Direct gas-fired technology is the most energy efficient choice for heating make-up air because it does not use a flue or heat exchanger. The gas is burned directly in the air stream being heated. That means 100% of the available heat (every BTU of gas burned) is then delivered directly and efficiently to the building.
Indirect gas-fired make-up air heating equipment is only 60% to 80% efficient due to heat exchanger inefficiency and heat lost up the flue. It is also possible to experience condensation within heat exchanger sections of indirect-fired units in cold climates causing corrosion and premature failure.
Do not substitute any type of indirect gas-fired make-up air heater for a Cambridge M-Series direct gas-fired heater. The technologies are different. Performance will be jeopardized and energy costs will increase.