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 puts 15.4 kW into its water — at its rating point of 7 °C outside. At −12 °C, which is what a Berlin winter is designed against, it makes 9.3 kW: 40 % less, exactly where the heat is wanted, and its heating COP falls from 3.8 to 2.5 on the way. Size the building against the badge and it is short by two-fifths on the coldest morning. Sized against the design condition instead, every flat is held — one unmet hour in each corner flat over the year.

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 the same comfort (only 32 °C leaves the corner flats short, 35 hours) the seasonal efficiency reads 2.85 at 32 °C flow, 2.56 at 35, 2.21 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 4 351 kWh at a seasonal COP of 2.49 against 5 480 on the fixed 45 it replaces. A fifth 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	4.4 MWh — seasonal COP 2.49 on a 45 → 30 °C heating curve (5.5 MWh on the fixed 45 it replaces)
Capacity	15.4 kW to the water at +7 °C, 9.3 kW at −12 °C — the number to size on
Comfort	21.0–30.3 °C, one unmet heating hour in each corner flat; 56–59 hours over 28 °C, one to four over 30
Flow temperature	2.85 / 2.56 / 2.21 / 1.98 seasonal COP at a fixed 32 / 35 / 40 / 45 °C (32 leaves the corner flats 35 hours short); the curve takes 21 % 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 9 kW against a 15 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

• Air-to-water heat pump plant, 15 kW class — open in the builder
• Flat: radiators, 12 kW at ΔT 50 — open in the builder

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.