Updated on 24.8.2026
When the heat recovery cell becomes frozen, what should I do? Is it normal that the ventilation unit collects water? Why is it that a mechanical ventilation unit can become frozen?
Is the condensing water tube blocked?
Highly efficient ventilation units produce large volumes of condensing water in cold weather. Before the cold weather sets in, it should be checked that the condensing water pipe is open and not e.g. insects have accumulated in the siphon during the summer months. Condensing water is generated when the humid indoor air comes into contact with the cold heat recovery cell in below-zero weather. The amount of condensing water does not affect the humidity of the air that is blown indoors. As the outdoor air contains very little water when the outdoor temperature is below zero, the indoor air is always dry in below-zero weather.
Has the outdoor air grille become frosted?
Frost can accumulate on the outdoor air grille through which the ventilation unit sucks outdoor air. If tree branches are covered with beautiful frost, the grille should be checked and cleaned e.g. by using a brush. However, the grille must never be covered with a filter or an insect net! Snow can also penetrate into the filter of the ventilation unit. In such a case, it is easy to remove the filter, clean it of snow, and install it back into the unit. The blocking of the grille or the filter increases the underpressure of the apartment. A clogged grille or filter increases negative pressure in the dwelling, whereas in MyVallox CFi units equipped with the constant airflow feature, it results in increased fan power and, consequently, higher energy consumption.
Is the condensate insulation of the ventilation ducts in order?
The purpose of the condensate insulation of the ventilation ducts is to prevent the condensing of water inside cold ducts. The closed cell insulation must extend from above the unit all the way to the vapour barrier (also between the unit and the ceiling mounting plate). If the outdoor and exhaust air ducts located in warm premises have no condensate insulation, humidity and ice can collect on their outer surface. When the outdoor temperature rises, the ice will melt and the melting water will run on top of the ventilation unit and finally down between the sheet metal surface. This can cause the impression that the unit is leaking water from its bottom corners even though the cause is the condensing water of the ducts located above the unit. This can also cause the unit to get broken.
Is the thermal insulation of the extract air pipe sufficient?
Vallox ventilation units are designed to utilise the heat contained in the extract air as effectively as possible. For this reason the exhaust air that is blown outside in below-zero weather is very low, even less than –10°C. In below-zero weather, the ceiling feedthrough or the air blow-out unit mounted on the wall can become frosted. Even the extract air duct located in the attic can become frozen. Ventilation that is insufficient in relation to the humidity load of the apartment contributes to their freezing. Ice contained in the exhaust air duct reduces the extract air flow and can lead to an incorrect operation of the freeze prevention automatics and, eventually, can cause the heat recovery cell to become frozen.
If the exhaust air duct becomes frozen repeatedly, its insulation should be checked. The problem area is often immediately below the roof at the connection point of the extract air duct and the ceiling feedthrough. Water condenses and freezes on the inner surface of an uninsulated duct that is located in a cold attic. The part of the feedthrough that is above the roof must also be insulated and the extract air opening must be sufficiently large. The ceiling feedthrough must usually be 1-2 sizes larger than exhaust air duct. So if the diameter of the exhaust air duct led from the ventilation unit is 160 mm, the rated size of the ceiling feedthrough must be 200 mm or 250 mm.
How the exhaust air duct can be defrosted if ice has accumulated inside the duct?
The exhaust air duct and exhaust air terminal can be defrosted by blowing warm air from a fan heater into the exhaust air duct from the ventilation unit. In newer Vallox unit models, defrosting can be carried out using the unit’s own fan. The latest Vallox ventilation units are equipped with automation that detects freezing in the exhaust air terminal and, if necessary, initiates an enhanced defrosting cycle. Please check that your ventilation unit has the latest software version.
Why does the ventilation unit become frozen?
The ventilation unit must also operate in cold weather and the ventilation must be turned on also in below-zero weather. If the ventilation unit becomes frozen, the reason for this must be examined. The heat recovery cell can become frozen due to incorrect unit settings or a faulty thermostat or sensor. In case of newer ventilation units, however, it is normal that there is some ice in the heat recovery cell from time to time. This enables making the unit as efficient as possible.
The defrosting settings of Vallox ventilation units that are equipped with preheating or the stopping of a supply air fan are, depending on the unit model, e.g. stopping of the supply air fan, preheating, and hysteresis (the difference between the starting and stopping temperatures of the fan or the preheating resistor). Factory settings can be found in the manual of the ventilation unit. If frosting continues to occur, the above setting values can be increased by one degree from the factory settings.
In ventilation units that are equipped with a MyVallox control panel, the settings of the defrosting automatics are the relative humidity of the indoor air (factory setting 0%) and the outdoor temperature (factory setting 20 C). More detailed instructions can be found in the MyVallox WebHelp by searching for “Defrosting settings”.
No changes should be made to the unit settings unless their purpose is fully understood. If you have questions or problems related to the operation of the ventilation unit in winter, please contact an authorised Vallox service centre.
How does the defrosting work?
All new Vallox ventilation units are equipped with demand-controlled defrosting automation that defrosts the heat recovery cell only when necessary. This ensures a high annual heat recovery efficiency.
The automation continuously monitors temperatures around the heat recovery cell and determines when ice begins to accumulate. When excessive ice has formed, the cell is defrosted. When the heat recovery bypass function is enabled, the ventilation unit bypasses the supply air side of the heat recovery cell during the defrost cycle, and the heat contained in the extract air is used to melt the ice. Supply air continues to flow past the cell throughout the defrost cycle, so the balance between supply and extract air remains unchanged.
During the defrost cycle, the supply air is heated by the unit’s post-heating element. Larger models are also equipped with an additional heating element, preventing cold outdoor air from entering the building during the cycle.
During a defrost cycle, the supply air temperature may temporarily fall below the set value. However, the supply air continues to pass through the filters as normal. Ice may occasionally be present in the heat recovery cell, and condensation water may accumulate at the bottom of the unit. This is a normal part of the unit’s operation. Alternatively, the supply air fan stop function can be selected as the defrosting method. In that case, negative pressure within the building may increase during the defrost cycle.