8.4.5 Second external heat source
For dual-mode heating systems, always connect the heat pump
into the return of the second heat source (e.g. oil fired boiler).
High heating water temperature: In dual-mode heating systems,
the return water from the second heat source can flow through
the heat pump, immediately after it has been switched off, with
a max. temperature of 60 °C. The temperature may be 70 °C no
earlier than ten minutes after the heat pump has been shut down.
8.4.6 Heat meter
Observe the additional pressure drop when installing heat meters
on the heating side. The sieves inside the heat meters are easily
blocked by dirt particles in the heating circuit, further increasing
the pressure drop.
8.4.7 Oxygen diffusion
Steel radiators, pipes or steel containers can corrode through
oxygen diffusion if permeable plastic pipes are used in underfloor
or open heating systems.
The product of corrosion, i.e. rusty sludge, can settle inside the
heat pump evaporator and can result in output loss through a
reduction of cross-section or in a heat pump shutdown triggered
by the high pressure limiter.
Therefore it is advisable to avoid open heating systems or the
installation of steel pipes in conjunction with plastic pipes that are
permeable to oxygen in underfloor heating systems.
8.4.8 Scaling
Water quality, operating conditions and the water volume are
decisive factors for the extent of scaling. To prevent damage to
valves, heat exchanger and heating elements, check the water
condition and assess it in accordance with VDI 2035 [or local
regulations].
Note: An awareness of the hardness in accordance with the
Detergent Act [Germany] is insufficient. Decisive for scaling is the
concentration of calcium hydrogen carbonate; relevant detailscan
be supplied by your water supply utility.
www.stiebel-eltron.com WPF 20/27/40/52/66 | 11
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