Summary
Welcome to another Adveco hot water Q&A session. This month, we respond to questions on hybrid approaches for sustainability, transitioning from gas to electric and things to be aware of when using electric for DHW, plus our thoughts on the current generation of high-temperature heat pumps...
Welcome to another Ask Adveco hot water Q&A session. This month, we respond to questions on hybrid approaches for sustainability, transitioning from gas to electric and things to be aware of when using electric for DHW, plus our thoughts on the current generation of high-temperature heat pumps…
Why are hybrid water heating systems more sustainable?
Despite the best efforts of manufacturers, in terms of sustainable alternatives to gas water heating, there is still no single, cost-effective technology that meets the exacting demands for safe hot water provision. That is, providing the water at high enough system temperatures (+60°C) to ensure there is no danger of a Legionella outbreak. This is why the hybrid approach to water heating is so advantageous since it combines multiple technologies, such as electric and heat pumps and/or solar thermal, maximising efficiency to reduce energy demands, carbon emissions and running costs.
Air source heat pumps (ASHPs) have become the popular option for heating and hot water.
The technology works the same way as a fridge, only in reverse, drawing heat from often cold ambient air by compressing a refrigerant. This means the technology is very efficient when operating at lower temperatures, making it excellent for space heating where system temperatures can be low (35-40°C). The technology will struggle to meet the higher temperatures required for water heating, increasing the demand for electricity as efficiency drops, thus costing more to operate. The sweet spot is to push the heat from the ASHP to 50-55°C, giving the best value in terms of consistent heat generation versus cost. This is fed to the hot water system as preheat and then topped up to meet peak demands by the primary electric boiler, or an immersion, which acts as a system backup. This is an optimal hybrid system for small to mid-sized buildings, as it maximises the heat pump, enabling smaller, more compact, quieter and lower-cost ASHPs to be implemented. Larger, more complex structures can also integrate solar thermal as a mid-heat function.
We are considering an electric and heat pump system for my building for water heating. Are there any common pitfalls we need to avoid?
The value of air source heat pumps (ASHP) is that they can achieve carbon reductions of 42-47%, whilst saving 25-35% of the energy costs when compared to an equivalent-sized direct-electric only system (i.e., from the grid). It is, however, a technology that operates most efficiently at lower temperatures. Still, domestic hot water (DHW) systems for commercial properties require a 60°C working flow for safe operation and anti-legionella processes. This does not prevent the use of ASHPs as they can be pushed to deliver a higher percentage contribution, generating working temperatures of 45-50°C for preheat. This is attained at the cost of performance efficiency, requires electrical energy, and has operating cost implications.
Your system will still be required to top up the heat to the necessary 60°C, which is best achieved with an electric boiler. It is important to recognise that this, combined with the heat pump’s reduced operational efficiency, means it will still be much more expensive to run than an equivalent-sized gas-fired system. The recommendation in this case is to keep electrical demand down by increasing the size of the hot water storage, which is then heated more slowly. A 30kW energy source can heat 750 litres/hour by 34°C, so when the system draws hot water at a faster rate than it can be heated to 44°C, such as for hot showers, you would get complaints that the water is ‘cold’. Integrating a larger volume cylinder helps to overcome this undersizing, allowing for a two-hour reheat cycle that maintains enough water at 60°C to meet daily demand, whilst slowly heating reserves through the night when demand is minimal to meet the morning peak. This is a very different approach to the high energy input and low storage seen with traditional gas-fired systems.
What do you think about high-temperature heat pumps for DHW?
There are new ‘high temperature’ heat pumps available on the market, which use either CO₂ or propane (R290) as natural refrigerants, but both technologies have issues that require overcoming. CO₂ systems still struggle to attain efficiency and have a tendency to shut down when there is a warm system return, which is common in commercial hot water systems.
Propane in heat pumps is currently unrestricted, which has raised a red flag with BESA, as it has concerns over the transportation, storage, and use of the technology due to its high potential for flammability. Given the latest building regulation changes in the wake of the Grenfell enquiry, Adveco is currently advising against the integration of propane-based heat pumps until regulations on their use are clarified.
We are investigating the options for moving from an existing gas water heating system to a more sustainable technology. What do we need to know?
It’s important to realise that carbon savings and costs are not aligned. As an example, if we take a building with an average occupancy rate of 23.5 with provision of basins, and shower/wet rooms, typically seen in student accommodations, care homes or boutique hotels, the yearly running costs resulting from a change from gas to direct electric would increase from £1019 to £3019 (based on electricity on average currently costing as much as 3.8 times that of gas). Even with an ASHP operating at optimum efficiency (for 35% recorded reduction in energy), costs would be £2862. Close to three times that of gas alone, so it is inherently important to consider the nominal value of the carbon reduction, especially if planning a refurbishment from gas to electricity. Ensure your sustainability strategy takes into account both the capital investment and running costs when balancing against carbon reduction gains.
As you are already gas connected, it may be worth considering updating your gas water heating and supplementing it with either solar thermal or air source heat pumps to offset primary heating demands, to lower energy consumption, costs and carbon e missions. This approach provides a level of futureproofing and a bridge to new technologies, such as green gas, expected to develop in advance of the 2050 net zero deadline.
If you have a question you would like answered by the team, feel free to send it to marketing@adveco.co, and we will aim to include it in future Q&As.
