This is one of the most technically advanced units on the market, direct vent and thermistor regulated. Free stainless steel coaxial vent Pipe, ideal for indoor use for whole house, mobile homes, etc. The heating chamber of the water heater is sealed air-tight. Air for combustion is completely drawn into the heating chamber from outside and, at the same time, waste gases are 100% blown to the outside through the direct vent system. There is no pollution or consumption of air in the house and, therefore, no space requirements for combustion, making this water heater one of the safest to use. It is equipped with anti-freezing device to prevent the water heater from freeze damage by lighting up the burner and heating up the heat exchanger when temperature in the environment falls below 42* F (patented technology). Overheating prevention device shuts off the water heater to prevent overheating caused by overly high water temperature or by dry burning when water flow is too low. Flame failure detection device instantly shuts down the gas valve when flame is unexpectedly out. Water pressure releasing valve/water drain plug relieves water pressure when it exceeds the allowable limit in order to prevent water heater from being damaged; free 1/2" BSP (Metric) to 1/2" NPT adapters for gas/water fittings ($45 value).