Grant does not list a dedicated low water pressure fault code for the Aerona3 in the documentation we could confirm. Low pressure on an Aerona3 is normally spotted on the system gauge, and if it leads to a fault it tends to appear as a pump or water-flow related code such as P1 or FU. Check the gauge first, and call an engineer 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 Grant Aerona heat pump error codes guide covers every other code.
Please read first: this is a guide only. We make every effort to keep it accurate, but manufacturers update manuals and codes differ between models and software versions, so always check the official Grant Aerona manual for your unit. Do not attempt anything dangerous or anything that could void your warranty.
Is there a Grant Aerona error code for low water pressure?
Not that we could verify. Grant’s own support page explains that the Aerona3 shows a letter-and-number code with a warning icon, that the red on/off LED flashes, and that you should notify your installer. The full table sits in the Fault Finding section of the installation and servicing manual. We could not find a code in the published tables that is described as low water pressure, so we are not going to invent one.
What the manual does give is useful pressure information. It lists the system water pressure as 0.1 to 0.3 MPa (1 to 3 bar), with a maximum of 3 bar and a safety valve set to operate at 3 bar. It also asks for a 0 to 4 bar pressure gauge, and says to fill a cold system until the gauge reads between 0.5 and 1.0 bar. The cold fill should be 0.2 to 0.3 bar above the expansion vessel charge. Typical figures it mentions are about 0.5 bar for a bungalow and 1.0 bar for a two-storey house.
Some websites quote a different target, such as 1.5 bar. Where sources disagree we take the cautious reading and follow Grant’s manual, plus whatever fill pressure your installer recorded at commissioning.
The Aerona3 also depends on a minimum water flow, listed in the manual as 5 L/min for the smaller unit up to 15 L/min for the larger ones. When pressure is low, air gets drawn in and the pump struggles, so flow-related faults can follow. Installer guides that reproduce Grant’s fault table describe P1 as a pump error and FU as the high pressure switch operating, with a clogged water circuit among the listed causes. FU refers to the refrigerant side switch, so low water pressure is only an indirect link, but the advice to check the flow and return temperature difference is useful.
Why does a Grant Aerona heat pump lose water pressure?
A sealed Aerona3 system should hold its pressure for months. If it does not, look at these likely causes:
- A leak at radiators, valves, joints, underfloor manifolds or the connections near the heat pump.
- An expansion vessel with the wrong or lost air charge. Grant specifies the charge relative to the static head at the vessel, so a vessel that has been recharged incorrectly can cause pressure problems.
- Air in the system after filling or maintenance, which can mimic a drop and affect flow.
- The safety valve discharging at 3 bar after the pressure has risen too high, which then drops the gauge once it reseats.
- Recent bleeding or radiator work that removed water from the circuit.
- A leaking automatic air vent.
Nothing in the heat pump electronics warns you about a slow loss, so a regular look at the gauge is the most useful habit.
What can you safely check yourself?
These checks need no tools and no panels removed. If you are not comfortable with any step, stop and call an engineer.
- Read the gauge when the system is cold. Compare it with the cold fill range in the Grant manual, 0.5 to 1.0 bar, and with any figure your installer recorded.
- Check for a fault code on the remote controller and see whether the red on/off LED is flashing. Write down the code before you do anything else.
- Look for drips and damp patches under radiators, around the pipework and near the pressure relief valve discharge.
- Make sure radiator and zone valves are open and nothing has been isolated.
- Top up only if the manual allows it and you are competent, using the filling loop slowly and never going above the maximum. Close the loop fully afterwards.
- Check the gauge again after a day. If it has fallen, you have a leak or a vessel problem to investigate.
Our guide to why a heat pump keeps losing pressure goes into the repressurising process and what a normal rate of loss looks like.
What not to do:
- Do not attempt any refrigerant work. Refrigerant handling is for F-Gas certified engineers only, and a low water pressure fault is not a refrigerant fault.
- Do not remove the outdoor unit or indoor unit panels. Live electrics, moving parts and pressurised components sit behind them.
- Do not keep over-pressurising to be safe. Pushing the gauge towards the maximum can lift the pressure relief valve and stress joints.
- Do not ignore repeated top-ups. If you need to refill the system more than once or twice a year, water is escaping somewhere and it needs finding.
When should you call an engineer?
- The gauge keeps falling after you top it up.
- A fault code such as P1 or FU shows on the controller and returns after being reset.
- The safety valve has discharged or is dripping.
- Heating output is poor, the flow and return temperatures are far apart, or the unit is noisy.
- You can see water leaking from the system.
Booking early matters here. A small leak caught in week one is usually a cheap fix, while months of topping up can let air and fresh oxygen into the circuit and cause sludge and corrosion.
How can you stop it happening again?
- Book an annual service. The annual service checklist includes gauge reading, expansion vessel check, strainer clean and a leak inspection.
- Do not ignore slow drips. Read our guide on a heat pump leaking water to see what to look for.
- If the circuit is dirty or flow is poor, a heat pump powerflush can help clear the system.
- If the unit stops running properly, our guides on a heat pump not working and a heat pump not heating cover wider troubleshooting.
- Record the cold gauge reading once a month.
Other Grant Aerona codes you might see with low pressure
Grant lists no low pressure code, so this table shows the related water-circuit codes reported in installer guides based on the Aerona3 manual. Treat the meanings cautiously and check them against your own manual.
| Code | Meaning | First step |
|---|---|---|
| P1 | Pump error. Listed causes include a pump fault, a clogged pump or water circuit, or a main PCB fault. | Check there is water in the system and the gauge is normal, then book an engineer to test the pump voltage. |
| FU | High pressure switch operating (refrigerant side). Listed causes include outside air recirculation or a clogged water circuit. | Make sure nothing blocks the outdoor unit, then book an engineer to check flow and return temperatures and the strainer. |
The wording of these entries comes from installer guides rather than a table we could read directly in Grant’s PDF, which is another reason to check your own manual.
Need help with low pressure on a Grant Aerona heat pump?
UK Heat Pump Servicing engineers find the leak, check the expansion vessel and air vent, and carry out heat pump repairs with a 12-month guarantee. We repair Grant Aerona systems and can trace the leak, test the expansion vessel charge against the manual and set the fill pressure correctly. Our annual heat pump service catches pressure problems before they stop your heating, and we also offer cover plans if you would rather spread the cost of looking after the system. If your gauge is low or a code is showing, contact us or call 0333 041 5130.
Frequently asked questions
What does the red flashing light on my Grant Aerona mean?
According to Grant, a flashing red on/off LED together with a code on the controller means the heat pump has detected a fault. Note the code and contact your installer or an engineer.
Can I repressurise my Aerona3 myself?
If you are competent and the manual allows it, you can top up through the filling loop to the correct figure. If it keeps dropping, that is a sign of a leak and not something to keep fixing yourself.
Why does the gauge on my Grant system read higher when the heating runs?
Warm water takes up more space, so the pressure rises a little, and the expansion vessel absorbs the difference. The maximum is 3 bar, and anything approaching that needs checking.
What is the minimum flow rate for an Aerona3?
Grant lists 5 L/min for the HPID6, 10 L/min for the HPID10 and 15 L/min for the HPID13 and HPID17. Poor pressure or a blocked strainer can pull the flow below that.
Where is the error code table for my Aerona?
It is in the Fault Finding section of the Grant installation and servicing manual for your model, and it also lists how each code resets.
