Pb means your Worcester Bosch heat pump is in antifreeze mode, which is normal protection rather than a fault, while Hb is a freeze-risk alarm raised after repeated water flow problems in cold conditions. These codes come from a Worcester Bosch heat pump alarm table whose model range is not stated, so they apply to some Worcester Bosch and Bosch ranges only; check your own manual.
If Hb appears, or Pb comes with other alarms, book an engineer rather than resetting it again and again. You can book a heat pump repair online or call 0333 041 5130, and our full Worcester Bosch heat pump error codes guide covers every other code.
Please read first: this guide is here to help you understand what your Worcester Bosch heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official Worcester Bosch manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
What do Pb and Hb mean on a Worcester Bosch heat pump?
Pb is antifreeze mode and Hb is a protective alarm linked to freeze risk. Both come from the alarm table in the manufacturer's manual.
| Code | Meaning in the alarm table | What you will notice |
|---|---|---|
| Pb | Antifreeze. Not a protection; the alarm light does not flash | An antifreeze icon on the display |
| PP | Abnormal temperature difference between entering and leaving water | An alarm, often in cold weather |
| Hb | Three consecutive PP faults with leaving water (Twout) below 7C | An alarm; the manual says it is reset by a power failure |
Put simply, Pb is the heat pump protecting itself as designed. Hb means water flow has been abnormal three times in a row while the water leaving the unit was cold, which is where freezing becomes a real risk.
Is ice on a heat pump normal?
Yes, a light layer of frost on the outdoor coil is normal in cold, damp weather, and the unit clears it with regular defrost cycles. During a defrost the fan may stop, you may see steam and water will run from the base. Our guide to the heat pump defrost cycle explains what normal looks like.
It becomes a fault when ice builds into a solid block, the fan is encased or alarms such as Hb, P0 or H6 appear. Our guide to a frozen outdoor unit covers what to do when the ice will not clear.
Why do Pb and Hb appear in cold weather?
Pb appears because the unit switches into antifreeze mode when it gets cold, and Hb appears when water flow problems (PP) keep happening while the leaving water is below 7C.
From the manual: Pb is antifreeze operation, not a protection. Hb follows three consecutive PP faults with leaving water below 7C. The manual gives descriptions only, with no causes.
General engineering knowledge: the usual underlying causes of PP and Hb are:
- Low flow from sludge, a blocked strainer or filter, or air locks
- Closed or partly closed isolation valves
- Low system pressure
- A pump running at the wrong speed or failing
- A faulty water temperature sensor giving false readings
- A frozen or blocked condensate drain, letting ice build up under the coil
- Snow or leaves blocking airflow, or a unit sitting too low to the ground
- A failed defrost sensor or valve
- Low refrigerant charge, which only an F-Gas engineer can check
What can you safely check yourself?
You can make a few simple checks from outside the unit, without tools and without opening any panels.
- Keep the power on. Antifreeze mode and frost protection need electricity, so do not switch off at the isolator.
- Clear snow, leaves and debris from around and under the outdoor unit, working from outside the casing.
- Check the condensate drain path is clear and not frozen. If it is iced, thaw it with lukewarm water only, never boiling.
- Check the pressure gauge is in its normal range and that isolation, radiator and zone valves are open. If pressure keeps dropping, see our guide to a heat pump losing pressure.
- If you are going away, use holiday or frost settings rather than turning the system off. Our guide to heat pump holiday settings explains how.
- Pb needs no reset. For Hb, the manual says it is cleared by a power failure, so after the checks above you can interrupt the power once and switch it straight back on; if Hb returns, leave it for an engineer.
What not to do:
- Never chip or scrape ice off the coil, as the fins and pipes damage easily
- Never use boiling water or a heat gun on the coil
- Never remove panels or covers
- Never touch the refrigerant circuit, which is F-Gas certified engineer work only
When should you call an engineer?
Call an engineer if Hb returns after one reset, or if PP keeps appearing, because both point to a flow problem that can lead to freezing. Other clear triggers are:
- Pb shows alongside other alarms
- The outdoor unit stays a solid block of ice after a defrost cycle
- Heating or hot water has stopped
- Pressure keeps falling or you can see a leak
- P0, H6 or HH appear in the same cold spell
Refrigerant checks and repairs must only be done by an F-Gas certified engineer. For wider symptoms, see our guide to common air source heat pump problems.
How can you stop it happening again?
Most freeze alarms can be prevented with routine care. The points below are general engineering knowledge, as the alarm table gives no maintenance advice.
- Annual service: an engineer checks flow, pressure, sensors and the condensate drain. Our annual service checklist shows what is covered.
- Clean filters: a blocked magnetic filter or strainer is a common cause of PP, and heavy sludge may need a heat pump powerflush.
- Freeze protection: keep the correct antifreeze or inhibitor concentration, or fit freeze-protection valves where the installation design calls for them.
- Insulation: outdoor pipework should be fully insulated and weatherproofed.
- Raised feet and drainage: the unit should sit clear of the ground on a free-draining base.
- Location: keep space clear around the unit and avoid spots where snow drifts or roof water drips onto it.
Our heat pump winter tips and guide to running a heat pump in cold weather cover more.
Other Worcester Bosch codes you might see in a cold snap
These codes from the same alarm table can appear in cold conditions. Each links to our full Worcester Bosch error code guide.
| Code | Meaning | First step |
|---|---|---|
| PP | Abnormal water temperature difference | Check valves and pressure |
| P0 | Low pressure protection | Clear the coil surroundings, then call an F-Gas engineer |
| H6, HH | DC fan failure, repeated fan failure | Look for ice around the fan from outside |
| E0, E8 | Water flow failure | Check valves and pressure, then call an engineer |
Need help with a frozen Worcester Bosch heat pump?
If your Worcester Bosch heat pump has frozen or keeps showing Hb, we can help. Our F-Gas certified engineers work on Worcester Bosch and Bosch systems across the UK, and our emergency heat pump repair service offers same-day visits where possible.
We trace the flow problem behind PP and Hb, from sludge to a failing pump, then carry out heat pump repairs with a 12-month guarantee. An annual heat pump service catches most of these issues before winter, and our cover plans combine servicing with repairs.
See the Worcester Bosch heat pumps we work on, contact us or call 0333 041 5130.
Frequently asked questions
Why does my manual not list Pb or Hb?
The alarm table we used comes from one manual whose model range is not stated, and alarm lists differ between Worcester Bosch and Bosch ranges. Always go by the manual for your exact model, which you can find from the rating label on the unit.
What does Twout mean?
Twout is the leaving water temperature, the water flowing out of the heat pump to your heating. Hb looks at this reading, which is why it only triggers when the water leaving the unit is below 7C.
Does antifreeze mode use extra electricity?
It can, as the unit runs to keep water moving and warm enough to avoid freezing. That small cost is far lower than repairing a frozen heat exchanger.
Can I cover my heat pump in winter?
No, do not wrap or cover the outdoor unit, because it needs free airflow to work and to defrost. A purpose-made shelter can help with snow, but only if it keeps the clearances in your installation manual.
Is Hb dangerous for my home?
Hb is a protective alarm that flags poor flow while the water is cold. The main risk is to the heat pump itself if the flow problem is ignored, so get it checked promptly.
