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Three different investigations on intermittent heating in well insulated buildings were carried out, two of them dealing with buildings insulated according to the current Building Regulations BR-82 (a school and an office building) and one dealing with a one-family low-energy house having a transmission heat loss that is only 30-35% of the loss from a similar house insulated according to BR-82. The objective was to examine to which degree demands on a reasonably short re-heating period should be included in the design criteria for heating systems. Presently, heating systems in Denmark are designed to meet a steady-state heat load, calculated with a design outdoor temperature of -12 deg. C. Experiments with cooling-off and heating-up periods were carried out in rooms in the three buildings, and monitoring results were used to adapt computer models to an acceptable agreement between calculated and measured temperatures. The models were used to analyze heating-up sequences under different external conditions and heat emitter ratings. The need was evident for larger power ratings than those specified in the current design rules, meeting the steady-state demand at the set design temperatures, typically at least twice the steady-state value to obtain satisfactory re-heating after night set-back, especially for buildings that just meet the demands in the Building Regulations or are only slightly better insulated. Here, the temperature might drop 4-6 K during the set-back period, and the re-heating period can be unreasonably long, if the heating system is not over-sized compared to the present steady-state design rules. If a low-energy house has an even lower available re-heating power subsequent requirements can be supplied by a small portable fan-coil heater. (AB)