In-situ comparison between the energy performance of an in-slab gas heating system and a split system air conditioning in classrooms

Publication Name

Energy and Buildings


Sub-metered energy use of an in-slab gas hydronic heating system and a newly installed split system air conditioner in thermodynamically-equivalent classrooms are compared during unoccupied (with artificially imposed internal heat gains) and occupied periods. A preliminary comparison is also undertaken to quantify the heating penalty for the existing gas-fuelled hydronic heating systems when windows remain open (e.g. to comply with COVID-19 guidelines in schools) against the energy needed for heating by the same system in a control classroom with windows closed. The performance evaluation of the heating systems during three unoccupied days where internal heat gains were artificially imposed demonstrated that the gas hydronic system used 41.5 kWh of metered energy while the AC split system used 7.2 kWh, with the AC split system being able to maintain the internal conditions above the set point by operating for approximately-one third of the time the hydronic system was operating. Occupied tests confirmed the observations of the unoccupied tests of equivalent operation. Over a period of 2 occupied winter weeks, the in-slab gas hydronic heating system used 274.6 kWh of metered energy and operated for 35¼ hrs, while the AC split system used 34.7 kWh and operated for 22¼ hrs, albeit on some occasions the occupants in the AC split system's classroom chose not to turn on the system. After selecting days from this occupied period when both systems were used for long periods, it was observed that the AC split system used approximately 4 to 7 times less metered energy than the in-slab gas hydronic system (or 3.3 times in terms of primary energy when worse-case primary energy factors were used). Finally, from the preliminary test, the parallel measurements also confirmed that the additional ventilation in schools through opening of windows increases the energy use of the hydronic heating system; 66.8 kWh were measured for heating needs over a period of 3-days as opposed to 25 kWh for the windows open and closed cases respectively. These precise values are, however dependent on the season and building type.

Open Access Status

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Article Number


Funding Sponsor

Australian Renewable Energy Agency



Link to publisher version (DOI)