Home / system #620

Vaillant Arotherm+

Oxford, Oxfordshire · Semi-detached · 100 m² · 1900 to 1939 · 5 kW · radiators · installed by Boxt

Last 30 days Last 7 days 5 Jan – 4 Feb 2026 (winter reference)1–31 Dec 2025

Last 7 days

169 hourly rows · MID-class heat meter present: yes

4.02efficiency (COP) over 169 hours, from the heat meter
better than 77% of the metered fleet in January 2026

Electricity used: 47 kWh
Heat delivered: 189 kWh (heat meter)
Estimate without heat meter: flow sensor 3.59 · temperature model 4.52
Mean outdoor temperature: 8.3 °C · 96 space-heating hours
Flow temperature at 0 °C outside: 34.5 °C (curve slope -0.66 per K)
Flow − return while heating: 1.3 K
House needs at −3 °C: 2.0 kW (unit rated 5 kW) · fit r² 0.20
Hot water: 27% of electricity, COP 3.43

warningOnly 1.3 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 5 kW but the house needs about 2 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: “Not recording negative heat on heat meter”

Day by day

electricity per daydaily COP3.0 floor

Daily COP needs a heat meter; days without one show electricity only.

The installed heating curve

one heating hourfitted curve50 °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.