SCHOOLS | ICT COOLING COST OF RETROFITTING HTM UNITS Quotes were taken from two leading HTM and two air conditioning contractors (early 2019 prices): Ground-floor room (easy access for installation): total installed cost of HTM unit 5,585 v split air conditioning unit 2,533. At more than twice the cost, we calculated it would take the lower energy HTM unit more than 50 years to pay for itself First-floor room (more difficult access for installation): at 7,789 v 2,533, it would take the HTM unit around 100 years to pay for itself. An ICT room at Parrs Wood High School with a split air conditioning unit rooms on a cold morning, but the zone-thermostat shuts it off once other heat gains become significant. Clustering all computer classrooms together does pose difficulties: each of the 10 subject faculties requires one or two such rooms in their area of the school. There would also be increased installation costs, as the majority of heating zones do not have enough spare fuse ways to support the additional 32A ring main required for each computer room. An additional distribution board would have to be installed and wired back to the main incomer. Method 4: Provide comfort cooling with HTM ventilation units After the launch of the Priority Schools Building Programme, the guidelines for thermal comfort in schools were reviewed in 20164 and the adaptive thermal comfort criteria of TM52 were adopted. The new guidelines aim to avoid overheating using low-energy solutions a combination of natural and mechanical ventilation instead of mechanical cooling. Additionally, outside air should be premixed with room air to avoid draughts.5 As a result, several companies have developed hybrid thermal mixing (HTM) units that cool classrooms by mixing in outside air. Cooling results from increased air velocities and, as theres no compressor to power, they are cheaper to run than traditional air conditioning. Unlike air conditioning, they are also intended for use with open windows. The guidelines apply to new schools, but a comparison was carried out to see if HTM units offer a viable, low-energy alternative in existing schools. The costs of installing both types of cooling were assessed in two computer rooms: one on the ground floor, with easy access and a working height of less than three metres, and one on the first floor, requiring scaffolding or a hydraulic lift. HTM units channel large quantities of outside air into a room and need an air duct through the external wall that is considerably larger than the refrigerant feed pipes of a split air conditioning unit in this case, 900mm by 300mm. Forming the opening and installing the outer cowling increases installation cost, especially at heights above three metres (see panel, Cost of retrofitting HTM units). Compared with an air conditioning unit, the payback times are far longer than the 20-year life expectancy of the HTM unit. Additionally, for energy efficiency Salix funding, the payback times exceed the maximum of eight years and, at 338 per lifetime tonne CO2 saved, it exceeds the Salix criteria of 200 per tonne. National trials of HTM units confirmed improved thermal comfort levels and reduced CO2 levels compared with natural ventilation and identified a potential to lower capital costs. However, this was based on the reduction in building costs from avoidance of central AHU ventilation feeding large numbers of rooms,5 as opposed to the cost of retrofitting a single air conditioning unit. The trial report also expected maintenance costs to be lower for HTM units.5 However, maintenance contracts offered in this study were about the same price as for standard air conditioning. Some of our air conditioning units are 15 years old and have not required any mechanical repairs, only the annual service. HTM units also work out expensive in rooms without wet central heating, such as new-build extensions. A heater in the HTM unit would need to be powered by electricity which would be more expensive to run than a heat pump, as it would not benefit from a coefficient of performance or it could be heated by a heat pump, which could be used on its own to condition the space, so would be the cheapest option. Conclusions When converting classrooms into computer suites in existing wet heating zones, the most cost-effective way of minimising comfort cooling costs is to use rooms with low heat gains. Use of a run-on timer and setpoint control is also good practice and can reduce the running cost of a 2.5kW unit by about 47% (66) a year. Avoiding cooling fighting heating by clustering computer rooms into one heating zone is effective, but generally impractical because of other logistical considerations. HTM units have several benefits and offer an excellent solution in new-build schools. However, the capital cost is currently prohibitively high and they incur greater installation costs when retrofitting. The low-cost solutions outlined above are set to become more effective as computers become more energy efficient and are run increasingly by power over ethernet fed from solar PV installations, further reducing heat gains. CJ CHRIS BAKER is a science teacher and coordinator of the student CO2 sustainability team. c.baker@parrswood.manchester.sch.uk References: 1 Sung-Min Hong et al, UCL. (2014) p7. Improved benchmarking comparability for energy consumption in schools. London: bit.ly/CJSep20CvH1 2 HM Government. (2010). L2B Conservation of fuel and power in existing buildings other than dwellings. London: The Stationery Office. 3 HSE. (2018). Risk of falling from windows. www.hse.gov.uk/healthservices/fallswindows.htm. HSE. 4 DfE. (2016). Building Bulleting 101 Update. London: DfE. 5 DfE BB101 draft. (2017). BB101 Draft version 1.3. 54 September 2020 www.cibsejournal.com CIBSE September 2020 p52- 54, 56 School cooling and covid.indd 54 21/08/2020 15:17