Designing Commercial Heat Pump Systems for Domestic Hot Water: Key Engineering Considerations
- MTS DNC ENERGY CONSULTANTS LIMITED
- Jul 6
- 5 min read
As Ireland transitions towards low-carbon buildings, heat pumps are becoming the preferred heating solution for commercial developments. While many engineers focus on sizing heat pumps for space heating, designing a system to produce domestic hot water (DHW) is often the more demanding task.
Unlike space heating, domestic hot water requires consistently high temperatures throughout the year, regardless of the weather. This places additional demands on the heat pump, affects efficiency, and requires careful consideration of storage, controls, refrigerants, and system configuration.
This article explains the key engineering principles behind commercial hot water heat pump design and why proper sizing is essential for achieving both performance and energy efficiency.
Why Domestic Hot Water Is More Difficult Than Space Heating
Space heating demand changes throughout the year. During milder weather, modern heat pumps can reduce their flow temperature through weather compensation, significantly improving seasonal efficiency.
Domestic hot water, however, follows very different rules.
Stored hot water must normally be maintained at 60°C or above to minimise the risk of Legionella bacteria. Hotels, hospitals, leisure centres and industrial facilities may require even higher storage temperatures depending on operational requirements and water hygiene strategies.
Producing higher water temperatures requires the compressor to work harder, increasing compression ratios and reducing the Coefficient of Performance (COP).
For this reason, domestic hot water often represents the most challenging aspect of commercial heat pump design.
Understanding Hot Water Demand
One of the most common design mistakes is selecting a heat pump based solely on the building's daily hot water consumption.
Good engineering design focuses on when the hot water is required.
Examples include:
Hotels with large morning and evening demand peaks.
Offices with relatively low hot water usage.
Sports centres with high demand immediately after events.
Healthcare facilities requiring continuous hot water availability.
Engineers therefore create hourly demand profiles rather than relying only on total daily consumption. This approach ensures the heat pump and storage system can meet demand without unnecessary oversizing.
The Importance of Thermal Storage
Commercial heat pumps achieve their highest efficiencies when operating steadily for long periods.
Rather than continuously starting and stopping, they work best by charging a hot water storage cylinder, which acts as a thermal battery.
A correctly sized storage vessel should:
Supply hot water during peak demand periods.
Allow sufficient recharge time between demand peaks.
Reduce compressor cycling.
Improve seasonal efficiency.
Maintain hygienic storage temperatures.
The objective is to optimise the relationship between storage volume, recharge time and installed heat pump capacity.
How Is the Outdoor Heat Pump Sized?
Selecting the correct outdoor unit involves much more than choosing the largest available model.
Several engineering factors influence sizing.
1. Building Heat Loss
Although domestic hot water is the primary focus, the overall heating demand of the building must also be considered, particularly where one heat pump serves both space heating and DHW.
2. Domestic Hot Water Consumption
Engineers estimate the daily hot water requirement based on occupancy, fixture types and building use.
Different building types have very different demand profiles, making accurate occupancy assumptions essential.
3. Peak Demand
Rather than sizing for the total daily volume, engineers calculate the maximum hot water demand during the busiest periods of the day.
This determines how much energy must be available from storage and how quickly the system must recover afterwards.
4. Recovery Time
Recovery time refers to how quickly the heat pump can reheat the storage cylinder after hot water has been used.
A shorter recovery time generally requires a larger outdoor heat pump.
Conversely, increasing storage volume may allow a smaller heat pump to be installed while still meeting the building's operational requirements.
5. Outdoor Design Temperature
Air source heat pumps produce less heating capacity as outside air temperatures fall.
For this reason, equipment is typically selected using the winter design temperature rather than average annual conditions.
This ensures reliable operation throughout the heating season.
6. Required Flow Temperature
Producing water at 35–40°C for underfloor heating is relatively straightforward.
Producing 60–65°C for domestic hot water is significantly more demanding and generally results in a lower COP.
Higher flow temperatures increase compressor work and therefore influence both equipment selection and operating costs.
Should You Install One Large Heat Pump or Several Smaller Units?
Many commercial projects benefit from using multiple modular heat pumps instead of a single large unit.
Advantages include:
Higher part-load efficiency.
Built-in redundancy during maintenance.
Easier future expansion.
Reduced compressor cycling.
Improved operational flexibility.
If one unit requires maintenance, the remaining heat pumps can continue operating, improving system resilience.
High-Temperature Heat Pumps
Many existing commercial buildings were originally designed to operate with heating water temperatures between 70°C and 80°C.
Modern high-temperature heat pumps using refrigerants such as R290 (propane) or R744 (carbon dioxide) can achieve these temperatures more efficiently than conventional systems.
These technologies are particularly suitable for refurbishment projects where replacing radiators or terminal units is not practical.
Intelligent Controls Are Essential
The efficiency of a commercial heat pump system depends not only on equipment selection but also on its control strategy.
Modern control systems monitor:
Outdoor air temperature.
Storage cylinder temperatures.
Compressor operation.
Pump sequencing.
Defrost cycles.
Building demand.
Features such as weather compensation, variable-speed compressors and intelligent staging significantly improve seasonal performance while reducing energy consumption.
Accurate Heat Loss Calculations Matter
Heat pump performance begins with an accurate understanding of the building.
Oversized systems can:
Increase capital costs.
Reduce seasonal efficiency.
Cause excessive compressor cycling.
Shorten equipment lifespan.
Undersized systems may struggle to maintain hot water temperatures during peak demand.
Accurate heat loss calculations, occupancy assessments and dynamic thermal modelling are therefore essential before selecting any heat pump system.
🏗️ About Us
At MTS DNC Energy Consultants, we provide:
Heat pump assessments
Part L compliance reports
Building energy modelling
Through NEXUS M&E Design, we also deliver:
Heat loss calculations
Heat pump sizing
Radiator and underfloor heating design
Mechanical ventilation design
We ensure every system is designed for maximum efficiency, compliance, and long-term performance.
Heat pumps work differently. Rather than generating heat through combustion, they transfer heat from the outside environment into a building. By exploiting the vapour compression refrigeration cycle, heat pumps can deliver three to five times more heat energy than the electrical energy they consume, making them one of the most energy-efficient heating technologies available.
📍 Disclaimer
The content shared in these posts is intended for informational purposes only and should not be interpreted as design advice, specifications, or a calculation template. For professional guidance or design services, please contact us through our contact form.
