Protherm F.37 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.37 fault code mean?
The F.37 code appears on Protherm boilers when the pressure-monitoring circuit detects that water pressure in the central heating system has dropped to a dangerously low level — typically around 0.4 bar or below. The boiler locks out and shuts down to protect itself from running dry. On Protherm displays, the letter 'F' flashes alongside '37' to indicate a permanent fault condition. The code can be triggered either by genuinely low system pressure or by a faulty pressure sensor sending incorrect readings even when the actual pressure is adequate. In either case, the boiler will not provide central heating or hot water until the fault is resolved.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
Common causes
- Low system pressure due to water loss Common
The most frequent trigger. If the pressure gauge on the boiler reads below 1 bar (ideally it should sit between 1 and 1.5 bar when cold), the system simply does not have enough water in it. This can happen gradually over time through micro-leaks, or more suddenly after radiators have been bled without topping the system back up afterwards.
- Faulty or weeping pressure relief valve (PRV) Common
The 3-bar PRV is designed to release water if pressure gets too high, but a valve that has started to weep or stick open will slowly bleed water from the system. If your pressure keeps dropping within days or a week of topping up, a degraded PRV is a likely culprit and needs engineer inspection.
- Failing expansion vessel Sometimes
The expansion vessel absorbs pressure fluctuations as water heats and cools. If its internal diaphragm has perforated or the pre-charge pressure has dropped, the system pressure can swing wildly or steadily decrease. A vessel in this condition will cause recurring F.37 faults even after repeated top-ups.
- Faulty water pressure sensor Sometimes
If the pressure gauge appears normal (1–1.5 bar) but F.37 still shows, the pressure sensor itself may have failed and is sending an incorrect low-pressure signal to the PCB. Sensors can also have blocked sensing tubes that give false readings. An engineer can test the sensor output voltage (expected 4.9–5.1 V DC) to confirm.
- Internal heat exchanger leak Rare
A pinhole or crack in the primary heat exchanger can cause a slow but persistent loss of water into the condensate or flue system. This is harder to spot visually but will cause chronic pressure loss that returns shortly after re-pressurising. This is a more serious and costly fault.
- Blocked circulation pump impeller Rare
A seized or partially blocked pump can disrupt water flow to the point where the pressure sensor registers an anomalous reading. The pump may also have wiring or connection faults that affect the sensor circuit indirectly.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge (or digital pressure display) on your Protherm boiler. If it reads below 1 bar, low system pressure is almost certainly the cause. If it reads a normal 1–1.5 bar, the fault is more likely a sensor or component issue and you should call an engineer.
- Re-pressurise the system using the filling loop DIY safe
Locate the filling loop — usually a short braided hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches approximately 1.2–1.5 bar, then close the valve(s) firmly. Do not over-pressurise beyond 1.5 bar. Once correct pressure is restored, attempt a boiler reset.
- Reset the boiler DIY safe
Once pressure is at the correct level, press and hold the reset button (consult your Protherm manual for its exact location on your model) until the boiler attempts to restart. If it fires up and runs normally, monitor the pressure over the following days. Do not reset more than two or three times in succession — repeated lockouts indicate a deeper problem.
- Check for obvious leaks around the system DIY safe
Walk around your home and inspect visible pipework, radiator valves, and the boiler casing for any signs of drips, damp patches, or corrosion marks. Also check underneath the boiler and around the pressure relief valve discharge pipe (usually exits outside or to a drain). Report any findings to your engineer.
- Have a Gas Safe engineer inspect the pressure relief valve and expansion vessel Gas Safe engineer
If the pressure drops again within a week or two of topping up, a Gas Safe engineer should test the PRV for weeping and check the expansion vessel pre-charge pressure using a suitable gauge. The vessel may need re-pressurising or replacing, and the PRV may need replacing if it is no longer sealing correctly.
- Have the pressure sensor tested and replaced if faulty Gas Safe engineer
An engineer can disconnect the pressure sensor and measure its output voltage. A reading outside the 4.9–5.1 V DC range indicates a defective sensor. The sensor and its connecting cables should be inspected and replaced as needed. This is an electrical task on a live boiler component and must not be attempted by an unqualified person.
- Have the circulation pump and heat exchanger inspected Gas Safe engineer
If all of the above checks are clear, the engineer should assess the circulation pump for blockages or seizure and check the primary heat exchanger for signs of internal leakage. A leaking heat exchanger in particular is a significant repair that requires proper diagnosis before any work is carried out.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If re-pressurising does not clear the fault, if F.37 returns repeatedly, or if you cannot identify a visible cause, contact a Gas Safe registered engineer. Continuing to top up a system with an undiagnosed leak wastes water, can introduce air into the system, and masks a potentially serious underlying problem.
Parts you may need
- Boiler water pressure sensor · from £25
- Pressure relief valve (3-bar PRV) · from £20
- Expansion vessel (wall-hung boiler size) · from £55
- Filling loop assembly · from £18
- Circulation pump · from £85
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Protherm F.37 fault myself?
If low pressure is the cause — which it most commonly is — yes, topping up the system via the filling loop is a straightforward homeowner task. Re-pressurising to 1.2–1.5 bar and resetting the boiler is free and takes only a few minutes. However, if the fault returns within a week or two, or if the pressure gauge already reads normal when F.37 appears, you need a Gas Safe registered engineer to investigate the underlying cause.
How often should I need to top up my Protherm boiler pressure?
A healthy sealed heating system should need topping up no more than once or twice a year, and even that is on the high side. If you find yourself re-pressurising every few weeks or the pressure drops noticeably between monthly checks, something is wrong — likely a weeping pressure relief valve, a failed expansion vessel diaphragm, or a slow leak somewhere in the system. An engineer should investigate rather than you continuing to add water indefinitely.
What does it cost to repair a Protherm F.37 fault in the UK?
If low pressure is the only issue, topping up yourself costs nothing. If an engineer visit is needed, most straightforward repairs — pressure sensor replacement, PRV swap, or expansion vessel re-pressurising — typically fall in the £80–£350 range including labour. Expansion vessel replacement usually costs £150–£300 depending on the vessel size and access. More serious faults such as a leaking heat exchanger can cost significantly more (£500–£1,000), but these are much less common causes of F.37 and would be identified during an engineer's diagnosis.
Why does F.37 appear even though my pressure gauge looks normal?
This usually points to a faulty pressure sensor rather than an actual pressure problem. The sensor may have failed electronically and is sending a false low-pressure signal to the boiler's control board, or the small sensing tube connecting the sensor to the system may be blocked, preventing it from reading accurately. In either case, a Gas Safe engineer should test the sensor output voltage and replace the component if it is out of specification. Do not keep resetting the boiler repeatedly if the pressure gauge appears fine — it will not resolve a sensor fault.