Heat pump modes
Heat pumps can work in various operating modes.
Monovalent
The heat pump the only source of heat for a
building all year round. This mode is particularly
suitable for heating plants with low supply-water
temperatures and is primarily used in combination
with brine/water and water/water heat pumps.
Single energy source
The heat pump has an E-heater to handle peak
loads. The heat pump covers the majority of the
required heating power. Occasionally, when it is
extremely cold outside, an electrical booster-
heating system switches on as required in order to
support the heat pump.
Bivalent parallel
The heat pump provides the entire heating energy
down to a predetermined outdoor temperature. If
the temperature drops below this value, a second
heat source switches on to support the heat pump.
There is a distinction to be made here between
alternative operation
with oil- or gas heat and
regenerative operations
with solar energy or
wood-fired heating. This mode is possible for all
heating systems.
Layout
A precise calculation of the building's heating load
according to EN 12831 is required for the design
and dimensioning of a heating system. However,
approximate requirements can be determined
based on the year of construction and the type of
building. The table shows the approximate specific
heating loads for a number of building types. The
required heating system output can be calculated
by multiplying the area to be heated with the given
values
For a precise calculation, various factors must be
considered. The transmission-heat requirement,
the infiltration heat-loss and an allowance for water
heating comprise the total heating output which the
heating system must provide.
The total area of the floor surfaces, exterior wall
windows, doors and roofing is required in order to
determine the transmission heat requirement. In
addition, information about the materials used in
the building is required, as these lead to extremely
varied thermal transmission coefficients (the so
called K value). Also required are the room tem-
perature and the standard outdoor temperature,
that is, the lowest outdoor-temperature on average
that will occur during the year. The equation for
calculating the transmission-heat requirement is
Q=A x U x (t
R
-t
A
) and must be calculated sepa-
rately for all room-enclosure surfaces.
The infiltration heat requirement takes into consid-
eration how often the heated room air is
exchanged for cold external air. The room volume
(V), the air exchange frequency (n) and the spe-
cific heat capacity (c) of the air is also required in
addition to the room temperature and average low
temperature. The equation is: Q=V x n x c (t
R
-t
A
).
An approximate allowance for heating water - per
person according to VDI 2067: 0.2 kW
Example
A residential home comprised of 115 m² living-
space and a heat requirement of 100 W/m² has
been selected for the example design. A total of
five persons live in the house. The heat load
amount to 11.5 kW. Adding a drinking water allow-
ance of 0.2 kW results in a required heat capacity
of 12.5 kW. Depending on the power company, an
additional charge must then be made in order to
factor in the service time-out period. The rating and
determination of the heat pump's balance-point
temperature derives graphically from the heat
pump's temperature-specification heat-output dia-
gram. (In the example, 35 °C for a floor heating-
system). Next, the heat load for the standard out-
door temperature (the lowest temperature of the
year locally) and the heat threshold are marked on
the graph (Fig. 27). The outdoor-temperature-
dependent heating requirement, simplified here as
a straight-line relationship between heat-load and
the start of the heating season, is recorded in the
graph of heat-load curves. The intersection of the
two straight lines with the rated heat-load curve is
plotted on the X axis, where the balance-point tem-
perature is read. (in the example, ca.-3°C) The
least load of the 2nd heat source is the difference
between heat load and the heat pump's maximum
heat output on these days. (In the example, the
capacity necessary to cover peak loads is ca. 4
kW.)
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