The answer
Is the lowest possible flow temperature always best?
A lower flow temperature usually helps a heat pump run more efficiently, but only if the radiators or floors can still keep every room comfortable in cold weather. The design temperature is a result of the heat-loss and emitter calculations, not a sales target.
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What does the number actually describe?
Flow temperature is the temperature of water leaving the heat pump for radiators or underfloor heating. Return temperature is what comes back after those emitters have released heat. It is not the room thermostat setting, and the hot-water cylinder may need a different, higher-temperature operating period.
The harder the heat pump must work to lift its heat source to a high water temperature, the more electricity it generally needs for the same heat delivered. Energy Saving Trust describes typical heat-pump radiator flow temperatures around 35–45°C, but that is guidance, not a promise that every existing radiator will heat every home at 35°C. A cold room with too little emitter area can set the temperature requirement for the whole circuit.
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How is the design temperature chosen?
Calculate each room's heat loss and then check what its radiator or floor can emit at a proposed flow and return temperature. If a room falls short, consider a larger radiator, a sensible fabric improvement or a different layout before simply lifting the temperature for every room. The design should also check water flow and pipes; an output printed in a radiator catalogue is not enough by itself.
Ask the installer to show the cold-day design case and the estimated seasonal performance used for running-cost figures. A quoted low flow temperature is only useful if the home can maintain comfort at it. Conversely, specifying a high temperature merely to avoid all emitter work may increase the bills for years.
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Will it use the same temperature all winter?
Usually it should not need to. Weather compensation changes the water temperature as the outside temperature changes: cooler water on mild days and warmer water when needed. The installer should set the heating curve, explain what it does and arrange a way to correct it if the first cold spell reveals a room that is too cold or a system that runs hotter than necessary.
Avoid changing several controls at once when you first move in with a heat pump; otherwise it is hard to see what fixed or caused a problem. Note the outdoor weather, the room that is uncomfortable and the displayed flow temperature, then ask the installer for a targeted adjustment. A survey should put these settings in the context of your actual rooms, not sell one 'magic' temperature.
Where this guidance comes from
Check the original guidance.
These primary sources support the technical or scheme points above. Rules can change; follow the current source before making a commitment.
Apply this to your property
See what temperature your home needs.
FLO can check heat loss and radiator output to design for comfort and sensible operating efficiency, rather than choosing a flow temperature in isolation.