Which type of appliance does not mix flue gases with the medium being heated?

An indirect fired appliance is designed specifically to heat a medium, such as water or air, without allowing the flue gases produced during combustion to come into direct contact with that medium. This is achieved through the use of a heat exchanger, where the combustion gases heat a separate fluid that then transfers the heat to the intended medium. This design enhances safety and efficiency by preventing flue gas contamination in the heated medium.

In contrast, direct fired appliances combust fuel and directly utilize the resulting flue gases to heat the medium. This can pose risks of contamination and inefficiencies due to mixing. Condensing appliances also allow for the mixing of gases but focus on recovering heat from flue gases to improve efficiency. Combination appliances often provide multiple functions and may have arrangements where flue gases can enter the medium, depending on their design. Thus, the distinct characteristic of indirect fired appliances is their operational design that keeps the flue gases separate from the heated medium.