The rule underneath all of it
A boiler cannot see a leak. It can only see three things that a leak eventually affects: the water pressure in a sealed system, the flow of water past the heat exchanger, and in a few cases wet electrics inside its own case. Everything else it reports concerns gas, combustion, fans, sensors and boards.
That gives a test you can apply to any code on any brand. If the code is about water pressure, the system may be losing water and the question is how fast. If it is about ignition, flame, flue or overheating, it is a heating engineer's problem and a leak firm has nothing to look for.
Codes that point at water loss
| Brand | Code | What the manufacturer publishes |
|---|
| Baxi | E118 | "Water pressure LOW (<0.5 bar)". This is the low pressure code Baxi publishes in its technical support list. |
| Baxi, Potterton | E119 | Not defined separately in Baxi's published table. Baxi names it in its guidance on frozen condensate pipes, and appliances that display it are treated in the field as being in a pressure lockout. Read the gauge and check the condensate pipe. |
| Glow-worm | F22, F9 on older appliances | "Safety switch-off. Low water pressure in the boiler." Causes given as no or insufficient water in the product, a defective water pressure sensor, or a loose or defective cable to the pump or sensor. |
| Glow-worm | F24, F75 | Water pressure sensor faults rather than water loss: F.24 covers a sensor not connected or short-circuited, F.75 a defective pressure sensor or switch. Worth knowing so you are not sold a leak search. |
| Viessmann | F2, or 345 on the current Vitodens 100-W | "Temperature limiter has responded." An overheat lockout, but Viessmann's first listed remedy is to check the heating system fill level, so water loss is a possible indirect cause. |
Notice how short that list is, and how qualified. Two of the five entries are sensor faults that produce a pressure code on a system that is perfectly full, and the Viessmann entry is not a pressure code at all.
Codes that do not mean a leak
| Brand | Code | What it actually is |
|---|
| Baxi, Potterton | E133 | "Gas supply fault. Check the power supply, sensing electrode, spark electrode and cable." Gas Safe engineer. |
| Baxi, Potterton | E23, E53, E128 | Named by Baxi alongside E133 and E119 as codes a frozen condensate pipe can produce. |
| Glow-worm | F1, and the current F28 and F29 | Ignition fault, ignition unsuccessful at start-up, and flame loss. Glow-worm links them to gas supply, including a prepayment meter out of credit, or a frozen or blocked condensate pipe. |
| Glow-worm | F61 to F68 | Main board faults, per Glow-worm's own homeowner guidance. |
| Glow-worm | Err | A user interface problem. |
| Viessmann | F9, FA, FC | Fan speed, fan not at standstill, and gas valve or blocked vent system faults. |
If your boiler is showing one of these and someone has quoted you for leak detection, ask them which part of the code they are responding to. There is no honest answer.
The frequency test, which matters more than the code
Once you know you have a genuine pressure code, the diagnosis is not the number on the display. It is how long the system holds pressure after you top it up.
| How often you refill | What it means |
|---|
| Once or twice a year | Normal for a sealed system. No action. |
| Every month or two | A weeping joint, valve or towel rail, or a failing expansion vessel. Worth investigating before it gets worse. |
| Weekly | An active leak, usually under a floor, behind a wall or inside the boiler case. |
| Daily, or it will not hold at all | A significant leak, or a pressure relief valve dumping to the outside discharge pipe. |
The reason weekly matters is not the inconvenience. Every refill puts fresh, oxygenated mains water into a circuit designed to be closed, which corrodes radiator steel and copper joints from the inside and produces the next pinhole. Meanwhile the water that has left is in a joist, a screed bed or a ceiling void.
Two things that empty a system and are not leaks
Rule these out before paying anyone to search. If pressure climbs steeply while the heating runs and then sits below normal once it cools, the expansion vessel has probably lost its air charge and the system is venting through the relief valve every cycle. Viessmann lists checking the vessel pre-charge among its own remedies for the limiter fault, and Baxi names the expansion vessel and a debris-fouled pressure relief valve among the reasons a boiler keeps losing pressure.
The second is the relief valve itself. If the discharge pipe on the outside wall drips whenever the boiler fires, the water is going outside and the gauge will keep falling whatever you do. Both are component repairs, not detection jobs.
The pages behind each code
- Baxi E119 — what Baxi actually publishes, and the two checks that split the fault.
- Baxi E133 — a gas supply fault, what you can check safely, and when to stop.
- Glow-worm F22 — the low water pressure shutdown, and telling a leak from a sensor.
- Glow-worm F1 — ignition, gas supply and the condensate pipe.
- Viessmann F2 — the temperature limiter, and why fill level is the first thing Viessmann checks.
- How to repressurise a boiler — the method, and how often is too often.
Other brands use their own numbering for the same handful of faults. The brand pages in this hub cover them: Vaillant, Ideal and Worcester Bosch. If the gauge falls and nothing anywhere is wet, start with a system losing pressure with no visible leak, which is the commonest version of this problem and the one people waste the most money on.
What a detection visit does that guessing does not
The circuit is isolated and pressure tested first, which confirms the loss is real and sizes it, then traced with thermal imaging through screed and tile and, on a drained circuit, with tracer gas, which finds what thermal cannot. The floor is opened at the point of the leak rather than searched. We work to a fixed hourly rate, settled before we attend rather than after, and if we do not find the leak you do not pay for the time we spent looking. The repair and the making good afterwards are ours as well.
And if the code on your display is in the second table, none of that applies. Call a Gas Safe registered engineer instead, and check their registration on the register's own site before they start.
