Why does ice accumulate in the roof fan and the exhaust pipe, with water dripping onto the stove?

Warm air normally rises via ducts and gaps in a building’s internal structures, while cold air moves downwards. When a cold surface and warm air come together, and the relative humidity is 100 per cent, a dew point is formed, causing water to drip from the ducts.

How to solve the problem:

  • Check that the ducts are properly insulated. Insulation thickness on ducts in cold spaces must comply with building regulations. The thickness of the rockwool insulation on the ducts must be at least 150 mm.
  • Check that ventilation in the apartment has been adjusted correctly. Usually, ventilation should be adjusted so that there is about 3–5 Pa of negative pressure. If the problem cannot be eliminated by adjusting ventilation and insulating the ducts, the range hood should be fitted with a fully closing damper. This prevents humidity from rising into the duct and condensing.

If the fan has frozen up and cannot rotate freely, the automatic thermal relay switches on to prevent the fan from breaking down. When the ice has melted, the thermal relay is reset, and the fan is again operable. The easiest way to defrost a frozen fan is to blow warm air into the duct: for example, with a hairdryer.

If the roof fan is switched off immediately after cooking, a large amount of steam and warm air will remain in the ducts. Water vapour condenses in the ducts and the fan motor, forming ice in cold weather. The next time the fan is used, the ice melts, and water drips from the ducts onto the stove. The fan should be kept running as long as possible after cooking so that the ducts have enough time to dry. The best way to prevent condensation is to keep the fan on constantly at low power.