Why Domestic Hot Water Is Often the Hardest Part of Heat Pump Design
When people discuss heat pump projects, most of the focus is usually on space heating. But in many commercial buildings, domestic hot water can be the more difficult problem.
This is especially true in buildings such as leisure centres, schools, sports facilities, care buildings, hotels and accommodation blocks, where hot water demand can be significant.
The challenge is simple: domestic hot water often needs higher temperatures than space heating, and heat pumps become less efficient as the required temperature increases.
Why DHW is different
Space heating systems can often operate at lower temperatures, particularly if the heat emitters are suitable. Domestic hot water systems, however, usually need to store or deliver water at temperatures that manage hygiene and legionella risk.
This creates a design challenge. A heat pump serving domestic hot water may need to operate at 60°C or above. Some systems may require additional pasteurisation or backup heating.
The higher the required flow temperature, the lower the heat pump efficiency is likely to be.
The coil and heat exchanger problem
One of the most common issues is undersized heat transfer.
If a cylinder coil or plate heat exchanger is too small, the heat pump cannot transfer its heat into the stored water properly. The return temperature rises, the heat pump reaches its operating limit, and the unit cycles on and off.
From the client’s point of view, the system may appear to be installed correctly. But in operation it may:
Run at a low output
Cycle repeatedly
Deliver poor COP
Struggle to recover the cylinder
Rely on immersion heaters or boilers
Cost more to run than expected
This is not always obvious without monitoring.
Temperature difference matters
Heat pumps generally prefer a stable flow rate and a sensible temperature difference across the system. If the return temperature is too close to the flow temperature, the heat pump may not be able to reject enough heat into the system.
For example, trying to deliver a large heat output into a cylinder already close to 60°C can be difficult unless the coil or heat exchanger has been selected specifically for the heat pump temperatures.
A coil rated for a gas boiler system may not provide the same output when used with lower heat pump flow temperatures.
Storage volume and recovery
DHW design is also about storage and recovery.
A system needs to consider:
Peak hot water demand
Storage temperature
Cold water inlet temperature
Recovery time
Heat pump output
Backup heat source
Cylinder volume
Simultaneous demands
Legionella strategy
User profile
In leisure centres, for example, showers, pool backwash, washdown and top-up requirements can create large and variable demands. A system that works on a steady average load may still fail during peak use.
Why monitoring helps
DHW heat pump performance should be monitored separately from space heating where possible.
Useful data includes:
DHW heat pump electrical consumption
DHW heat output
Cylinder temperatures
Flow and return temperatures
Heat pump status
Immersion or boiler contribution
Recovery periods
COP
Cycling frequency
This can quickly identify whether the issue is heat pump selection, coil sizing, control strategy, storage volume or user demand.
Jupiter Engineering’s view
Domestic hot water should not be treated as an afterthought in heat pump design. In many buildings, it is the part of the system that carries the greatest performance risk.
Good DHW heat pump design needs proper load assessment, correct heat exchanger selection, realistic storage strategy and post-installation monitoring.
At Jupiter Engineering, we help clients assess, design and monitor commercial heat pump systems, including domestic hot water applications.
If your DHW heat pump is cycling, underperforming or costing more than expected, Jupiter Engineering can help diagnose the issue using design review and real performance data.