Design flow temperature

The design flow temperature is the temperature of the water leaving the heat pump when the heating system is meeting its design load. It links three parts of the design:

• how much heat the selected heat pump can produce in the design outdoor conditions
• how much heat the radiators or underfloor circuits can release into each room
• the seasonal efficiency used in the customer’s performance estimate

It is not simply a convenient installer setting. Selecting it without a room-by-room heat loss and emitter check leaves the main system design unresolved.

Use the lowest temperature that completes the design

A heat pump generally works more efficiently when it produces cooler heating water. Cooler water also reduces the output available from each radiator, so the temperature cannot be lowered independently of the emitters.

The design process is iterative:

1. Calculate each room’s heat loss at the internal and external design conditions.
2. Propose a flow and return temperature.
3. Check every emitter’s output at that condition.
4. Improve any room that falls short through emitter or fabric changes.
5. Confirm the heat pump’s capacity and electrical input at the same outdoor and water temperatures.

The aim is the lowest practicable design temperature at which every room and the heat pump satisfy their loads. One poorly served room should not raise the temperature of the entire system until alternatives for that room have been assessed.

State where the temperature is measured

As at 22 July 2026, MIS 3005-D Issue 3.0 requires the customer to receive the design flow temperature of the water leaving the heat pump before any blending valves. It also requires the design emitter temperature based on the worst-performing room.

Those can differ in a mixed-temperature system. For example, a blending valve may supply an underfloor circuit at a lower temperature than the main flow. Quoting only the blended circuit temperature could make the heat pump appear to operate more efficiently than the actual design allows.

The design should state:

• heat-pump leaving-water temperature
• return-water temperature or design temperature drop
• emitter circuit temperature after any blending
• outdoor and room conditions to which those values apply

Heat-pump output changes with the conditions

A heat pump’s nominal model size is not proof of output at the design point. Air-source output can change with outdoor temperature, leaving-water temperature and defrost operation. Ground-source output depends on source-side temperatures and flow as well as the heating-water condition.

Selection must use manufacturer performance data for the proposed unit at, or conservatively interpolated to, the actual design conditions. For a non-hybrid hydronic system, MIS 3005-D requires at least 100% of the calculated design heat load without counting a supplementary electric heater.

The same data should show electrical input and efficiency. Generic claims about a fixed efficiency penalty for each additional degree are not reliable across products and operating points.

Emitter output changes too

Radiators are rated against a temperature difference between their mean water temperature and the room. At lower heat-pump temperatures, their output is lower than the familiar catalogue figure unless that figure was declared at the same condition.

For a radiator circuit:

mean water temperature = (flow temperature + return temperature) ÷ 2

mean temperature difference = mean water temperature − room temperature

The appropriate manufacturer correction data then gives the output at that temperature difference. Underfloor heating and fan convectors require their own product and system data.

Emitter selection should therefore be completed at the agreed design temperature. Choosing emitters at one condition and lowering the heating curve later can create a genuine cold-weather shortfall.

Design temperature and weather compensation

The design flow temperature is the cold-weather endpoint of the space-heating design, not a temperature the system should maintain throughout winter. Weather compensation lowers the target as outdoor conditions become milder.

A well-set curve aims to supply only enough heat to replace the building loss. Running continuously at the design temperature in mild weather can reduce efficiency, increase cycling and cause room controls to close.

Commissioning usually starts from the calculated curve and then makes measured, gradual adjustments. Lowering it until rooms fail and then increasing it slightly can be a useful optimisation process, provided the installer preserves hygiene, frost-protection and product requirements and records the final settings.

Domestic hot water is a separate duty

Space heating and cylinder charging usually need different water temperatures and operating periods. MIS 3005-D requires a heat pump serving domestic hot water to be capable of at least 55°C flow at the design conditions. That requirement does not mean the space-heating circuit should also be designed at 55°C.

The performance estimate should keep space-heating efficiency, hot-water efficiency and any immersion-heater energy distinct. A single headline COP cannot describe all three duties.

Designs above 55°C

MIS 3005-D says high-temperature heat pumps should be avoided unless the application requires a flow temperature above 55°C. Where the proposed system exceeds 55°C, the customer must also receive an alternative design at 55°C or lower, with the efficiency and energy-use differences explained.

That threshold is a requirement of the named standard, checked as at the review date. It is not a claim that every system at or below 55°C is efficient or that every existing radiator will work there. The actual design should still seek the lowest viable temperature.

Design-temperature evidence

A sound proposal connects the selected temperature to evidence. It should show:

• room-by-room heat losses and the external design condition
• an emitter schedule with corrected outputs
• proposed flow and return temperatures
• the worst-performing room
• heat-pump capacity and electrical input at the design point
• the seasonal performance value used for the estimate
• separate treatment of blended circuits and domestic hot water
• the intended weather-compensation settings or design basis

A bare statement such as “designed for low temperature” is not enough. The temperatures and the product data are what make the claim testable.

Related entries

• Heat loss survey
• Room-by-room heat loss
• Heat pump sizing margin
• SCOP performance estimate
• Radiator emitter upgrades
• Weather compensation controls