Two checks, in this order
- Read the pressure gauge cold. If it sits below about 0.5 bar the boiler will not fire. Baxi's own technical list defines its low water pressure code as "Water pressure LOW (<0.5 bar)", and that is the threshold the appliance is watching.
- Find the condensate pipe. It is the white or grey plastic pipe, usually 21.5mm or 32mm, running out of the bottom of the boiler and through the wall. Baxi's published homeowner guidance groups E119 with E23, E53, E133 and E128 as codes a frozen condensate pipe can produce. In a cold snap this is the more likely of the two.
Those two checks separate the whole problem. A low gauge is a water question and eventually ours. A normal gauge with a frozen outside pipe is a twenty-minute job with a hot water bottle.
What Baxi actually publishes, and what it does not
Most pages about E119 state flatly that it means low water pressure. Baxi's own documents are more careful than that, and the difference is worth knowing before you pay anyone.
| Code | Baxi's published wording | Where it appears |
|---|
| E118 | Water pressure LOW (<0.5 bar) | Baxi's technical support error code list |
| E133 | Gas supply fault. Check the power supply, sensing electrode, spark electrode and cable. | Baxi's technical support error code list |
| E119 | Named alongside E23, E53, E133 and E128 as a code shown when "your condensate pipe may be frozen". No separate definition given. | Baxi's homeowner FAQ |
So E119 is not in Baxi's published fault table with a meaning of its own, and the low pressure code Baxi does publish is E118. In the field E119 appears on Baxi and Potterton appliances and is treated as a pressure lockout, and the first thing every engineer does is read the gauge. That is a sensible starting point, not a documented definition, and you should not let anyone quote for a leak search on the strength of the code alone. Baxi and Potterton are both Baxi Heating brands, which is why the same code lands on both badges.
If the gauge is low: top it up, then count
Repressurising clears the lockout on most Baxi and Potterton combis. Switch the boiler off, open both ends of the filling loop until the gauge reads between 1.0 and 1.5 bar cold, close them fully, and restart. Some models use a built-in key or lever instead of a braided hose. The full method, including where people usually go wrong, is worth reading once.
The number that matters is not the pressure. It is how often you have to do this.
- Once or twice a year. Normal for a sealed system. Nothing to investigate.
- Every few weeks. Something is losing water. A weeping valve, a towel rail, a corroded radiator or a pinhole under a floor.
- Weekly, or it will not hold at all. An active leak, and by then it is usually doing damage somewhere you cannot see.
Every top-up pushes fresh mains water into a system designed to be closed. That water carries oxygen, oxygen corrodes steel radiators and copper joints from the inside, and the corrosion makes more pinholes. Topping up weekly is not a fix. It is a way of feeding the fault while the escaping water soaks a joist.
Where the water actually goes
Check the things you can see first, because half the time the answer is there. Look at the pressure relief discharge pipe on the outside wall: if it drips whenever the boiler fires, the system is dumping water outside and no amount of topping up will hold. Then look under the boiler case, at every radiator valve spindle and tail nut, at the underside of radiators where a rust bloom forms, and at bathroom towel rails, which fail more often than anything else on a heating circuit.
If all of that is dry, the loss is under a floor, inside a wall, or in the boiler itself. A pinhole in a 15mm heating pipe under screed can empty enough pressure to trip an E119 every week while producing no visible damp for months, because the screed holds the water and the heating dries the surface. On underfloor heating the same thing happens across a much longer pipe run, which is why guessing is expensive there.
If the gauge is fine: the condensate pipe
A condensing boiler produces acidic condensate and has to drain it. When the outside section of that pipe freezes, the condensate backs up, a sensor detects it, and the boiler shuts down rather than letting it overflow inside the case. Baxi's advice is to switch the boiler and its electrical supply off, warm the frozen section with a hot water bottle or a heat pack rather than boiling water, and restart once it has cleared. It may need repeating.
If it freezes every winter the pipe run is the problem, not the weather. A longer external run, an undersized diameter, a shallow fall or an open outlet in the wind will all do it, and re-routing or lagging it properly is a plumbing job rather than a detection one.
Two faults that look exactly like a leak
Before anyone lifts a floor, rule these out. If the pressure climbs steeply when the heating runs and then drops below normal as the system cools, the expansion vessel has probably lost its air charge. The system is not losing water, it simply has nowhere to put the expansion, and it vents through the relief valve each cycle. That is a recharge or a new vessel.
The second is the pressure relief valve itself. A speck of debris on the seat lets it weep continuously to the outside discharge pipe. Both faults empty a system convincingly and neither needs a leak detection visit, which is why the first thing we do on a heating call is a pressure test rather than a search.
When it is worth calling us
Call when the pressure will not hold for a full heating cycle, when you are topping up weekly, when there is a warm or damp patch on a floor, or when E119 returns after every reset with a gauge that keeps falling. Do not call us for a frozen condensate pipe, and do not call us if the boiler has a gas or ignition fault rather than a pressure one.
What a visit does: the circuit is isolated and pressure tested to confirm the loss is real and to size it, then traced with thermal imaging and, on a drained circuit, tracer gas, so the floor is opened at one point rather than searched. We charge a fixed hourly rate, agreed before we attend, 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 too, which is the part most boiler and heating leak callouts leave to somebody else.
If your boiler is not a Baxi, the same logic applies with different numbering. Which fault codes actually mean water loss sets the brands side by side.
