On a Vaillant aroTHERM, F.22 means the heating circuit water pressure is too low, and F.73 means the water pressure sensor or its wiring has a fault. Check the pressure gauge on the system first: if it has dropped, there is a leak or an air problem to find, and if the gauge looks normal, the sensor is the more likely culprit and an engineer should check it.
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 Vaillant 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 Vaillant manual for your unit. Do not attempt anything dangerous or anything that could void your warranty.
What do F.22 and F.73 mean on a Vaillant heat pump?
Vaillant’s installation manual for the aroTHERM VWL /2 A series lists these as F.022 and F.073, which many displays and homeowners shorten to F.22 and F.73. F.022 is described as “Water pressure too low”, with the cause given as insufficient water in the heating system. F.073 is described as a fault on the water pressure sensor, with the cause given as a broken line or short circuit between the control and the sensor.
The fixes are different. F.22 is a real loss of water. F.73 is an electrical or sensor problem, so the water may be fine but the heat pump can no longer read it. The gauge tells you which.
Boiler fault code lists are not the same as heat pump lists, so do not rely on a boiler guide for your aroTHERM. We have only used codes we could confirm in Vaillant’s heat pump documentation.
On pressure figures, the manuals disagree slightly between models. The VWL /2 A manual gives a minimum operating pressure of 1.0 bar and a maximum of 3.0 bar for the heating circuit, while Vaillant’s quick guide for the aroTHERM plus lists 0.5 bar minimum and 3.0 bar maximum. We take the cautious reading: treat anything below 1 bar as too low until your engineer confirms the figure for your model.
Why does a Vaillant heat pump lose water pressure?
Heating systems are sealed, so a drop in pressure always means water or air is going somewhere it should not. The usual reasons on an aroTHERM installation are:
- A small leak at a radiator valve, pipe joint, underfloor manifold, or the connections around the heat pump itself.
- A failing expansion vessel that has lost its air charge, so pressure swings wildly between cold and hot and the gauge sinks overnight.
- Air in the system, often after filling or draining, which can make the gauge read oddly and starve the pump of water.
- A weeping pressure relief valve that is discharging a little each time the system heats up, usually outside where nobody sees it.
- Recent radiator work or bleeding, which removes water and drops the gauge until someone tops it up.
- A faulty water pressure sensor or cable, which is what F.73 points to and can also trigger a false low reading.
Vaillant also lists F.532 (building circuit flow rate too low) with low water pressure as one of its possible causes, alongside trapped air, a missing or clogged dirt filter and a pump fault.
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 pressure gauge on the heating circuit when the system is cold. Compare it with the minimum in your manual, and treat anything under 1 bar as low.
- Look for visible drips or damp patches around radiators, valves, the cylinder, the hydraulic module and the pipes under the outdoor unit.
- Check that radiators and zone valves are open and that nobody has closed a lockshield or isolation valve, which can cause flow faults as well as odd readings.
- Top up only if your Vaillant manual or installer has shown you how and you are competent to do it. Use the filling loop slowly, watch the gauge, close both valves fully and disconnect the loop. If the pressure falls again, stop topping up and call us.
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?
- F.22 or F.73 returns after you have reset the unit once.
- The gauge drops again within days of a top-up, or you can see water escaping.
- The pressure relief valve is dripping or has discharged.
- F.73 appears while the gauge reads normal, which points to a sensor or wiring fault that needs a qualified engineer.
- Heating or hot water has stopped, or you also see a flow fault such as F.532.
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. A proper check includes gauge pressure, expansion vessel charge, air vent condition and a look for weeping joints. Our annual service checklist shows what should be covered.
- Treat small drips seriously. Our guide to a heat pump leaking water explains what is normal condensate and what is not.
- If the system is dirty or flow is poor, ask about cleaning. A heat pump powerflush can clear sludge that blocks filters and strains the pump.
- Make sure it was set up properly in the first place. The commissioning checklist includes filling, flushing and recording the cold fill pressure.
Other Vaillant codes you might see with low pressure
These are the Vaillant codes we could confirm in the aroTHERM documentation that relate to water pressure, flow or the pressure sensor.
| Code | Meaning | First step |
|---|---|---|
| F.22 (F.022) | Water pressure too low. Listed cause: insufficient water in the heating system. | Read the gauge, look for leaks, then call an engineer if it falls again after a top-up. |
| F.73 (F.073) | Water pressure sensor fault. Listed cause: broken line or short circuit to the sensor. | Check the gauge. If it reads normally, book an engineer to test the sensor and wiring. |
| F.532 | Building circuit flow rate too low. Possible causes include air, a blocked dirt filter, low water pressure and a pump fault. | Check the gauge and that valves are open, then book an engineer to bleed and inspect the filter and pump. |
| F.86 (F.086) | Contact thermostat has opened. Listed causes include low heating circuit flow or a closed underfloor circuit. | Make sure underfloor and radiator circuits are open, then call an engineer if it repeats. |
Codes can be shown with or without the leading zero depending on the display and software, so F.22 and F.022 are the same thing. If your display shows something not in this table, our full error codes guide is the next place to look.
Need help with low pressure on a Vaillant 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 work on Vaillant aroTHERM systems regularly and will pressure test the circuit, check the sensor and cable if F.73 is showing, and put the system back to the correct fill pressure. 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
Can I run my Vaillant heat pump with F.22 showing?
The unit will normally lock out or limit operation because it is protecting itself from running with too little water. Repeatedly restarting it does the pump and heat exchanger no favours, so it is better to leave it off and get it checked.
Why did the pressure drop after my Vaillant heat pump was serviced or drained?
Any time water is removed, the pressure falls until the system is refilled and bled. If it keeps falling days after the work, a joint disturbed during the job may be weeping.
Does an aroTHERM need a different filling pressure in winter?
The cold fill figure comes from your installer and manual rather than the season. What does change is the gauge reading, because water expands as it heats, so always compare readings at the same temperature.
Will a pressure fault affect my warranty?
Unreported leaks and DIY repairs can cause problems with a warranty claim, so keep your service records and avoid altering the system yourself. A qualified engineer’s report is the safest evidence if the manufacturer ever asks.
