If roof stacks have been fitted to the air intake and outlet, check that they are not blocked with birds'nests, leaves or other dirt which can hamper air passage.If, instead of roof stacks, grilles have been mounted in facades or eaves, check that they are notclogged with leaves or dirt. Grilles are particularly likely to become clogged.Check ventilation ductsIt is important for operation of the unit that there is free air passage through the ventilation ducts.After some years of operation, dirt will attach itself to ventilation ducts or tubes, and accumulationsmay lead to higher pressure drop in the ducts, leading to higher power consumption. It is thereforeimportant to clean out the ducts when too much dirt has collected.After attending to the inlet and outlet valves, it will be advisable to have them adjusted again, toensure optimum operation of the ventilation system.However, it will not be necessary to clean ducts more than every few years.The heat pumpThe heat pump must be inspected in accordance with applicable laws and regulations, such that it iskept in good condition and meets safety and environmental requirements.The installer is obliged to inform the owner/user about applicable laws and regulations.16