A Victorian terrace that heats at 33°C

Took an 1874 terrace back to brick and rebuilt it around a heat pump that keeps it warm with 33°C water.

The house is an 1874 mid-terrace. We took it back to bare brick, and used that moment to do the things that are cheap only when walls and floors are already open: insulation everywhere, and underfloor heating in every room, deliberately oversized so the water in it never has to be hot. A 7 kW heat pump now heats the whole house with water at about 33°C on the coldest days, and has delivered 4.9 units of heat for every unit of electricity in both of its full winters so far.

The idea

A heat pump is most efficient when the water it makes is only lukewarm. So the whole renovation was designed backwards from one number: the lowest flow temperature we could get away with.

What we built

• Insulation: 100 mm of wood fibre on the walls — it lets moisture through, which suits old brick — and 200 mm of PIR in the roof. The staircase is insulated all the way down to the cellar.
• Heating: underfloor pipes set in screed in every room, 16 mm pipe at 100 mm spacing. That is far more pipe than usual, on purpose: more pipe means cooler water does the same job.
• Heat pump: a Vaillant Arotherm+ 7 kW running on R290, plumbed as one open system with nothing between it and the floors.

What it measures

The system is monitored with OpenEnergyMonitor, so these are measured, not brochure numbers:

• Flow temperature on the coldest day: 32.6°C average, at about 0°C outside.
• Heat loss of the whole house: 4.1 kW.
• Heating efficiency: a seasonal COP of 4.9 in both full winters so far, 2024/25 and 2025/26.

Heating the house now causes about 94% less carbon than it would uninsulated and on gas. The working is here.