3.4 Notes for using pressure maintenance systems
Pressure maintenance systems in hot-water heating systems keep the required
pressure within predefined limits and balance out volume variations caused by
changes in the hot-water temperature. Two main systems are used:
Compressor-controlled pressure maintenance
In compressor-controlled pressure maintenance units, a variable air cushion in the
expansion tank is responsible for volume compensation and pressure maintenance. If
the pressure is too low, the compressor pumps air into the tank. If the pressure is too
high, air is released by means of a solenoid valve. The systems are built solely with
closed-diaphragm expansion tanks to prevent the damaging introduction of oxygen
into the heating water.
Pump-controlled pressure maintenance
A pump-controlled pressure maintenance unit essentially consists of a pressure-
maintenance pump, relief valve and an unpressurised receiving tank. The valve
releases hot water into the receiving tank if the pressure is too high. If the pressure
drops below a preset value, the pump draws water from the receiving tank and feeds it
back into the heating system. Pump-controlled pressure maintenance systems with
open expansion tanks (e.g. without a diaphragm) introduce ambient oxygen via the
surface of the water, exposing the connected system components to the risk of
corrosion. These systems offer no oxygen removal for the purposes of corrosion
control as required by VDI 2035 and in the interests of corrosion protection should not
be used.
3.5 Return temperature control
If the hot water return is below the minimum return temperature, some of the hot water
outfeed will be mixed in. This is done by the return temperature control, which is
integrated in the hydraulics on the side panel of the boiler.
3
Notes for operating a heating system
Notes for using pressure maintenance systems
22 Froling GesmbH | A-4710 Grieskirchen, Industriestraße 12 | www.froeling.com
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