The inherent characteristic of rotary heat exchangers to carry small amounts of the exhaust fluid into the supply fluid poses a problem in some air-to-air heat exchanger applications .. In situations where either a high degree of purity is required for the supply fluid (e.g., hospital supply air) or where the exhaust fluid is bacteria laden or contains noxious gases (e.g., exhaust from industrial or chemical processes) actual levels of contamination caused by the carry-over become important.

Supply air contamination can result from a combination of several transfer mechanisms. The exhaust air can be trapped within the matrix as it rotates from the exhaust stream to the supply stream. This transfer is termed "carry-over11 and is directly proportional to the volume of the matrix voids and the matrix speed. It is essentially independent of the pressure differential between the two ducts and can be maintained at a minimum by operating the matrix at the lowest practical speed and by the use of a purge device.

Another contaminating effect is the leakage of exhaust air across the radial and circumferential seals. · Its magnitude depends on the type, the condition and the adjustment of the seals as well as the pressure differential between the two streams. Leakage of exhaust air into the supply air may be prevented by using a fan arrangement that maintains the exhaust duct at a lower static pressure than the supply duct.