Why Flow Temperature Matters: Getting the Most From Your UFH and Heat Pump

What Is Flow Temperature?

Flow temperature is the temperature of the water leaving your heat pump (or boiler) and entering the heating circuit. For a heat pump with wet underfloor heating, flow temperature is typically set between 30°C and 45°C. For a boiler with radiators, flow temperature is typically 60–80°C.

This is the single most important setting on a heat pump installation. Get it wrong — set it too high — and you lose most of the efficiency advantage the heat pump provides. Set it correctly and the system achieves SCOP 3.5–4.0+, meaning extraordinary value from every unit of electricity.

Why Lower Flow Temperature Improves Heat Pump Efficiency

A heat pump works by extracting heat from the outdoor air and upgrading it to a useful temperature. The harder the heat pump has to work to reach the target temperature, the less efficiently it does it. The relationship between flow temperature and SCOP is roughly:

Flow temperature (°C)	Approximate SCOP (mild climate)	Electricity for 10,000 kWh heat
35°C	4.2	2,380 kWh (£571 at 24p)
40°C	3.8	2,630 kWh (£631 at 24p)
45°C	3.4	2,940 kWh (£706 at 24p)
50°C	3.0	3,333 kWh (£800 at 24p)
55°C	2.6	3,846 kWh (£923 at 24p)

Running at 35°C instead of 55°C delivers the same heat at 62% of the cost. For a house with 12,000 kWh annual heating demand, that's a difference of £420 per year.

The Right Flow Temperature for UFH

Wet underfloor heating works at flow temperatures as low as 30°C and typically performs well at 35–42°C. The right temperature depends on:

• Pipe spacing — tighter spacing (150mm centres) requires lower flow temperature to deliver the same heat flux per m². Wider spacing (250mm) needs higher temperature.
• Floor finish — tiles (low thermal resistance) work at lower flow temperatures. Engineered wood (higher thermal resistance) needs a few degrees higher.
• Heat loss of the room — rooms with poor insulation or large windows lose heat faster and may need higher flow temperature or more zones.
• Outdoor temperature — in mild weather (above 10°C outdoor), 35°C flow temperature often provides comfortable room temperature. In cold snaps (below 2°C), 42–45°C may be needed.

A correctly designed UFH system for a Cornwall property should deliver comfortable 20–21°C room temperature at a flow temperature of 38–42°C in the coldest week of the year (typically December or January).

Weather Compensation: The Smart Approach

Weather compensation automatically adjusts flow temperature based on outdoor air temperature. When it's mild (15°C outside), flow temperature drops to 32–35°C. When it's cold (0°C outside), flow temperature rises to 42–45°C. The heat pump always runs at the lowest possible flow temperature for the current weather.

This is by far the most efficient operating mode for heat pump + UFH systems. Modern heat pumps from Mitsubishi, Vaillant, Daikin, and others all support weather compensation — but it needs to be correctly configured during commissioning. A heat pump installed without weather compensation being set up is leaving significant efficiency on the table.

We set up weather compensation curves for every heat pump installation. The curve is calibrated to your specific house — its heat loss, insulation levels, and UFH design temperatures.

When Your UFH Can't Reach Temperature

If your UFH-heated room is cold despite the heat pump running, the most common causes are:

• Flow temperature set too low for the heat loss — the manifold zone may need slightly higher temperature than others.
• Blocked or restricted zone — actuator stuck, air in system, or sludge blocking the loop.
• Insufficient insulation below the UFH — heat escaping downward rather than upward.
• Oversized room, undersized circuit — design error that requires additional pipe loops.
• Floor finish blocking heat transfer — thick carpet over UFH dramatically reduces output.

All of these are diagnosable and fixable. They're never a reason to raise the overall system flow temperature as a blunt correction.

Commissioning and Setting Flow Temperature Correctly

Correct UFH + heat pump commissioning involves setting:

1. Maximum flow temperature (typically 45°C for UFH — never set above 50°C for a pure UFH system)
2. Minimum flow temperature (typically 25–30°C — prevents the heat pump cycling off unnecessarily in mild weather)
3. Weather compensation curve — the relationship between outdoor temperature and target flow temperature
4. Zoning setpoints — each manifold zone's target temperature and schedule
5. Legionella protection — periodic hot water cylinder boost to 60°C (separate from UFH circuit)

We provide full commissioning documentation for every UFH + heat pump installation, including weather compensation curve settings and recommended seasonal adjustments for Cornwall's mild winter climate.

See our underfloor heating page, our guide to UFH with heat pumps, and UFH running costs for more detail. Contact us for installation, commissioning, or system optimisation across Cornwall.