperature, divide quoted radiator output by adjustment factor.
1268 / 0.406 = 3123.1 Watts
To adequately heat the room using a MWT of 45°C a radiator that achieves an output
of at least 3123 Watts in the brochure will be required.
K2 (double panel/double convector) 600mm x 1800mm, output = 3200 Watts (at ∆T 50).
It may not be practically possible to fit a radiator this size in the desired position in which case adding an
additional radiator should be considered in order to achieve the required total output.
FTC4 Installation Manual
18
T Adjustment Factor
5 0.05
10 0.123
15 0.209
20 0.304
25 0.406
30 0.515
35 0.629
40 0.748
45 0.872
50 1
55 1.132
60 1.267
65 1.406
70 1.549
75 1.694