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• The outside temperature is higher than approximately 19°C for a sufcient time (how long the bypass stays
on depends on the exact outside temperature).
The outside temperature and the inside temperature are measured in the HRU ECO 4. The development of the
temperatures allows the fan to recognise a summer day. The bypass will be closed automatically as soon as
one of these conditions is no longer met.
The power with which the bypass valve discharges heat depends on the temperature of the air inside and the
temperature of the air outside. It also depends on the air volume discharged by the HRU ECO 4. The rule of
thumb for the HRU ECO 4 is to discharge 50 W thermal power per degree of temperature difference at 150
m3/h (medium mode). If the inside temperature is 25°C and the outside temperature is 15°C, 500 W will be
discharged in the medium mode.
Frost Protection
The heat recovery unit HRU ECO 4 is delivered as standard with frost protection. The frost protection consists,
amongst other things, of a unique frost valve integrated at the top of the unit. This protection prevents the unit
from freezing (internally) during the winter. The operation of the frost protection is fully automatic.
Frost Protection Operation
The air discharged from the house is used to heat up the fresh air coming from outside. The air in the heat
exchanger, therefore, cools down. During a period of frost, the frost protection ensures that the heat exchanger
in the unit cannot freeze.
If the temperature of the discharge air in the heat exchanger nears freezing, the device will open the frost valve
at the top of the unit regularly and take in warm air from the area in which the unit is located. This warm air is
mixed with the cold air coming from outside.
At the same time, the intake fan will turn faster (the fan is revved up) to ensure the volume of fresh outside air
remains the same. The pre-heating of the fresh cold outside air ensures that the warm air discharged from the
home does not have to warm the cold outside air too much. The temperature of the discharge air in the heat
exchanger thus remains safely above freezing.
Should the outside air temperature become even lower, the intake fan will turn more slowly (the fan is revved
down) until it reaches a minimum. If the temperature drops even
further, the discharge fan will be revved up and the intake fan will
keep turning at the minimum rate.
If the outside temperature drops to an extreme low, the intake fan
will be turned off but the discharge fan will remain in operation. The
frost valve will also be closed. After some time, the intake fan will
turn at minimum speed and the frost valve will be reopened to check
whether the risk of freezing has passed. If the outside temperature
rises, the above measures are taken in reverse order until the risk
of freezing has passed. The resident “always” determines the air
volume that is discharged.
The frost valve can be easily removed from the top of the HRU ECO
4 without tools. Push the frost valve open (into the unit) using four
ngers against each other (with ngernail side up). Make sure your
thumb presses the black foam outside the unit (between the valve
housing and the adjacent nozzle). Carefully pull the valve housing
