Biasi ER13 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER13 fault code mean?
The ER13 code on a Biasi boiler signals that the boiler's PCB has detected an abnormal temperature differential between the flow (delivery) and return pipes of the central heating circuit — specifically, a gap exceeding 40°C (40 Kelvin) between the two. This temperature imbalance suggests that water is not circulating properly through the system, causing the flow side to overheat while the return stays comparatively cool. As a safety measure, the boiler locks out to prevent damage. On certain Biasi model ranges, ER13 can alternatively point to a flame detection issue — where the PCB either cannot confirm a stable flame after ignition or loses the flame signal unexpectedly. In either interpretation, the boiler enters lockout and will not restart until the underlying 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
- Blocked or slugged central heating circuit Common
Sludge, limescale, and debris accumulating in the pipework and radiators restrict water flow around the system. When circulation slows, the flow temperature climbs rapidly while the cooler water returning to the boiler barely rises — widening the differential until the PCB triggers a lockout. This is the most frequent root cause of ER13 on older or unmaintained systems.
- Seized or failing circulation pump Common
If the pump is not moving water effectively — whether due to a seized motor, worn impeller, or airlock within the pump itself — the heat exchanger cannot shed heat fast enough. The resulting temperature spike between flow and return is a classic trigger for this fault code, and the pump may be found to be warm but stationary when inspected.
- Faulty flow or return NTC temperature sensor Sometimes
The PCB relies on readings from two NTC thermistor probes — one on the flow pipe and one on the return — to calculate the temperature differential. If either probe has drifted out of calibration, developed a short, or failed entirely, the PCB may calculate a false differential and lock the boiler out even when circulation is perfectly normal.
- Blocked primary heat exchanger Sometimes
Limescale build-up inside the heat exchanger restricts the passage of water, reducing flow rate and causing localised overheating on the flow side. This is more common in hard-water areas and in boilers that have never had inhibitor added to the system.
- Airlock in the heating circuit Sometimes
Air trapped in the system — particularly after a refill or partial drain-down — can interrupt water flow in sections of the circuit. Like sludge, an airlock prevents adequate circulation and can cause the flow temperature to spike, triggering the differential fault.
- Flame detection or PCB connection fault (select models) Rare
On some Biasi model ranges, ER13 relates to a flame-sensing failure rather than a temperature differential. A contaminated or damaged ionisation electrode, unstable gas supply, or a loose connection between the ionisation circuit and the PCB can prevent reliable flame confirmation, causing a lockout after each ignition attempt.
How to fix it
- Note the exact fault code displayed and attempt a single boiler reset DIY safe
Press and hold the reset button for 3–5 seconds as described in your boiler's user manual (the button is usually marked with a flame or restart symbol). If the boiler fires up and runs normally, monitor it over the next hour. If ER13 returns within a short time, a reset alone will not solve the problem — do not repeat the reset more than two or three times in total, as repeated lockouts signal an underlying fault that needs professional diagnosis.
- Check system pressure on the boiler's pressure gauge DIY safe
With the boiler cold, the pressure gauge should read between 1.0 and 1.5 bar. If it has dropped below 1 bar, top up via the filling loop (usually a braided flexible hose beneath the boiler with a valve at each end) until the gauge reads approximately 1.2 bar, then close the valves. Low pressure can contribute to poor circulation. If pressure is already within range, move on to the next step.
- Bleed any radiators that feel cold at the top DIY safe
Cold patches at the top of radiators indicate trapped air, which can disrupt circulation. Using a radiator bleed key, open the bleed valve on affected radiators until water (not air) flows out, then close the valve. Re-check the boiler pressure afterwards and top up if it has dropped. This is straightforward and safe for a homeowner to carry out.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the fault persists after the checks above, an engineer is needed. They will test the circulation pump (checking it is running and delivering adequate flow), measure the resistance of both NTC probes with a multimeter and replace any that are out of specification, inspect the heat exchanger for limescale or sludge restriction, and check all PCB connections for corrosion or looseness. If sludge is confirmed, they will recommend a powerflush. On models where ER13 relates to flame detection, the engineer will also inspect the ionisation electrode and gas supply stability.
Parts you may need
- NTC flow temperature sensor (thermistor probe) · from £25
- NTC return temperature sensor (thermistor probe) · from £25
- Central heating circulation pump · from £75
- Ionisation/flame detection electrode · from £20
- Biasi PCB (printed circuit board) · from £180
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler myself to clear the ER13 fault?
Yes, a single reset is a sensible first step and is safe for a homeowner to try. Press and hold the reset button for 3–5 seconds. If ER13 clears and the boiler runs normally, keep an eye on it. However, if the fault returns within a short period, repeated resets will not fix the underlying problem — you'll need a Gas Safe engineer to investigate the cause.
Why does my Biasi show ER13 — is it definitely a flow/return temperature issue or a flame fault?
It depends on the model. The Biasi Rinnova service manual specifically describes ER13 as a 'Delta T M-R greater than 40K' fault — meaning the temperature difference between the flow and return pipes has exceeded 40°C. This is the most widely documented meaning and is the primary cause to investigate. On some other Biasi models, ER13 has been associated with flame detection problems. If your engineer cannot find a circulation or sensor fault, they should also check the ignition and ionisation circuit.
How much does it cost to fix an ER13 fault on a Biasi boiler?
Most homeowners pay between £150 and £380 for the repairs most commonly linked to ER13. Replacing an NTC sensor typically costs £140–£250 including labour, while a new circulation pump usually comes to £150–£300. If the system is heavily sludged and needs a powerflush, that adds £300–£600 on top. In rare cases where the PCB needs replacing on an older Biasi boiler, the cost can make a new boiler more economical — worth discussing with your engineer.
Is the ER13 fault covered by my Biasi warranty?
Biasi boilers generally carry a manufacturer's warranty of between 5 and 7 years depending on the model. If your boiler is still within this period, contact Biasi directly before commissioning any paid repairs — warranty work must be carried out by an approved engineer and attempting DIY fixes can invalidate the cover. Have your boiler's model number and installation date to hand when you call.