The efficiencies of a new oil fired boiler and new natural gas burners in one family houses were measured for long monitoring periods [2]. The boilers were the only available heating source in the investigated houses. In Fig 1, system efficiencies from 3 different sites on monitored during the last 2 years are shown. All boilers are of modern technology and they all were installed in the year 2002 and later. The system efficiency is calculated as (QDHW: Domestic hot water demand [kWh], Qsh: Space heating demand [kWh], Qfuel: Gas/Oil

consumption [kWh]): Qdhw + Qsh

Qfuel