From time immemorial, there have been 3E problems of energy, economy and environment. In particular, problems of energy and environment become increasingly serious after the industrial revolution. The demand for gas as an eco-friendly energy source is also increasing. With the demand, the installation and the use of gas boilers have also increased, so the damage to human life by the waste gas (CO and $CO_2$) continues increasing every year. Hence, the aim of this study was to investigate the concentration of CO (Carbon Monoxide) by the length and the variation of the bent tube of the exhaust pipe by installing a boiler with the same method as a household boiler and to discover the harm to humans. For the effect of the length, the allowable concentration of CO is 50ppm, and the 3m of the once bent tube starts exceeding the allowable concentration of CO after 5 minutes, and the 4m and 5m starts exceeding after 3 minutes. In addition, the 1m of three times bent tube starts exceeding the allowable concentration of CO after 3 minutes.