States that most large fuel oil boilers are equipped with two-stage burners. Part-load control consists in cycling the burner between 'off' and 'low' and between 'low' and 'high' regimes. Boilers are usually sized for water supply and exhaust temperatures of 60degC and 80degC respectively, in full load. In part load, the water exhaust temperature can be reduced to a minimum, which is usually around or above 50degC. Describes the results of more than 300 test and simulation results obtained over several years. A few results are also taken from the literature. Focuses on the sensitivity of boiler efficiency to water temperature and part load. Maintains that with the results presented in this paper, it is possible to make a fairly accurate estimate of fuel oil boiler efficiency in any regime. Details several levels of approximations which are possible according to the information available.

KEYWORDS: Boilers, testing, models, calculating, oil fired boilers, efficiency, water temperature, part load.