Heat Interface Units (HIUs) are central to the performance of heat networks. While much focus must be given to primary pipe sizing, central plant efficiency and system hydraulics, one area that is frequently underestimated is ‘keep warm control’. Too often it is dismissed as a “comfort setting” for residents, yet in practice it can strongly influence user satisfaction, energy efficiency, return temperatures, and long-term operating costs. Keep warm control must be treated not as a minor feature, but as a strategic control parameter for both occupant experience and network performance.
The growing deployment of district and communal heating systems across the UK is reshaping the way heat is delivered to multi-residential buildings. HIUs serve as the bridge between the central heat source and individual dwellings, providing space heating and domestic hot water (DHW) through indirect or direct arrangements.
A recurring challenge is balancing the need for responsive hot water delivery with the wider requirement for low return temperatures and energy efficiency. Keep warm functions are designed to ensure that DHW is available quickly – but if not carefully managed, they risk becoming uncontrolled bypasses that undermine the performance of the entire network.
WHY KEEP WARM MATTERS
At the simplest level, the purpose of keep warm is to reduce the waiting time for hot water at the tap. However, its role extends to several system-critical objectives:
• Tenant satisfaction: Occupants expect hot water on demand, while long delays lead to dissatisfaction and complaints.
• Energy efficiency: Responsiveness should be achieved without unnecessary standing losses.
• Low return temperatures: Keep warm must not compromise the differential temperature (ΔT) that underpins efficient network operation, in line with CIBSE CP1 guidance.
• Cost control: Heat wasted in bypass flow or preheating contributes directly to bills and undermines affordability.
• Flexibility: Networks vary, so control strategies must be adaptable to different operating temperatures, load profiles, and occupancy patterns.
By extension, keep warm should be considered not only as an end-user comfort measure but as a determinant of system-level performance.
THE RISKS OF POOR KEEP WARM CONTROL
Poorly controlled keep warm is one of the most common causes of high return temperatures in heat networks. The risks include: 1. High return temperatures: Uncontrolled circulation through
PHEs or pipework can increase the return temperature water to the central plant, reducing condensing boiler efficiency and shortening CHP runtimes.
2. Increased standing losses: Permanently hot distribution pipework raises thermal losses, particularly in laterals. These losses translate into wasted primary energy.
3. Apartment overheating: In modern airtight buildings, unwanted heat gains from pipework can worsen overheating risks, particularly in summer.
4. Increased pump energy: Circulating unnecessary flows increases the energy consumption of pumps, raising auxiliary electricity consumption.
5. Financial impacts: Every kWh of heat lost to poor bypass management is a cost borne by either operators or residents.
The combined effect is not only technical inefficiency, but also reputational risk for heat networks already under scrutiny for cost and performance.
THE TECHNICAL APPROACHES TO KEEP WARM
Modern HIUs provide a suite of functions to manage keep warm intelligently. These can be grouped into the following categories: 1. Exchanger preheating: The DHW plate heat exchanger (PHE) is held at temperature during periods of inactivity. This minimises response time when the tap is opened. Typical standby flows are 3–5litres/hr, but values depend on design and commissioning. 2. Time-based comfort modes:
Programmable scheduling allows keep warm to operate only during predictable demand periods (e.g. morning and evening). Outside these windows, the HIU remains idle, reducing standing losses.
3. Return temperature limitation: To avoid raising system return temperatures, keep warm cycles are limited to return flows of ~38–40°C. This protects ΔT while maintaining readiness.
4. Flow minimisation: Minimal bypass flows are maintained to keep PHEs primed without creating uncontrolled circulation. Flow limitation valves and electronic control loops can help optimise this balance.
5. Recirculation control: In some systems, HIUs manage apartment-level DHW recirculation rather than exchanger preheating. This provides fast response while allowing network-level oversight of flow and return behaviour.
6. Safety and monitoring: Integrated anti-Legionella cycles protect health, while Modbus or BMS integration enables remote monitoring of standby flows, return temperatures, and idle energy use.
CASE-BASED INSIGHTS
The BESA 2023 standard requires HIUs to deliver 45°C DHW within 15 seconds and evaluates standby energy use and volume-weighted average return temperatures (VWART). Keep warm strategies must therefore not only improve response but also prove compliance with these efficiency benchmarks.
The Altecnic SATK32107 HIU has been independently tested in accordance with the BESA UK HIU Test Standard (V3Rev001, Sept 2023) and has achieved full compliance. Results confirm that domestic hot water is delivered at 45°C in less than 10 seconds – well within the 15-second requirement. Standby energy losses remain below 1.0kWh/day, and VWART values are comfortably within limits, ensuring low return temperatures for network efficiency. These outcomes demonstrate both high performance and efficiency, making the unit ideally suited for modern low-carbon heat networks.
BEST PRACTICE RECOMMENDATIONS
To align keep warm control with industry best practice: 1. Prioritise configurability – commissioning engineers should be able to set delay periods, temperature caps, and flow limits to suit network needs.
2. Monitor and verify – Use metering and BMS integration to track idle flows and return temperatures in operation.
3. Optimise scheduling – Limit operation to likely demand windows, reducing unnecessary energy use.
4. Integrate with standards – Design and commissioning should explicitly reference CIBSE CP1 and BESA HIU testing outputs.
"A recurring challenge is balancing the need for responsive hot water delivery with the wider requirement for low return temperatures and energy efficiency. Keep warm functions are designed to ensure domestic hot water is available quickly."
5. Consider future networks – As supply temperatures fall in line with decarbonisation, the precision of keep warm control will become even more critical.
With heat networks playing a central role in decarbonising the UK’s built environment, achieving low return temperatures and efficient operation is a national priority. Keep warm control, often dismissed as a minor feature, is in fact a lever with system-wide implications.
Advanced HIUs with intelligent keep warm strategies provide engineers with the flexibility to meet user expectations while safeguarding network efficiency. In the transition to low-temperature heat networks, the ability to fine-tune standby performance will be essential.
Keep warm is more than a convenience. It is a strategic function in HIUs that determines whether networks deliver on the promise of efficient, affordable, and reliable heat. Poorly managed, it creates bypasses that raise return temperatures, waste energy, and frustrate residents. Well-controlled, it provides responsive DHW, lowers operating costs, and supports compliance with CIBSE CP1 and BESA benchmarks.
The future of heat networks will depend not only on central plant design but also on details like keep warm control at the dwelling level. Engineers and operators must therefore approach it with the same rigour applied to pumps, pipes, and plant, recognising that comfort and efficiency are inseparable goals. www.altecnic.co.uk