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Water heating
disruptive, a cause for concern if refurbishment work is scheduled into the narrow window afforded by the school holidays. This can best be avoided by collecting live onsite data first. A valuable, non-
invasive, and low cost exercise, it should be undertaken to assess actual usage, including time and duration of peak demands which is critical for correct sizing. When assessing a school’s domestic hot water (DHW) usage, it is important to also establish basic information on energy sources, be they gas or electric, planned use of renewables such as heat pumps or solar thermal and the level of system redundancy and backup. This helps steer the design of the replacement system. This approach has been applied to several public sector sites in the UK
where there is strong impetus from the government for properties to be rapidly decarbonised in line with net zero strategies. Data collected by Adveco has enabled our application design team to provide recommended alternatives that avoid common issues arising from oversizing. One recently assessed dormitory site was operating two 50kW output gas-fired
Size right for cost-effective outcomes
Do your homework before replacing gas-fired water heating in schools, advises Bill Sinclair, technical director at Adveco
T
he most consistent issue we see in school hot water systems is oversizing, whether through lack of understanding of application design or concerns over providing suitable back up to ensure
system continuity. The result of oversizing is however always the same, unnecessary capital costs for system supply and installation, and ongoing excess operational costs associated with higher energy demands and therefore greater carbon emissions. As schools plan to adopt greener building operations, replacing old gas-
fired systems with like-for-like electric is another guaranteed way to gain an oversized system. This is especially the case if theoretical hot water use is in excess of actual usage, which occurs more often than realised. In this scenario, the school is already over-paying on running costs. The replacement system exacerbates that problem, whilst adding excess capital costs to the replacement project in terms of size and number of water heating appliances and complexity of installation. That in turn can also become more time consuming and
water heaters and a pair of 140 kW boilers. Replacement plans included making use of two additional electric boilers with a heat pump to heat the building. Live metering indicated that property exhibited an average daily usage of 1793 litres with a maximum daily recorded usage of 2407 litres, though averaging out to 2003 litres. The single peak, spread between 7am and 10am, was contrary to the perceived dual morning and evening peak. With a long, low single morning peak the theoretical design day hot water consumption could be set at 2789 litre. On this basis, with a 20% uprate added to ensure excess demand is
accommodated in the system design, requirements can be met by a 24kW electric system with 250 litre storage. This assumes 40°C preheat feed from the air source heat pump, 63°C storage and supply at 60°C. The recommendation would be for a 500L twin coil cylinder, with the bottom half preheated by heat pump and the top half heated by a 24kW electric boiler, such as Adveco’s ARDENT. The storage cylinder is derated by 50% as only the top half is guaranteed to store water at a usable 60°C. A smaller boiler could be specified and would cope as well, but without the ASHP preheat the full 24kW would be required increasing operational costs. The new design will have an estimated annual consumption of 616,564 litres. With an estimated 16,544 kWh thermal energy demand for the year, carbon emissions fall from 8340 kg for the existing gas fired system to 3250 kg for the new electric heated system. However, annual electric running costs, despite a 25-35% offset in energy from the heat pump, would be an estimated £2813, compared to lower priced gas costs of £689. Replacing gas fired water heating with an electric system still has several
cost implications. Correct sizing with metered data can reduce costs of purchasing and installing new hardware, potentially saving tens of thousands of pounds depending on the scale and complexity of the DHW application. Operational costs do however climb and will continue to do so while grid electric prices remain much higher than those of gas grid supplies. The application of renewables including heat pumps and solar thermal can reduce, but not completely offset those direct electric costs. The advantage is clearly defined in the reduction of carbon emissions, and, as work continues to decarbonise the electricity grid, the emission reduction figures supplied in the new system design should improve considerably, adding further environmental value to the system over the course of its operational lifespan. Decarbonisation of hot water still comes with implicit operational costs. This needs to be recognised and factored into the school energy budget as part a successful decarbonisation strategy.
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April 2023
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