Nibe does not list a dedicated “low water pressure” alarm in the installer manuals we checked, so a loss of system pressure usually shows up indirectly, as a low flow or poor circulation alarm such as 162, 220 or 523, or as a plain low reading on the pressure gauge. Do not confuse these with Nibe’s “Low pressure alarm” (221), which is about the refrigerant circuit, not your heating water. Check the gauge, look for leaks and call an engineer if it keeps dropping.
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 Nibe 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 Nibe installer manual for your model. Do not attempt anything dangerous or anything that could void your warranty.
Is there a Nibe error code for low water pressure?
Not a dedicated one, as far as the manuals we reviewed show. We checked Nibe’s F2040, F2050 and S2125 documents and the Nibe alarm overview, and none of them lists an alarm that is simply “heating system pressure too low”. We could not find that wording in Nibe’s own manuals, so we are not going to invent a code for it. If you have seen a different message on your display, check it against the alarm list in your own manual.
What Nibe does list are alarms where poor flow or poor circulation is a named cause. These are the ones that matter if you are losing pressure:
- Alarm 162, High condenser out: the temperature out of the condenser is too high. Low flow in heating operation is a listed cause, and it resets itself.
- Alarm 220, HP alarm: the high pressure switch has tripped repeatedly. Listed causes include low or no heating flow, a defective circulation pump and a closed service valve.
- Alarm 523, Low defrosting flow (S2125): the manual says the flow is low and to check the particle filter and pump.
- Alarm 425, repeated LP/HP/flow alarms (S-series): listed causes include poor heating medium flow, which can follow air or low water in the circuit.
The confusing one is alarm 221, which Nibe’s F2050 manual calls “LP alarm”: too low a value on the low pressure sensor three times in 60 minutes. The causes are a faulty sensor or wiring, a defective control board or a problem with a refrigerant sensor. That is the refrigerant side, which only an F-Gas engineer should touch. Some Nibe models also show a low pressure alarm at number 51, again for the refrigerant or brine side. If your display says “low pressure”, do not assume your radiators are short of water.
Nibe’s manuals also disagree on system pressure figures between models. One lists a maximum heating system pressure of 6 bar, another 4.5 bar, and none gives a single filling figure, simply telling the installer to fill to the necessary pressure. For that reason we do not quote a target. Ask your installer what your system was filled to, and note it next to the gauge.
Why does a Nibe heat pump lose heating water pressure?
- A leak at a joint, radiator valve, pump or the safety valve.
- A faulty expansion vessel, so pressure spikes when the water heats and then falls.
- Air in the system, released by the air vent or when you bleed radiators.
- The safety valve discharging and not reseating.
- Recent bleeding or radiator work without a top-up.
- A blocked particle filter or filterball, which restricts flow while the gauge may look fine. Nibe’s manual lists a clogged filterball as a cause of poor hot water and says to switch off and clean it.
- A circulation pump fault or closed valves, which produce the same flow alarms.
A practical way to tell the difference is to look at the gauge first. If the gauge has dropped and radiators are cold or noisy, you are dealing with water in the heating circuit, and a leak, air or a vessel fault is likely. If the gauge looks normal but the display shows a pressure-related alarm, the cause is more likely a blocked filter, a pump problem, a sensor fault or the refrigerant circuit. Telling your engineer which of these you saw saves time on the visit and often means they arrive with the right parts.
It also helps to know that Nibe air-to-water units need a minimum volume of water in the system and a minimum flow during defrost. The F2050 manual gives a minimum system volume of 20 litres for the smaller model and 50 litres for the larger one, and the S2125 manual gives 120 litres. A system that has lost water, or has too many zones closed off, can fall below what the heat pump needs even if the gauge is not at zero.
What can you safely check yourself?
- Look at the pressure gauge and note the reading, and whether the system is hot or cold.
- Look for visible drips, wet floors and damp patches around the heat pump, pipework and radiators.
- Check that radiator and zone valves are open, as closed valves reduce flow.
- Note the alarm number and text on the display and when it happened.
- If radiators are cold at the top, air may be the cause. Nibe’s manual suggests bleeding for that. Do this only if you are confident, and check the gauge afterwards.
- Top up through the filling loop only if the manufacturer’s manual allows it and you are competent. Read our guide on a heat pump losing pressure first.
What not to do:
- Do not touch refrigerant pipework or connections. Refrigerant work is for F-Gas certified engineers only.
- Do not remove panels or covers on the heat pump.
- Do not keep over-pressurising the system.
- Do not ignore repeated top-ups.
- Do not keep pressing reset on a repeating alarm. Repeated resets can lock the compressor out.
When should you call an engineer?
- Pressure drops again within days or weeks of a top-up.
- Alarm 162, 220, 523 or 425 appears more than once.
- You can see water where it should not be, or the safety valve is dripping.
- You see a “low pressure” alarm such as 221. It is a refrigerant-side fault.
- Radiators remain cold after bleeding.
- You have lost heating or hot water.
Book early. Low flow makes the compressor work harder than it should, and repeated alarms can block the compressor until an installer resets it.
How can you stop it happening again?
- Book an annual service. Our annual service checklist covers pressure, filters, air vents and the expansion vessel.
- Have the filterball or particle filter cleaned regularly, and consider a heat pump powerflush if the system is sludgy.
- Follow up small drips fast. Our heat pump leaking water guide explains where to look.
- If the heating is not warming up as it should, see heat pump not heating for the checks.
- Check the gauge monthly and keep notes.
Other Nibe codes you might see with low pressure
These are the alarms we could verify in Nibe’s manuals where low flow or pressure appears. Numbers and wording vary between models, so check your own manual.
| Code | Meaning | First step |
|---|---|---|
| 162 | High condenser out; low flow in heating operation is a listed cause | Check the gauge and valves, then call an engineer if it repeats |
| 220 | HP alarm; low or no heating flow, defective pump or closed valve among causes | Do not keep resetting. Book an engineer |
| 523 | Low defrosting flow (S2125) | Book an engineer to check the particle filter and pump |
| 425 | Repeated LP/HP/flow alarms; poor heating medium flow among causes | Check pressure and valves, then book an engineer |
| 221 | LP alarm; low pressure sensor value (refrigerant circuit) | Call an F-Gas engineer. Not a water pressure fault |
Need help with low pressure on a Nibe heat pump?
UK Heat Pump Servicing engineers find the leak, check the expansion vessel and air vent, clean filters and carry out heat pump repairs with a 12-month guarantee. We also offer an annual heat pump service and cover plans. Contact us or call 0333 041 5130.
Frequently asked questions
Why does my Nibe say “low pressure” when the gauge looks fine?
The alarm is almost certainly about the refrigerant circuit, not the heating water. Leave it to an F-Gas engineer.
Does the pressure change when the heat pump heats up?
Yes, a small rise as the water warms is normal because the expansion vessel absorbs it. A large jump or a dripping safety valve is not normal.
Can a blocked filter cause alarms without pressure loss?
Yes. A clogged particle filter reduces flow even if the gauge is fine, and Nibe lists it as a cause of low defrost flow and poor hot water.
Will an alarm lock the heat pump?
Some alarms block the compressor until reset, and repeated alarms in a short window can block it more firmly. This is why repeated resetting is not recommended.
Is topping up myself covered by my warranty?
Topping up through the filling loop is usually acceptable if the manual allows it, but check your warranty terms. Anything beyond that, such as opening the unit, is for a qualified engineer.
