HOT WATER DISPENSERS | TIMER SWITCHES segment- or seven-day digital timers. If financial investment is required to facilitate the project, the anticipated cost savings can be quantified using granular electricity consumption data, which can help to support the business case. Assessing the energy-saving potential In a real-world project, the energy consumption of a typical hot water dispenser was monitored with a pre-programmed portable energy meter, to quantify its out-of-hours energy use and to calculate the associated running costs. Granular data was collected in half-hourly intervals over a period of five weeks. The flexible CT current sensor was installed by a qualified electrician to ensure no other power-consuming equipment would be monitored during the trial, and the collected data was later analysed in a spreadsheet. Figure 1 shows a typical week during the trial period. The plotted electricity data gives a clear indication of when the building is occupied generally between 7am and 6pm in this instance and what the baseload consumption is during out of hours and on weekends, when the building is empty. The analysis of the numerical data suggests that the daily average consumption of this particular unit is 0.6kWh from 00:00-07:00hrs, 1.95kWh from 07:00-18:00hrs and 0.46kWh from 18:00-00:00hrs on weekdays; and 2kWh from 00:00-23:59hrs on weekend days. The baseload consumption of the unit is 0.1kWh per hour (or 0.05kWh/30min). The equipment operating outside of the period 7am to 6pm wastes electricity and money, so installing a seven-day timer switch is a reasonable energy-saving intervention. To be able to accurately quantify the potential savings, it is worth noting that, with this measure, temperature loss over night and on weekends will occur. So extra energy will be required at the beginning of each work day to heat up the water to target temperature, and the savings calculation must account for this. To calculate the extra energy required, the following equation can be used: Q =c m T where: Q total heat required to reach target temperature c specific heat capacity of water m volume or mass of water dT temperature rise The electricity saving can be as much as 375kWh per annum, enough to boil 4,000 litres of water to make 1,600 cups of tea MONDAY 0.6 TUESDAY WEDNESDAY Superimposed over the original data, Figure 2 shows the modelled half-hourly electricity THURSDAY FRIDAY SATURDAY SUNDAY kWh / 30min 0.5 0.4 0.3 0.2 0.1 0 28/11/2016 00:00 28/11/2016 12:00 29/11/2016 00:00 29/11/2016 12:00 30/11/2016 00:00 30/11/2016 12:00 01/12/2016 00:00 01/12/2016 12:00 02/12/2016 02/12/2016 03/12/2016 03/12/2016 04/12/2016 04/12/2016 00:00 12:00 00:00 12:00 00:00 12:00 Figure 1: Time series of half-hourly energy consumption data (5 litre vessel capacity, 3kW tabletop water heater) Legend MONDAY Day 07:00 Time 0.61 Consumption in period without timer (kWh) Energy usage without timer Table 1 summarises the average electricity consumption during the different periods of the weekdays and weekends Out-of-hours consumption (average per weekday) 00:00-07:00 0.6 kWh/day Daily average consumption (weekday) 07:00-18:00 1.95 kWh/day Out-of-hours consumption (average per weekday) 18:00-00:00 0.46 kWh/day Weekend average consumption (average per day) 00:00-23:59 2 kWh/day Table 1: Summary of electricity consumption during different times of the day 56 April 2021 www.cibsejournal.com CIBSE April 21 pp55-57 Hot water dispenser.indd 56 26/03/2021 14:44