A heat exchanger unit for use with spas or hot tubs. The unit is constructed to be installed in a side wall of a portable spa or hot tub. The unit has an exposed front plate or panel, and the unit is constructed so as to limit the temperature to which this panel can reach. The heater unit includes a finned tube construction and a blower with a double impeller. Flue gases enter into one end of the blower unit, and fresh air enters this end and also the opposite end so that the flue gases mix with fresh air. The discharge from the blower is through louvers in the front panel. The temperature of the front panel does not rise above a limiting value. The heat exchanger unit is automatically controlled and controls the temperature of the water in the spa or hot tub. A control is provided including a pitot tube positioned in the blower outlet whereby to shut down the system in the event of absence or failure of flow of gases which could allow the temperature to rise above the limiting value.