Potterton E125 Fault Code: Primary Circulation Fault

What E125 means on a Potterton boiler

E125 means the boiler has not seen the water moving through it in the way it expects. Potterton's user guides for the Promax Combi HE Plus A (UK Comp No 5114443) and the Gold Combi 24, 28 and 33 call it "Circulation Fault (Primary Circuit)". The guides for the newer Promax Ultra Combi and the Assure Combi 25 and 30 (reference 7685721) list code 125 as "Circulation fault (primary)". The primary circuit is the sealed loop that carries hot water from the boiler to the radiators and, on a combi, through the plate heat exchanger that heats your taps.

There is no flow meter involved. The boiler judges circulation from its temperature sensors. Installer manuals for the Baxi boilers that share this platform explain the check: the code is raised if the temperature has not changed by 1 degree within 15 seconds of the burner lighting, or if it overshoots the set temperature by 30 degrees twice within 10 minutes. Either pattern tells the boiler that heat is not being carried away, so it stops before anything overheats.

E125 compared with the other Potterton heat codes

If you see E110 as well as E125, the overheat thermostat has tripped too. The two often share a cause, so mention both when you book.

What the Potterton guides tell you to do

The Gold Combi and Promax HE Plus A guides group E125 with codes that "indicate faulty components", and the instruction is to note the code and contact your installer or service engineer. On the Assure and Promax Ultra, the symbol beside the number decides it: the R symbol means a user reset is allowed, while a spanner means it cannot be reset by the user. Neither guide suggests resetting repeatedly, and the simple checks below are worth doing first, because some causes of poor circulation sit outside the boiler.

Why water stops circulating

Not enough water in the system

With very low pressure the pump has too little water to move. Your boiler will usually show its low pressure code first (E118 or 118 on newer models, E119 on older ones), but a system hovering close to the limit can produce E125 instead.

Air locked in the system or the pump

Air gathers at high points after a drain down, a radiator swap or a big loss of pressure. In the radiators you can bleed it out. Trapped in the pump, it stops the impeller pushing water, and that is engineer territory.

A pump that is sticking or tired

Potterton builds in pump protection to stop it seizing: the Gold Combi guide says the pump runs for about a minute if it has not run in the previous 24 hours, and the Assure guide says the pump starts if the system drops below 7°C. That only works if the boiler stays powered. Even so, pumps wear, and a weak one will trigger E125 under load.

Sludge and restrictions

Older systems collect magnetite sludge that narrows pipes, clogs filters and starves the boiler of flow. Radiators that are warm at the top but cold along the bottom are the classic sign.

Too few radiators open

If almost every thermostatic valve is shut, perhaps on a mild day, there may be nowhere for the water to go.

Safe checks you can do

1. Turn the heating off and read the pressure cold. If it is under 1 bar, top up to 1 to 1.5 bar with the filling loop and close it fully.
2. Run the heating briefly and feel the radiators. Turn it off, let things cool, then bleed any that are cold at the top. Recheck the pressure afterwards.
3. Open several radiators fully, including the one in the room with the main thermostat if it has a thermostatic valve.
4. Check the programmer and room thermostat are set normally.
5. On an Assure or Promax Ultra showing the R symbol, press Reset for about one second. Stop after two or three attempts.
6. Listen for humming or knocking from inside the boiler and note it for the engineer.

When to call a Gas Safe engineer

Call an engineer if E125 returns after the checks above, if it appears together with E110, or if the boiler is making grinding or loud humming noises. The engineer will usually confirm the pump is running at the right speed, vent air from it, check the valves under the boiler are open and look at the system water for sludge. They may also test the temperature sensors, because the circulation check depends on them reading correctly. If the code started within days of plumbing work, ask whoever did the work to return first, as venting the system is part of that job. Our boiler pump guide explains how the pump works and how it fails.

Typical repair costs in the UK

Keep the boiler powered through the summer so its pump protection can run, and keep the cold pressure between 1 and 1.5 bar. The same code on Baxi boilers is covered on our Baxi E125 page.