Overview
E37 on a Keston C36 combi normally indicates a problem with the heating system water pressure. On the C36 the manufacturer’s documentation commonly shows E37 as a water pressure error and the usual fix is to top up the system until the pressure is in the normal range. The boiler detects pressure that is outside its expected limits and locks out or displays the fault to prevent damage or unsafe operation. This fault is often low pressure (not enough water in the sealed heating system) and is usually not an emergency if the boiler has not lost heat entirely. It is a common, low-to-medium severity fault that homeowners can often correct themselves by re-pressurising the system using the filling loop. However, if pressure drops repeatedly, you may have a leak or a failed component (pressure relief valve, expansion vessel, pipework) and that requires a Gas Safe registered engineer. Also be aware that some service documents and third-party sites sometimes list different meanings for E37 (for example thermistor or control issues); if topping up pressure does not clear the code, the fault may be electrical or sensor related and will need a professional. In short: check and correct the boiler pressure first (DIY if you are comfortable and follow safety notes). If the pressure keeps falling, the code persists after correct pressure, or you are unsure about any step, stop and call a qualified heating engineer (Gas Safe registered) to investigate further.
Possible Cause: Water pressure error – increase water pressure
Troubleshooting Steps
Safety precautions:
1) If you smell gas or suspect a gas leak, leave the building immediately and call the gas emergency number. Do not attempt repairs. 2) Turn the boiler off before you start working on it (use the on/off switch). 3) Do not remove the boiler cover or work on gas or electrical components unless you are a Gas Safe registered engineer. 4) Protect floors and carpets when bleeding radiators or draining water.
Initial checks a homeowner can do:
1) Look at the boiler display and pressure gauge. The gauge is usually on the front and will show the system pressure in bar. Typical cold pressure target is around 1.0–1.5 bar (manufacturer text also references 1.0–2.0 bar; aim for about 1.0–1.3 bar as a good nominal value). 2) Some C36 units allow you to display the exact pressure by pressing the installer button repeatedly until the number “5” appears — wait a few seconds and the display will show the system pressure in bar. 3) Visually check pipes, radiators and under the boiler for obvious leaks or drips. 4) Note whether this is a one-off low-pressure event or if the pressure has been falling repeatedly over days or weeks.
Diagnostic and fix steps (what to do to top up pressure and test):
1) Locate the external filling loop: this is a small braided hose or flexible pipe under the boiler connecting the mains cold water to the heating return. Some systems have a permanent braided loop with two isolation valves; others use a removable loop. 2) Ensure both filling loop valves are closed before you start. 3) Slowly open the filling loop valves (or open the single filling valve) to allow mains water into the heating system. Do this slowly and watch the pressure gauge. 4) Stop when the gauge reaches about 1.0–1.5 bar (aim for around 1.3 bar). Do not over-pressurise; if you go above the recommended limit, close the valve and release pressure by bleeding a radiator or using the system drain cock. 5) Close the filling loop valves fully. If the loop is removable, remove and stow it (or ensure its isolating valves are fully closed). 6) Turn the boiler back on and press the reset or standby button as required. Demand central heating or run a hot water cycle and watch the pressure: it will rise slightly as the system heats (a small rise of 0.2–0.3 bar is normal). 7) Bleed radiators to remove trapped air and re-check pressure. After bleeding, repressurise if needed. 8) Inspect for leaks while the system is up to temperature and as it cools down.
What to do if problems continue or if you see other symptoms:
1) If the pressure keeps dropping repeatedly (you need to top up more than a couple of times over a short period), do not continue to keep topping up without investigation. Persistent loss usually indicates a leak in pipework, radiators, radiator valves, the boiler heat exchanger, or a failed pressure relief valve/expansion vessel. 2) If E37 remains showing after you have set correct pressure, or if the boiler reports other errors, the fault may be a faulty sensor (pressure switch, return thermistor) or control board issue — these require a Gas Safe engineer. 3) If you accidentally overfill and cannot reduce pressure with radiator bleeding or the drain cock, switch the boiler off and contact a qualified engineer. 4) If you see water on the floor around the boiler or from any components, isolate the boiler electrical supply and call a Gas Safe engineer.
When to call a professional:
1) Call a Gas Safe registered heating engineer if there is a persistent pressure loss, visible leakage, you cannot locate or operate the filling loop, or if the fault code does not clear after correct pressure is restored. 2) Call a professional for any internal repairs, sensor, PCB, gas valve, or heat-exchanger work, or if you are uncomfortable performing the filling procedure. 3) If the boiler shows different fault patterns or additional codes after topping up, have a qualified engineer inspect and test the system.
Additional notes:
1) E24 or a logged history of repeated low-pressure events may indicate a leak and will commonly need a service visit. 2) Some third-party sites list alternate meanings for E37 (sensor or airflow related), but for a Keston C36 the manufacturer documentation primarily refers to E37 as a water pressure error — always start by checking the pressure. 3) Never attempt gas or sealed-system component repairs yourself; always use a Gas Safe registered engineer for those tasks.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.