Heat pumps

Hot water not hot enough from heat pump

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)

Your taps or shower are only producing lukewarm water (e.g. 35-40°C) instead of hot water. You may find that you run out of hot water very quickly, even if the tank is supposedly “full.”

What’s normal vs not normal

Normal:

  • Hot water taking longer to reheat than with a gas boiler (e.g. 60-90 minutes).
  • Water being slightly cooler than a gas boiler (e.g. 50°C vs 65°C).
  • The heat pump pausing central heating while it focuses on the hot water tank.

Not normal:

  • Water that never gets above 40°C even after a long reheat.
  • The heat pump controller showing a “Hot Water Fault” or “Tank Sensor Error.”
  • Hot water only being available if the immersion heater (backup) is turned on manually.

Likely causes (ranked)

  1. Low setpoint: The target temperature for the hot water tank is set too low in the controller settings (e.g. 40°C instead of 50°C).
  2. Sensor position: The temperature sensor (thermistor) is loose or located too high in the cylinder, giving a “false” high reading.
  3. Diverter valve fault: The valve that sends heat to the cylinder is stuck, meaning the heat is going to the radiators instead.
  4. Cylinder coil too small: The internal coil in the hot water tank is too small to transfer the heat from a heat pump efficiently.
  5. Legionella cycle confusion: The system is waiting for a high-temperature “disinfection” cycle that hasn’t triggered.

Quick checks you can do safely

  • Check the setpoint: Go to your controller and check the “Domestic Hot Water” (DHW) target temperature. Try increasing it to 50°C.
  • Check the timer: Ensure the hot water “on” periods are long enough to allow for a full reheat.
  • Feel the pipes: When the heat pump is in hot water mode, feel the pipes going into the cylinder. They should be very hot (about 50-55°C).
  • Check the immersion: If your system has an emergency immersion switch on the wall, turn it on to see if that provides hot water (this confirms the tank itself is okay).

When to call an installer

  • If the heat pump runs but the cylinder pipes stay cold.
  • If you see “Diverter Valve Error” or “Tank Sensor Fault” on the screen.
  • If you have increased the setpoint to 50°C+ but the water at the tap remains cold.

Questions to ask your installer

  • “Is the hot water cylinder a high-recovery ‘heat pump ready’ model?”
  • “Is the tank sensor positioned in the middle pocket of the cylinder?”
  • “Is the immersion heater set to take over if the heat pump cannot reach the target temperature?”

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FAQs

Why is it cooler than my old boiler? Boilers often heat water to 65°C, which is actually dangerously hot at the tap. Heat pumps aim for 45-50°C, which is more efficient and still perfectly hot for a shower or bath (which is usually taken at 38-40°C). Is it safe at 50°C? Yes, but you must ensure your system has a regular “Legionella cycle” (anti-bacterial) which raises the temperature to 60°C once a week using the immersion heater. Do I need a special tank? Yes. A standard gas boiler cylinder has a small heat exchanger coil. A heat pump requires a “high-recovery” cylinder with a much larger coil to transfer heat at lower temperatures. How much hot water can I get? Because the water is stored at a lower temperature, you may need a slightly larger tank than you had with a boiler to get the same amount of “usable” hot water once mixed with cold. Can I use a heat pump for instant hot water? No. Heat pumps (like most systems except combi boilers) require a storage cylinder for hot water.\n

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