Worcester Bosch’s heat pump user manuals do not list a numeric fault code for low water pressure. What they do say is that if the gauge reads below 0.5 bar you should fill slowly to a maximum of 2 bar, and that water from the safety valve below 2 bar is something to report to your installer. Check the gauge, look for leaks, and get an engineer in if the pressure keeps falling.
If it keeps coming back, book an engineer rather than topping up or restarting repeatedly. 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 article is a guide only. We make every effort to keep it accurate, but manufacturers update manuals and codes can differ between models and software versions, so always check the official Worcester Bosch manual for your exact heat pump. Do not attempt anything dangerous or anything that could void your warranty.
What does a low pressure fault mean on a Worcester Bosch heat pump?
Worcester Bosch sells several heat pump families, and their documents do not all say the same thing. We read three of them and found no pressure-specific fault code in the user manuals. Where a fault appears, the manuals tell you to note the fault code, the sub-code and the user interface ID and give them to your installer or an authorised contractor. We have therefore written this as a general low pressure article and not tied it to a single code.
Here is what the manuals do document:
- Compress 3400i and 7001i user guides: check the pressure on the gauge on the indoor unit. If it is below 0.5 bar, increase it slowly by adding water through the filling valve, to a maximum of 2 bar.
- Safety valve: check it once or twice a year. Water coming from it when pressure is below 2 bar should be reported to the installer. Never close the valve off.
- 7001i/7400i guide: a triggered alarm or controller message can mean “too low pressure in the system”, because a pressure switch is wired in series with the overheat protection. It resets itself when pressure is enough.
- Dirty particle filter: in the same guide, dirt on the magnetite bar can cause low or poor flow alarms.
- Compress 2000 AWF installation manual: filling to approximately 1.8 bar, keeping pressure above 0.3 bar to prevent air entering, and a maximum of 3 bar. It also notes the unit can discharge water through the relief valve and that pressure should be checked regularly.
Those figures are different because the products are different. Do not pick a number from this list. Use the one in the manual for your own heat pump.
Why does low water pressure happen on a Worcester Bosch heat pump?
- Leaks at radiator valves, joints, underfloor manifolds, the pump or the cylinder connections.
- Expansion vessel faults. The Compress 2000 AWF manual gives a vessel pre-charge of 1 bar, and if that charge is lost the pressure behaves badly as the system heats and cools.
- Air in the system, which can come back after a drop in pressure.
- An automatic air vent that is weeping or has failed.
- Safety or relief valve discharge that has emptied some water out of the system.
- Recent bleeding of radiators or other plumbing work without a top-up afterwards.
- A blocked particle filter, which causes poor flow messages that look like a pressure fault.
- A failed pressure sensor or switch, which reports a problem the system does not have.
What can you safely check yourself?
- Read the pressure gauge on the indoor unit with the system cold. Note the number and the date.
- Look for drips, water stains and damp patches around the indoor unit, cylinder, pipes and radiators. Check where the safety valve discharges.
- Check that radiators, TRVs and zone valves are open. A shut circuit can trigger flow messages.
- Only top up through the filling valve if your manual allows it and you are confident. Go slowly, stay within the manual’s limit, and close the valve fully after. Our guide to a heat pump losing pressure explains why repeated drops matter.
- Press the selector to confirm the fault on the display and write down the code and sub-code exactly.
- Check again in a day or two. If the pressure has dropped, you have a leak or a fault.
What not to do:
- Do not do any refrigerant work. F-Gas certified engineers only.
- Never remove panels from the outdoor or indoor unit.
- Do not over-pressurise. Going above the manual’s limit risks the safety valve lifting.
- Do not close or block the safety valve or its drain.
- Do not keep topping up without finding out where the water is going.
When should you call an engineer?
- Pressure falls again after a top-up.
- Water is coming from the safety valve, especially when the gauge is below 2 bar (on the 3400i and 7001i).
- A fault code or controller message returns after being cleared.
- You can see leaks or damp on pipes, floors or around the cylinder.
- Heating or hot water stops working.
- You are not confident using the filling valve.
- Your heat pump is still under installer or Worcester Bosch warranty. Contact them first so you do not affect cover.
Book early. A leak left through winter gets worse and can damage floors and ceilings, and engineers are busiest when everyone’s system starts working harder.
How can you stop it happening again?
- Follow Worcester Bosch’s own advice to check the gauge and safety valves once or twice a year, and look at it more often through winter. Our annual service checklist shows what a service should include.
- After a long absence, check the pressure. Worcester Bosch’s manuals say so specifically.
- Have the particle filter cleaned, and if it keeps clogging, ask about a magnetite filter. See our heat pump powerflush guide for dirty systems.
- Act on drips straight away. Our heat pump leaking water guide explains the usual causes.
- If your heat pump struggles in cold weather, see our heat pump winter tips.
Other Worcester Bosch symptoms you might see with low pressure
Because the user manuals give no pressure-specific codes, this table lists symptoms and messages the manuals mention, not codes.
| Symptom or message | Meaning | First step |
|---|---|---|
| Gauge below 0.5 bar | Manuals say to top up slowly to a maximum of 2 bar (3400i, 7001i) | Check for leaks, top up only if competent, then monitor |
| Controller message or alarm about low system pressure (7001i) | Pressure switch may have tripped; it resets when pressure recovers | Check gauge; call an engineer if it recurs |
| Low or poor flow alarm | Dirt on the magnetite bar or a blocked particle filter | Engineer to clean the filter |
| Water from the safety valve below 2 bar | Something is wrong with the valve or expansion vessel | Contact your installer |
Need help with low pressure on a Worcester Bosch heat pump?
UK Heat Pump Servicing engineers find the leak, check the expansion vessel and air vent, inspect the filter and carry out heat pump repairs with a 12-month guarantee. We also offer an annual heat pump service and cover plans, which keep an eye on pressure before it becomes a fault. Contact us or call 0333 041 5130.
Frequently asked questions
Which Worcester Bosch heat pump do I have?
Look at the label on the indoor or outdoor unit, or your commissioning paperwork. The 3400i, 7001i and Compress 2000 AWF have different documents and pressure figures, so you need the right one.
Why does the Compress 2000 manual give a higher fill pressure?
It describes filling until the gauge shows approximately 1.8 bar and keeping pressure above 0.3 bar. The user guides for other models say to top up from 0.5 bar to a maximum of 2 bar. The products are built differently, so follow your own manual.
Is water from the safety valve always a problem?
The valve should only release water when the pressure goes above its limit. If it drips below 2 bar, Worcester Bosch says to contact the installer, and you should never close the valve off.
Can a heat pump run with low pressure?
Low pressure can cause flow problems, noise and shutdowns. Running with air in the system strains the pump, so do not leave a falling gauge unchecked.
Does Worcester Bosch use the same codes as its boilers?
No. Boiler fault codes should not be used for a heat pump. Always take the code and sub-code from your heat pump display and look them up in the heat pump documents.
