# 6 calculating radiator sizes, Calculating radiator sizes – Grant Products HPAW155 User Manual

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Calculating

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Most existing wet heating systems will
use radiators as emitters. When the
original system was installed, the
according to the manufacturer’s
specifications. Typically, this would have
been 82ºC flow and 71ºC return with
the connections being flow at the top
and return at the opposite bottom
corner.

Existing systems

With the advent of condensing boilers,
most installations were found to have
oversized radiators and as such, little or
no adverse effects were found when
the system temperatures fell to 70ºC
flow and 50ºC return.

However, as heat pumps work at
temperatures lower than even this, it is
important that each radiator is checked
again for its suitability and replaced with
one of the correct size/output if
necessary.

As can be seen, the size of radiator
required will be larger than conventional
systems. This can be controlled to an
extent by choosing a suitable design
water temperature. The trade off will be
a slightly lower COP. As we have
running temperature, the harder the
heat pump has to work to reach the
desired temperature.

any case, the radiator will not get
‘hot’. The perception may well be
that the system is not working
only ‘warm’.

Below is a typical radiator conversation factor table* and a worked example of sizing radiators for use with a heat pump

A typical heat pump operating to feed radiators will run at a flow temperature of 50ºC
and a return temperature of 40ºC – giving a mean water temperature of 45ºC.

In the case of a system using both radiators and Underfloor heating (UFH) a flow of
40°C and a return of 30ºC – giving a mean water temperature of 35ºC – would usually
be preferred.

For a living room with a design temperature of 21ºC and heat loss of 1.8kW.

The

ΔT = 45ºC – 21ºC = 24ºC.

From the radiator manufacturers correction factor table: for

ΔT = 24°C factor ≈ 0.406.

For a design heat loss of 1.8 kW: the required corrected output is 1.8 / 0.406 = 4.43kW.

Select a radiator from manufacturer’s information that would give 4.43kW output (at
75°C mean water temperature) – this will give the required 1.8 kW output at 45°C
mean water temperature produced by the heat pump.

Similarly, for a bedroom with the same design heat loss but design temperature of 18°C.

The

ΔT = 45°C – 18°C = 27°C.

From the radiator manufacturers correction factor table: for

ΔT = 27°C factor ≈ 0.46.

For a design heat loss of 1.8 kW: the required corrected output is 1.8 / 0.46 = 3.48kW.

Thus, select a radiator from manufacturer’s information that would give 3.48kW output
to give the required 1.8 kW output at 45°C mean water temperature.

For an UFH system with a mean water temperature of 35°C.

For a design heat loss of 1.8kW and a design room temperature of 18°C.

The

ΔT = 35°C – 18°C = 17°C.

From the radiator manufacturers correction factor table: for

ΔT = 17°C factor ≈ 0.26.

For a design heat loss of 1.8 kW: the required corrected output is 1.8 / 0.26 = 6.92kW.

Thus, select a radiator from manufacturer’s information that would give 6.92 kW output
to give the required 1.8 kW output at 35°C mean water temperature.

information for the correction factors for different types of radiator.

ºC

Correction Factor

5

0.050

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

1.000

55

1.132

60

1.267

65

1.406

70

1.549

75

1.694