In recent years, energy saving in greenhouse cultivation has been an objective of many researches. One of measures for energy saving is an accurate control of a room temperature to avoid excessive heating. To achieve such an accurate control with small heaters used in greenhouses, it is necessary to repeat firing and extinguishing with short time intervals.
It has been said that the heat efficiency of the hot water boiler drops with the decrease of the heating load factor due to the increase of the heat loss from the boiler surface. Little is known, however, on the effect of the frequency of the intermittent firing on the heat efficiency, or on the difference of the heat efficiencies between intermittent firing and continuous firing. The objective of this study is to evaluate the heat efficiency at the intermittent firing.
First of all, we evaluated the heat loss caused by the unburned carbon particles, carbonhydrates and imperfect combustion at the start and end of burning and results were compared with those at the continuous combustion. The measurements showed that the unburned carbon particles appeared only at the start of firing and the period of the appearance was less than 1 second. This means that the effect of unburned carbon particles on the heat loss is negligibly small, if the heater burns for more than 1 minute. The heat loss due to unburned carbonhydrates and imperfect combustion which was estimated from their maximum concentrations during combustion was less than 0.5% and was also negligibly small, if the heater burns at the appropriate air ratio. These results show that firing or extinguishing brings about only a negligible heat loss.
After long period of the intermittent firing had been conducted, the heat efficiency at the continuous combustion dropped only 4-5% from the initial value for a type of the hot-air heater easily accumulating soot. It dropped less than 1% from the initial value for a type of the hot-air heater hardly accumulating soot. These results show that the effect of long term intermittent firing on the heat efficiency is less than the value so far estimated without measurements. One of the reasons of this phenomena may be that most part of the heat generated by the heater is exchanged on the surface of the combustion chamber where the soot accumulates hardly.

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