A heat exchanger for an apparatus including a burner has at least one tube extending along a centerline from an inlet end adjacent the burner to an outlet end. A plurality of indentations is formed in the tube adjacent the inlet end and extend radially inward towards the centerline. The indentations are formed in opposing pairs extending towards one another to a depth sufficient to create turbulent fluid flow through the inlet end of the tube.

A passive heat recovery or defrosting apparatus features an evaporator, a condenser, and vapour and liquid conveyance lines connected therebetween. The vapour and liquid conveyance lines respectively connect to upper and lower ends of the evaporator and condenser. The evaporator and/or condenser has a ring-shaped body for fitting around or inline with a pipe to achieve heat exchange relation with a fluid passing therethrough. The evaporator is installed on or inside a warm pipe or duct (e.g. waste drain pipe, clothes dryer exhaust duct, flue pipe, or indoor section of a sewer vent stack) at a lower elevation than the condenser. The condenser is placed on an outdoor end of either a sewer stack or air intake duct for defrosting purposes, or is placed on a water supply line or air intake of a hot water tank, clothes dryer, etc. Working fluid circulates passively between the evaporator and condenser.

The disclosure relates to the field of ventilation systems, specifically to ventilation systems in residential and non-residential (commercial) buildings. Specifically, the invention relates to decentralized supply and extract units with waste heat recovery and can be used for transmitting heat and power from an extraction air flow, i.e. from a flow of air extracted from a building, to a supply air flow, i.e. a flow of fresh air being drawn into a building, for the purpose of replacing spent air from the building.