MODELLING HVAC PREDICTED VS ACTUAL USE
designed to utilise heat recovery where heat energy, absorbed by units in cooling, is redirected to units in heating, and vice versa, through the BC controller.
Inputting data The geometry for the model was input using the floor plans. Photographs of the building were used to provide information on constructions, elevation details and building orientation. Internal conditions were assigned to
the spaces based on the designated use for the space, and were derived from the National Calculation Method (NCM) internal conditions. This gave the zones internal gains from occupants, equipment and lighting. The internal conditions also specified a fresh air requirement, which was met in the systems model. A value for infiltration was also included. A small aperture opening is added to
the window frames to allow for additional infiltration. An aperture is also added to internal doors to allow air to move between zones as if the doors were being opened and closed during the day. The only weather data that is available
from the study gives the peak dry bulb temperature each day. This is not sufficient for use in the simulation, so the Swindon Test Reference Year has been selected as a reasonable fit for the Chelmsford location. There are two main air-side systems. One supplies the ground floor with fresh air and the other, the first floor. The fresh air rate is 568 l/s for each system and the sensible heat recovery on the fresh air is 80%.
Simulation results The building simulation results give the zone loads for the spaces in the building. There are hours in the year when only heating or cooling are required. There are also hours in the year when both heating and cooling are required. This is illustrated below for zone loads on the First Floor. For example, heating and cooling are required on day 50, hour 12 (see page 66). These are the zone loads before adding fresh air.
Weekly consumption results The weekly consumption results allow the comparison of measured and simulated data. The consumption figures in the graphs on page 66 are for the 5 City Multi systems and their indoor units. There is a clear correlation between the two sets of results. The consumption in both cases for the three systems on the first floor is
www.cibsejournal.com Ground floor plan
roughly equal to the consumption of the two systems on the ground floor. The peak
weekly consumption in winter is around 1,400 kWh and peak weekly consumption in summer around 1,000 kWh.
Weekly cost results (assuming an historic (2004) time of use tariff of £0.07 per kWh) The consumption figures are taken and multiplied by a tariff of £0.07 per kWh to put them in context of the original report. Peak weekly cost in the winter is around £100 and about £70 in the summer.
Heat recovery The number of hours where there has been demand on the systems for both heating and cooling is given in the table on page
Annual consumption (kWh)
Simulation (installed equipment)
Measured
Simulation (2011 equipment)
42,494
46,821 27,810
Annual cost (£) Improvement (%)
2,982
3,277 1,947
Cost and consumption comparison between simulations and actual use N/A N/A 34.6%
System name Dark green Yellow Red Blue
Light green
Hours 334 407 43
959 956
Hours where there is demand for heating and cooling First floor plan
January 2013 CIBSE Journal 65
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