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Vaillant Arotherm Split

Swanage, Dorset · Mid-terrace · 70 m² · Pre-1900 · 7 kW · radiators · installed by Inafix

1–31 Dec 2025

745 hourly rows · MID-class heat meter present: yes
4.91efficiency (COP) over 745 hours, from the heat meter
better than 98% of the metered fleet in January 2026
Electricity used
91 kWh
Heat delivered
448 kWh (heat meter)
Estimate without heat meter
flow sensor 1.83 · temperature model 6.05
Mean outdoor temperature
8.4 °C · 115 space-heating hours
Flow temperature at 0 °C outside
24.4 °C (curve slope -0.55 per K)
Flow − return while heating
1.2 K
House needs at −3 °C
1.4 kW (unit rated 7 kW) · fit r² 0.03
Hot water
no hot-water flag in data
warningOnly 1.2 K between flow and return
Circulation pump may be set too high or emitters are undersized for the low flow temperature. Ask the installer to check pump speed.
infoUnit is rated 7 kW but the house needs about 1.4 kW at −3 °C
Oversizing causes cycling in mild weather. Cannot be fixed cheaply now, but the curve and minimum run time can limit the damage.
noteOwner’s note on the source site: “No heat on hot water due to split system”

Day by day

electricity per daydaily COP3.0 floor
electricity, kWh per day05102025-12-01: 1.8 kWh, outside 11 °C2025-12-02: 2.6 kWh, outside 8.3 °C2025-12-03: 2.8 kWh, outside 7.4 °C2025-12-04: 3.2 kWh, outside 7.2 °C2025-12-05: 3.5 kWh, outside 7.1 °C2025-12-06: 2.4 kWh, outside 10.7 °C2025-12-07: 1.4 kWh, outside 12 °C2025-12-08: 0.3 kWh, outside 12.5 °C2025-12-09: 0.9 kWh, outside 12.6 °C2025-12-10: 2 kWh, outside 10.5 °C2025-12-11: 2.1 kWh, outside 10.4 °C2025-12-12: 2.2 kWh, outside 10.5 °C2025-12-13: 3 kWh, outside 5.8 °C2025-12-14: 2.4 kWh, outside 10 °C2025-12-15: 1.9 kWh, outside 11.5 °C2025-12-16: 2.4 kWh, outside 9.1 °C2025-12-17: 2.9 kWh, outside 8 °C2025-12-18: 2.3 kWh, outside 11.2 °C2025-12-19: 2.5 kWh, outside 8.5 °C2025-12-20: 2.5 kWh, outside 8.7 °C2025-12-21: 2.5 kWh, outside 10 °C2025-12-22: 2.4 kWh, outside 9.9 °C2025-12-23: 2.7 kWh, outside – °C2025-12-24: 3.6 kWh, outside 5.2 °C2025-12-25: 4.5 kWh, outside 4.6 °C2025-12-26: 4.6 kWh, outside 5 °C2025-12-27: 4.2 kWh, outside 5.9 °C2025-12-28: 3.7 kWh, outside 7.7 °C2025-12-29: 3.9 kWh, outside 6.1 °C2025-12-30: 4.4 kWh, outside 5.3 °C2025-12-31: 9.5 kWh, outside 0.8 °C2026-01-01: 0.4 kWh, outside -1.1 °Cdaily COP (heat ÷ electricity)1352025-12-01: COP 5.72025-12-02: COP 5.122025-12-03: COP 5.122025-12-04: COP 4.862025-12-05: COP 4.732025-12-06: COP 5.612025-12-07: COP 5.582025-12-09: COP 5.782025-12-10: COP 5.142025-12-11: COP 5.122025-12-12: COP 52025-12-13: COP 4.632025-12-14: COP 5.182025-12-15: COP 5.462025-12-16: COP 4.922025-12-17: COP 4.82025-12-18: COP 5.592025-12-19: COP 4.882025-12-20: COP 5.022025-12-21: COP 5.42025-12-22: COP 5.152025-12-23: COP 52025-12-24: COP 4.752025-12-25: COP 4.782025-12-26: COP 4.842025-12-27: COP 5.12025-12-28: COP 5.212025-12-29: COP 4.982025-12-30: COP 4.942025-12-31: COP 3.7412-0112-0512-0912-1312-1712-2112-2512-29
Daily COP needs a heat meter; days without one show electricity only.

The installed heating curve

one heating hourfitted curve50 °C
2030405060-3-113579outdoor temperature, °Cflow temperature, °C
Each point is one hour of space heating. Lower and flatter is cheaper; well-set radiator systems sit around 35–42 °C at 0 °C outside.
Public system published by its owner on HeatpumpMonitor.org. Cycling (starts per hour) comes from the site’s 365-day statistic. Findings use installer rules of thumb; see methodology.