Pre-heating element for frost protection 800x500 - 21000W

If the unit has a high-efficiency counterflow heat exchanger (heat exchanger), ice may form in the exchanger during periods of cold weather. This will result in reduced heat recovery and potentially cause a draught in the dwelling at night. 

In order to prevent icing of the counterflow heat exchanger during cold periods, it is recommended that you install a pre-heating element for frost protection before the unit. The pre-heating element ensures that the outdoor temperature does not fall below -3 ºC. It regulates in steps to avoid unnecessary energy consumption. 

Duct connections 800x500 mm with rubber gasket
Output 21000W
Max. phase current 30,4A
Voltage 3x400V - 50 Hz
Airflow >1,5 m/s
Min. air volume 415 m3/h
Plate type Aluzinc steel plate
Tightness class IP44
Heating element Stainless steel ASI304
Max. thermostat Turns off if the temperature > 50 ºC (at vertical installation > 70 ºC)
Safety thermostat Turns off if the temperature > 100 ºC. Then reset manually

Calculation of correct pre-heating element​:

Δt * air density * air volume [m3/h]

e.g.: The pre-heating element must give frost protection down to -15°C and the air volume is m3/h.

15 * 0,34 * 100 = 510W

In the table below we have calculated the energy consumption when using a pre-heating element for frost protection. The calculations clearly show that it is worth using energy on frost protection because you achieve an overall reduction in energy consumption. 

Air volume 126 m3/h* 216 m3/h**
Frost protection at outdoor temperature -2 ºC -2 ºC
Hours per year *** 676 676
Energy for frost protection via pre-heating element 107 kWh/år 183 kWh/år
Energy loss when icing occurs 105 kWh/år 180 kWh/år
Energy loss when de-icing 200 kWh/år 343 kWh/år
Energy saved with frost protection​ 198 kWh/år 340 kWh/år

*Average for apartment
**Average for detached house
***Average calculated from Danish DRY weather data