Concepts · Chapter 09
Flow temperature — the master lever
The one number that decides whether a heat pump can even reach your old radiators — and that affects efficiency more than any other single figure.
Flow temperature is the water temperature a heat pump delivers to your radiators or underfloor heating. It keeps coming up throughout this guide because it determines two things at once: whether a heat pump can get your existing radiators warm enough at all — and how efficiently it does it.
Why the maximum flow temperature matters
Old radiators (1970s/80s vintage) were often sized for high flow temperatures — historically 70 °C or more. Whether a heat pump can work with them depends on the maximum flow temperature it can reach in the first place.
Modern high-temperature heat pumps using R290 (propane) refrigerant, in monobloc form, now reach 70–75 °C. An example from a real project (as of mid-2026): both the Stiebel WPL-A 10.2 Plus and the Buderus WLW186i reached 75 °C — both, in principle, suitable for old-building radiators. (A note from the same project: older or third-party documentation sometimes quoted only 65 °C for the Stiebel unit — the current datasheet for the ".2 Plus" variant states 75 °C. Always check the current datasheet for any specific model, not older summaries.)
Why lower is still better
Just because a heat pump can run at 75 °C doesn't mean it should. The "Why a heat pump doesn't behave like your oil boiler" chapter already did the math: every degree less flow saves electricity — roughly 8–12 % less consumption going from 50 °C to 45 °C. And the efficiency chapter showed why: SCOP55 (55 °C flow) is meaningfully lower than SCOP35 (35 °C flow) on every heat pump — lower flow means higher efficiency, almost without exception.
Remember: maximum flow temperature is a suitability question ("can this heat pump technically work with my radiators?"). The flow temperature you actually run is a cost question ("what does this cost me per year?"). The goal is always to run as cool as comfort in the house allows — not to default to the technical maximum.
How to get it low in an existing house
In a renovation you lower the required flow temperature by:
- Insulation and window replacement → lower heat load → a lower flow suffices.
- Larger or better radiators or surface heating (floor/wall) → more area delivers the same heat with cooler water.
- Hydraulic balancing (needed for the subsidy anyway) → each radiator gets exactly the right amount of water, instead of the first ones in the loop getting too much and the last too little.
Target, if feasible: design flow ≤ 45 °C.
The single-pipe special case
Old single-pipe systems (radiators wired in series rather than in parallel) tend to need high flow temperatures — the worst case for a heat pump. If that's your house, factor in converting to two-pipe or upgrading the radiators. Have every heat-pump installer assess the radiators specifically, not just quote a kW figure.
What this means for you
- Ask every offer for the maximum flow temperature of the specific model, backed by a current datasheet — not the salesperson's memory.
- At commissioning, start the heating curve low and nudge it up only as far as needed on a genuinely cold day (see "Why a heat pump doesn't behave like your oil boiler").
- If a later renovation is on the horizon (insulation, larger radiators, underfloor heating), the flow temperature you need drops further — and with it, running cost. How to size a heat pump correctly, against a renovated rather than the old heat load, is the subject of a later chapter.
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