Heat Transfer through Building Envelopes in the Conditions of Increasing Outdoor Air Temperature and Coolant Temperature Fluctuation

Introduction. With rising energy prices and stricter environmental regulations, it is critical to find hidden savings economy. Recently, for improving building energy efficiency the preference is given to intermittent operation heating systems among which low-frequency fluctuation of the coolant temperature in the heating system circuits occupies a special place. Aim of the Study. The study is aimed at evaluating the effect of an increase in outdoor air temperature and the coolant temperature fluctuation on a decrease in the density of heat flow through the envelopes of an individual design constructed building. Materials and Methods. The experimental study was conducted during increasing outdoor air temperature under the influence of solar radiation and coolant temperature fluctuation. Low-frequency coolant temperature fluctuation mode in the building heating system was simulated using the patented technology. To the heating system circuits, there was periodically supplied hot or cooled coolant, the frequency of which varied within 0.001–0.003 Hz and amplitude varied within 10–20 °C depending on the coolant temperature, and the outdoor and indoor air temperatures in a separate room of the residential building. The heating system was additionally equipped with a three-way control valve, a control unit with sensors for measuring coolant temperature in the supply and return pipes, and outdoor air temperature. The parameters of the coolant, the temperatures of the outdoor and indoor air in the room at the measuring points were monitored using an automated measuring system. Results. With the traditional method of coolant supplying to heating circuits, the temperature difference between indoor and outdoor air decreases by 4.6 °C, and with a fluctuation of the coolant temperature by 6.9 °C. When using the traditional method of supplying coolant to the heating system circuits, in the zones of measuring the envelope surface temperatures the excess of the average values of the heat flux density over the average values of the heat flux during the coolant fluctuation ranges from 34.49% for the zone 2.5 m from the floor to 47.42% for the zone behind the heater. Discussion and Conclusion. During the study, it was found that the decrease in heat flux density during the coolant fluctuation depends on the temperature measurement zone of the surface of the building envelope; the maximum value is fixed behind the heating device. Therefore, heating devices should be located on the internal building envelope.