Ferroli F37 Fault Code: Low Water Pressure
The F37 code appears when the boiler's water pressure sensor detects that system pressure has dropped to around 0.4 bar or below — too low for the boiler to operate safely. Ferroli classifies F37 as a blocking fault, meaning it will prevent the boiler from firing until the pressure issue is resolved. In many cases the boiler will recover automatically once the correct pressure is restored, without needing a full reset. Normal operating pressure for most Ferroli systems should sit between 1.0 and 1.5 bar when the system is cold, with 1.2 bar being the ideal target.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Ferroli F37 fault code mean?
The F37 code appears when the boiler's water pressure sensor detects that system pressure has dropped to around 0.4 bar or below — too low for the boiler to operate safely. Ferroli classifies F37 as a blocking fault, meaning it will prevent the boiler from firing until the pressure issue is resolved. In many cases the boiler will recover automatically once the correct pressure is restored, without needing a full reset. Normal operating pressure for most Ferroli systems should sit between 1.0 and 1.5 bar when the system is cold, with 1.2 bar being the ideal target.
Common causes
- System pressure gradually lost over time Common
The most frequent trigger for F37 is the boiler pressure naturally creeping downward. This can happen slowly over months through minor seepage at joints, valve glands, or radiator unions — often with no visible puddle. Topping up via the filling loop usually clears the fault straight away.
- Recent radiator bleeding Common
Bleeding radiators releases trapped air but also reduces the volume of water in the sealed system, which causes a pressure drop. If F37 appeared shortly after you bled your radiators, this is almost certainly why.
- Pressure relief valve discharging Sometimes
The pressure relief valve (PRV) is designed to open and dump water if pressure climbs too high, for example after a component failure. If the PRV has discharged water — look for a damp or wet pipe leading outside or to a drain — pressure will have dropped and the underlying overpressure fault still needs attention.
- A leak elsewhere in the heating system Sometimes
A slow drip from a radiator valve, pipework joint, pump seal, or heat exchanger can cause pressure to fall repeatedly. If F37 keeps coming back within days of repressurising, a leak is the most likely culprit and must be found and fixed rather than masked by topping up.
- Faulty water pressure sensor Rare
The sensor that monitors system pressure can fail or give inaccurate readings. If the system pressure appears correct on the gauge but F37 persists, or if the sensor voltage is outside the expected 4.9–5.1 V DC range when tested, the sensor itself may need replacing.
How to fix it
- Check the pressure gauge on the boiler front panel DIY safe
The gauge is usually on the front or underside of the boiler. If the needle is below 1.0 bar (or the display reads below 1.0 bar on digital models), low pressure is confirmed as the cause of F37. A reading near zero strongly suggests either a significant leak or a PRV discharge — look around the boiler and on exposed pipework for any signs of water.
- Top up the system using the filling loop DIY safe
Locate the filling loop — typically a short braided silver hose with one or two small taps, found beneath the boiler. Slowly open the tap(s) and watch the pressure gauge rise. Stop when the needle reaches 1.2–1.3 bar. Do not overfill beyond 1.5 bar. Close the tap(s) firmly. If your filling loop is an external clip-on type, remove it once the taps are closed.
- Reset the boiler DIY safe
Press and hold the boiler's reset button for around 5–10 seconds (refer to your model's handbook if you're unsure of the button's location). The F37 code should clear and the boiler should attempt to restart. If it doesn't restart after one or two resets, do not keep attempting — move on to the next step.
- Monitor the pressure over the following week DIY safe
Keep an eye on the pressure gauge daily. If it holds steady at around 1.2 bar without dropping, the repressurisation has solved the problem. If pressure falls again within a few days — particularly without any radiator bleeding — there is a leak somewhere that needs professional tracing. Do not simply keep topping up the system, as this can mask a fault and potentially cause further damage.
- Inspect for visible leaks around the system DIY safe
Check accessible radiator valves, pipework joints, and the area directly beneath the boiler for any drips, staining, or corrosion marks. Also check outside the property for a damp patch near the condensate or pressure relief pipe outlet — a dripping PRV pipe indicates the valve may have opened due to previous overpressure. Report any findings to your engineer.
- Call a Gas Safe registered engineer to investigate further Gas Safe engineer
If F37 returns repeatedly, if you cannot identify or access the source of the pressure loss, or if the fault persists after repressurising and resetting, you need a Gas Safe registered engineer to carry out a full system pressure test, trace and repair any leaks, and check the water pressure sensor. The engineer can also test the sensor's voltage output and replace it if it is giving false readings. Do not attempt to work on the pressure sensor, gas valve, pump, or any internal boiler components yourself.
Parts you may need
- Water pressure sensor · from £35
- Pressure relief valve (PRV) · from £25
- Filling loop (external type) · from £15
- Central heating pump seal kit · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £80–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Ferroli F37 fault myself?
In many cases, yes. If the cause is simply low system pressure, you can top up the boiler yourself using the filling loop and reset it — this costs nothing and takes about five minutes. However, if the pressure keeps dropping, or if there is a visible leak, a faulty pressure relief valve, or a failed pressure sensor, you will need a Gas Safe registered engineer to carry out the repair safely.
How often should I need to repressurise my Ferroli boiler?
A well-maintained sealed system should need topping up no more than once a year, if that. If you find yourself refilling the system every few weeks or months, there is almost certainly a slow leak or a pressure relief valve issue that needs professional attention. Repeated repressurisation without finding the cause is not a long-term fix.
What should the water pressure be on a Ferroli boiler?
When the system is cold, the pressure should sit between 1.0 and 1.5 bar, with 1.2 bar being the ideal target for most Ferroli models. When the boiler is up to full operating temperature, pressure will naturally rise slightly — typically to around 1.5–2.0 bar — and this is normal. If pressure regularly climbs above 2.5 bar when hot, or if your pressure relief valve is discharging water, that is a separate fault requiring an engineer.
Could the F37 code be caused by a faulty sensor rather than actual low pressure?
Yes, though this is less common than genuine pressure loss. If you top up the system to 1.2 bar, can see the gauge reads correctly, and F37 still appears or returns very quickly, the water pressure sensor may be giving false readings. An engineer can test the sensor's DC voltage output — it should read between 4.9 and 5.1 V — and replace it if necessary. Sensor replacement on a Ferroli typically costs around £80–£150 including labour, which is towards the lower end of the overall repair cost range.