Heat pumps

Underfloor heating slow to warm

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

You turn the thermostat up, but the floor remains cold for several hours. By the time the room reaches the target temperature, it’s often time to go to bed. The system feels unresponsive compared to radiators.

What’s normal vs not normal

Normal:

  • A “warm up” time of 2-4 hours for concrete (screed) floors.
  • A “warm up” time of 30-60 minutes for timber suspended floors or low-profile overlay systems.
  • The floor feeling “neutral” rather than “hot” (it should be around 25-27°C, not 40°C).

Not normal:

  • Rooms taking more than 6 hours to increase by even 1 degree.
  • Some zones (rooms) staying stone cold while others are warm.
  • The heat pump running at high power but the floor pipes at the manifold remaining cold.

Likely causes (ranked)

  1. System “Setback” too low: You are letting the house get too cold at night. UFH (Underfloor Heating) works best with a small “setback” (e.g. 18°C at night, 20°C by day).
  2. Manifold pump failure: The dedicated pump that circulates water through the floor loops is stuck or has no power.
  3. Air locks: Air is trapped in one or more of the floor loops, preventing water flow.
  4. Mixing valve set too low: The mechanical valve at the manifold is restricted, limiting the water temperature too much.
  5. Actuator failure: The small motors that open the valves for each room are not receiving a signal from the thermostats.

Quick checks you can do safely

  • Check the manifold: Find your UFH manifold (usually in a cupboard). Are the flow meters (small glass tubes) showing movement when the heating is on?
  • Check the pump: Is the pump at the manifold vibrating or making a slight hum?
  • Check the setback: Adjust your thermostats so the temperature never drops more than 2 degrees below your target.
  • Check the rug: Large, thick rugs or heavy furniture can “insulate” the floor and prevent heat from entering the room.

When to call an installer

  • If the heat pump is running but the manifold flow meters show zero flow.
  • If a specific room stays cold while the thermostat shows it is “calling for heat.”
  • If you see water leaking around the manifold valves or pump.

Questions to ask your installer

  • “Is the UFH pump speed correctly matched to the heat pump’s flow rate?”
  • “Are the floor loops balanced correctly to ensure even heat distribution?”
  • “Is there a ‘buffer’ or ‘low loss header’ to prevent the heat pump from short-cycling with UFH?”

Related NetZeroNow tools & guides

FAQs

Why is it so slow? Underfloor heating has a high “thermal mass.” You are heating up a giant slab of concrete or a whole floor’s worth of timber. This takes much longer than heating the thin metal of a radiator. Should I leave it on 24/7? Yes, ideally. With a heat pump and UFH, you should use “setback” temperatures rather than turning it “on and off.” This keeps the slab warm and allows the heat pump to run at its most efficient, low-power state. Can I have UFH and radiators? Yes, but they often require different water temperatures. A “mixing valve” at the UFH manifold will lower the temperature for the floors while the radiators get the full flow. What temperature should the water be? For UFH, the water usually only needs to be 30-35°C. This is why UFH is the perfect partner for a heat pump. Do I need special flooring? Stone and tile are best as they conduct heat well. Thick carpets and some laminates can act as insulators and make the system less effective.\n

Written by NetZeroNow | Last updated on 2026-03-02