This paper describes the results of a field measurement of a residential plumbing system recycling rainwater and greywater and its exergy analysis. The followings were found. The amount of rainwater harvested is 48∼98% of rainfall on the rooftop. Quality of greywater is temporally improved by monitoring of the plumbing system and the residents' operating a combined treatment septic tank. The amount of exergy of rainwater or city water is very small, so that they might be similar to environmental water rather than resource. Chemical exergy of fossil fuel of 1086 MJ/month is consumed by a power plant in order to make micro organisms in the combined treatment septic tank consume chemical exergy of wastewater of 365.6MJ/month. Exergy of faeces or garbage is very large and consumed at all. Therefore, it is important to convert the way to consume exergy of these organic matter from just consuming into consuming with producing farm products such as compost or vegetable.
