Heat pumps

Heat pump error code: High pressure

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)

The heat pump controller displays an error code (often “H9”, “E20”, or explicitly “High Pressure Fault” depending on the brand). The outdoor unit has stopped running, and your radiators or hot water are getting cold.

What’s normal vs not normal

Normal:

  • The system shutting down for a few minutes during a defrost cycle.
  • Pressure rising slightly (e.g., from 1.5 bar to 2.2 bar) as the water heats up and expands.

Not normal:

  • The system “locking out” and refusing to restart after a power cycle.
  • Persistent high-pressure errors occurring every time the system starts a high-temperature run (like hot water).

Likely causes (ranked)

  1. Poor water flow: The most common cause. If the water isn’t moving fast enough, the heat pump can’t shed its heat, causing pressure to spike.
  2. Dirty magnetic filter: The filter on your central heating pipework is clogged with sludge, restricting flow.
  3. Closed valves: TRVs (Thermostatic Radiator Valves) or zone valves have shut, leaving the heat pump with nowhere to send its heat.
  4. Pump failure: The internal or external circulation pump is stuck or faulty.
  5. Air in the system: Air pockets are blocking the flow of water.

Quick checks you can do safely

  • Check the magnetic filter: If you know how, safely clean the magnetic filter (e.g., Adey Magnaclean).
  • Open all radiators: Ensure all TRVs are turned to the maximum setting to allow maximum water flow.
  • Check the system pressure gauge: If the static pressure is above 3 bar when cold, the system may have been overfilled.
  • Check for air: Use a radiator key to bleed any air from the highest radiators in your home.

When to call an installer

  • If the error returns immediately after you have checked the filters and opened the valves.
  • If you hear a loud “banging” or “hammering” noise when the system starts.
  • If the circulation pump is hot to the touch but the pipes next to it are cold.

Questions to ask your installer

  • “Is the primary circulation pump set to the correct speed for this system?”
  • “Does the system have an appropriately sized volumiser or buffer tank to maintain flow?”
  • “Can you check the heat exchanger for debris or scaling?”

Related NetZeroNow tools & guides

FAQs

What does ‘High Pressure’ actually mean? It refers to the pressure of the refrigerant gas, not necessarily the water in your radiators. If the heat pump can’t transfer its heat into your water, the gas gets too hot and its pressure rises. Can I reset the error myself? Usually, yes, by turning the main power isolator off and back on. However, if you don’t fix the underlying flow issue, it will just trip again. Why does it only happen during hot water runs? Hot water requires the heat pump to work at its highest temperature and pressure. If flow is marginal, this is when it will most likely fail. Is my heat pump broken? Probably not. High-pressure faults are almost always “external” issues related to the pipework, filters, or valves in your home, rather than the heat pump itself. How often should I clean the filter? We recommend checking your magnetic filter at least once a year, ideally just before the heating season starts.\n

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