Worked cases · Berlin

The badge says twelve kilowatts. In January it is seven

Four flats on radiators and one air-to-water heat pump: sized on the rating it leaves the corner flats cold, and the flow temperature it runs at is a fifth of its running cost.

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Lindenhof is one floor of four 70 m² dwellings — a new, well-insulated German block — heated through radiators by a single air-to-water heat pump on a shared water loop. There is no cooling: radiators heat, and the flats ride the summer on their windows. It is the first example in the library where heat reaches a room through water rather than through a duct.

The first draft left the corner flats cold, and the reason is the whole of heat-pump sizing. This machine is a 12 kW machine — at its rating point of 7 °C outside. At −12 °C, which is what a Berlin winter is designed against, it makes 7.07 kW: 41 % less, exactly where the heat is wanted, and its COP falls from 3.77 to 2.64 on the way. Size the building against the badge and it is short by a third on the coldest morning. Sized against the design condition instead, every flat is held with no unmet hour.

The second number is what the radiators are for. Twelve kilowatts of radiators at the EN 442 rating carries a 2.6 kW flat on 40 °C water — four times the bank a 70 °C boiler would have needed, and that multiple is the capital cost of running cool. Swept over the whole year at identical comfort the seasonal efficiency reads 2.40 at 32 °C flow, 2.29 at 35, 2.13 at 40 and 1.98 at 45. But a fixed setpoint has to be the cold day's number — here 45 — so the plant runs a heating curve instead, 45 °C at −12 outside down to 30 at +15, and reads 4 830 kWh at a seasonal COP of 2.24 against 5 483 on the fixed 45 it replaces. Twelve per cent of the year's electricity, for nothing but a curve every hydronic controller already draws.

What it reads on Berlin

Heat delivered	10.8 MWh at the radiators
Electricity	4.8 MWh — seasonal COP 2.24 on a 45 → 30 °C heating curve (5.5 MWh on the fixed 45 it replaces)
Capacity	12.0 kW at +7 °C, 7.07 kW at −12 °C — the number to size on
Comfort	21.0–30.0 °C, no unmet heating hour; 51 hours over 28 °C, none over 30
Flow temperature	2.40 / 2.29 / 2.13 / 1.98 seasonal COP at a fixed 32 / 35 / 40 / 45 °C; the curve takes 12 % off the cold-day setpoint

What it taught

A heat pump is sized on what it makes at the design condition, not on the number in its name — here 7 kW against a 12 kW badge.
Oversized emitters are not waste: they buy a lower flow temperature, and a fifth of the running cost came from nothing else.
An uncooled dwelling's summer is decided by its thermal mass. On the model's default the same flats peaked at 38 °C; on ISO 13790's medium class — a concrete slab, which is what they are — they peak at 30.

The machines

Sources

ISO 13790 §12.3.1.2 — internal heat capacity by construction class
EN 442-2 — the radiators' rating law
PVGIS TMY, Berlin

Readings of the shipped model on the building's own PVGIS typical meteorological year; engineering estimates from catalog data and published correlations. The building runs on every plan, including Free, a few times a day.