Updated on 30.8.2026
The energy demand of post-heating elements in ventilation units has decreased as heat recovery efficiencies have improved.
Older Ventilation Units Feature a Separate Post-Heating Element or Water-Based Heating Coil
When mechanical ventilation became more common, the heat recovery efficiency of ventilation units was only around 40 to 50%, significantly lower than it is today. This meant that the ventilation unit alone could not warm the supply air sufficiently for residential spaces. To achieve a comfortable supply air temperature, ventilation units were therefore equipped with either an electric post-heating element or a water-based post-heating coil. For example, post-heating was required in the Vallox Digit2 SE unit.
The operating costs of a water-based post-heating coil were relatively low, but its higher investment and installation costs could rarely be justified within a reasonable payback period, especially in smaller properties. Electric post-heating was also a more reliable solution, as events such as power outages or circulation pump failures during freezing conditions could not damage an electric heating element.
The table below can be used to estimate the energy consumption of post-heating for older ventilation unit models. The calculation for Vallox 90 also applies fairly accurately to other smaller ventilation units equipped with a counterflow heat exchanger and no pre-heating. The calculation for Vallox Digit2 can similarly be used when assessing units equipped with a crossflow heat exchanger, such as MUH Ilmava, Ilmava 100 and 120, Ilmava TSK, Digit S, Digit SE, Vallox 75, 95, 130, and 132E. Of these models, only Digit2 and Vallox 132 are equipped with pre-heating. The calculation for Vallox 150 can be used for ventilation unit models featuring both pre-heating and a counterflow heat exchanger.
Here you can download a calculator for calculating the energy consumption of your ventilation unit.
Modern Ventilation Units Do Not Require Separate Post-Heating
All MyVallox ventilation units currently on the market feature a heat recovery efficiency of over 75%, enabling the unit to heat the supply air to above +17°C for most of the year using only the heat energy recovered from the extract air. As a result, additional supply air heating with a separate post-heating device connected to the ventilation unit is no longer necessary in modern systems.
Vallox CFi units are equipped with an integrated electric post-heating element, which operates when needed, for example during winter conditions when heat recovery alone is not sufficient to raise the supply air temperature to the desired level.
Electric post-heating and the primary energy indicator of the building
The new energy regulations “punish” electric heating with an energy factor of 1.7. However, the consumption of energy for post-heating and therefore its impact on the primary energy indicator is very small if the correct starting data and a sensible supply air temperature are used. An electric radiator is still the only sensible option for small-dwelling ventilation units as it is not the purpose to heat the dwelling.
How Should the Heat Recovery Function of a Ventilation Unit Be Used?
The ideal supply air temperature is largely a matter of personal preference. However, the supply air temperature setting should generally be kept as low as possible without causing a sensation of draught. The supply air temperature should never be set higher than the room temperature, as it is not practical or energy-efficient to heat a dwelling using the ventilation unit. In practice, a supply air temperature of approximately 16-18°C often provides a good balance between energy efficiency and indoor comfort, allowing the ventilation system to operate correctly without creating draughts.
Modern Vallox ventilation units feature automated heat recovery control. The units continuously monitor air temperatures and adjust heat recovery operation automatically. There is no longer any need to manually switch a summer/winter damper, as the unit bypasses heat recovery automatically whenever the outdoor air temperature exceeds the selected supply air temperature setting. The unit can also recover the cooling effect of summer nights when the cooling function is enabled and the outdoor temperature is lower than the set temperature. The selected temperature setting can be viewed and adjusted, if desired, via the MyVallox control panel or MyVallox Home.
If the air supplied through the ventilation valve feels uncomfortably cool (it should be noted that moving air below 37°C will always feel cool against the skin when standing close to the valve), it is advisable to measure both the air temperature leaving the ventilation unit and the temperature of the air supplied through the room valve. If the air cools by several degrees while travelling through the ductwork, the duct insulation may be inadequate. The direction of airflow from the valves, window performance, and the heating system can also contribute to the sensation of draught.
The Impact of Ventilation Ductwork on Heat Recovery Energy Efficiency
The placement of ventilation ducts plays an important role in the performance of heat recovery. If the ducts run within the building’s heated envelope, for example enclosed near the ceiling, the supply air will warm slightly as it travels through the ductwork due to internal heat gains within the building. This can improve indoor comfort without increasing energy consumption. However, in modern airtight and energy-efficient buildings, unnecessary heating of the supply air should be avoided, as overheating may become an issue.
If extract air cools down in poorly insulated ducts installed in an attic space, the energy efficiency of the ventilation unit is reduced because the air entering the heat recovery cell is cooler. When the extract air temperature drops by an average of one degree Celsius between the extract air valve and the ventilation unit, the annual heat recovery efficiency of the ventilation unit decreases by almost four percentage points.
When the ductwork is installed on the warm side of the vapour barrier, heat recovery operates efficiently and the temperature of the supply air delivered through the ventilation valve may even be higher than when it left the ventilation unit.
Actual energy consumption of post-heating radiators
The adjacent table shows the consumption of energy for post-heating in different ventilation units with different air flows and supply air temperatures. The calculation concerning Vallox 90 corresponds quite accurately with other smaller units with cross-counterflow cells, such as TSK Multi 50 and 80, 096, 121, 110 and 145. Correspondingly, the calculation for Digit2 can be used for estimating energy consumption for the models Vallox 75, 95 and 132E. Similarly, the calculation for Vallox 150 can be used for larger units, such as Vallox 245.