Off-line filtration is in many cases the only feasible principle to achieve the required system cleanliness in large air conditioning and heating systems.
An off-line filter is used to keep impurity levels in a medium within a required range. An example is e.g. 5–10% for a coolant which is filtered in parallel with the main line before being returned to the main line. An off-line filter collects the impurities in a filter element.