HERZ laying system

Dimensioning
Design and calculation of
surface heating systems
As with all heating systems an accurate
design is the key for the efficient functioning
of the underfloor heating systems. The
calculation and design is done according
to general rules and standards. This results
in a comfortable room temperature, efficient
system with low operating costs. The
dimensioning of the underfloor heating is
according to BS EN 1264 and the heat load
calculation is according to BS EN 12831.
The starting point for the calculation is
the heat requirement for one room. This
depends on the room’s location, building
materials, insulation, number of windows
and other given factors. With the heating
requirement known the dimensioning of the
underfloor heating can begin.
Floor excess surface temperature
In the calculation of the system, the floor
surface temperature should stay below the
physical comfortable temperature (stated
in BS EN 1264). Floor surface temperatures
above 25 °C are uncomfortable over
a period of time and can also lead to
health problems. As the maximum floor
temperature is only essential on a few days
during the year, a maximum temperature
of 29 °C can be applied in the calculation
process for living rooms. For zones which
are not constantly used, like peripheral
zones and transitional areas, a maximum
temperature of 35 °C is permissible.
These temperatures are set according to
BS EN 1264 which states the limit values for
floor excess surface temperatures (living
space 9K, peripheral zones 15 K).
If the heating requirement cannot be
achieved with the use of peripheral zones
then additional heat emitters will be
required.
By using appropriate insulation below the
pipes then the heat lost to the ground or
room below should be lower than 25 % of
the heating output, but definitely lower than
20 W/m 2 .
Calculation steps
Starting point is the heat requirement PN
(according to BS EN 12831)
1) Actual heat requirement
The heat loss from the floor surface can be
deducted from the total heat loss for the
room (as this is the heat emitter):
P NB
P N
P FB
Example:
P NB
= P N
− P FB
actual heat requirement (W)
standard heat requirement (W)
heat loss from the surface (W)
Nominal heat requirement of the standard room:
P N
= 1000 W
Heat Loss above floor surface:
P FB
= 150 W
Actual heat requirement:
P NB
= 1000 - 150 = 850 (W)
2) Calculation of the specific heat
requirement
With the actual heat requirement and the
available heating surface area (room floor)
the specific heat requirement is calculated:
q spec
specific heat requirement (W/m 2 )
P NB
actual heat requirement (W)
A R
room area (m 2 )
Example:
Actual heat requirement of standard room:
Room surface:
P NB
= 850 W
A R
= 15 m²
Specific heat requirement:
q spec
= 850 = 57 (W/m²)
15
For the calculation of the flow temperature
the room with the highest specific heat
requirement (not including bathrooms) is
used and called the standard room for the
calculation.
3) Calculation of the standard room
Only for the calculation of the standard room
the temperature difference between flow
and return is chosen
– according to BS EN 1264 σ < 5 K
Bathrooms are not taken into account as
standard rooms.
4) The average heating excess
temperature
The average heating excess temperature is
the average difference determined between
the average heating temperature and the
standard indoor temperature.
To calculate the standard room, the layout
average heating excess temperature
is used, which exists for selected heat
conductivity resistance for the floor covering
and the layout heat flow density.
The average heating excess temperature
can be viewed in the diagram (page 8).
The average heating under-temperature for
room cooling is then taken from the diagram
(page 8).
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