Biasi ER04 Fault Code: Primary Circuit / Water Pressure Error
The ER04 fault code on a Biasi boiler indicates a problem within the primary circuit — most commonly insufficient water pressure or a circulation failure. The boiler's safety controls have detected that water flow through the system is either absent or too weak to operate safely, causing a protective lockout. Note: a small number of sources link ER04 on certain Biasi models to a fan fault rather than a pressure or circulation issue, so it is worth cross-referencing your specific model's manual if the pressure-related checks below do not resolve the problem.
General guidance only — not a substitute for professional advice. 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.
What does the Biasi ER04 fault code mean?
The ER04 fault code on a Biasi boiler indicates a problem within the primary circuit — most commonly insufficient water pressure or a circulation failure. The boiler's safety controls have detected that water flow through the system is either absent or too weak to operate safely, causing a protective lockout. Note: a small number of sources link ER04 on certain Biasi models to a fan fault rather than a pressure or circulation issue, so it is worth cross-referencing your specific model's manual if the pressure-related checks below do not resolve the problem.
Common causes
- Low system pressure Common
If the water pressure in the heating system has dropped — typically below 0.5 bar — the boiler will lock out to protect itself. This is the single most common trigger for ER04. Pressure loss usually points to a slow leak somewhere in the system pipework, radiators, or inside the boiler itself.
- Faulty or blocked circulation pump Common
The pump is responsible for pushing hot water around the system. If it has seized, is running on too low a speed setting for the size of the heating system, or is partially blocked, flow will be inadequate and the boiler's flow sensor will trigger an ER04 lockout.
- Airlock in the primary circuit Sometimes
Trapped air can stop water circulating properly, making the boiler behave as though there is insufficient flow even when system pressure looks normal. Bleeding radiators and checking for airlocks at high points in the pipework can resolve this.
- Faulty pressure gauge or pressure switch Sometimes
A pressure sensor that gives an inaccurate reading can cause the boiler to report a pressure fault when the actual system pressure is within normal range. This requires an engineer to diagnose and replace the faulty component.
- Cracked or leaking heat exchanger Rare
A heat exchanger that has deteriorated and developed hairline cracks will leak water internally, causing a gradual pressure drop and potential flow restriction. This is a more serious and expensive fault that is typically found after simpler causes have been ruled out.
- Fan fault (model-dependent) Rare
On some specific Biasi models, ER04 has been linked to a malfunctioning fan rather than a circulation issue. A failed fan prevents safe flue gas expulsion, triggering a safety lockout. If the pressure and circulation checks below draw a blank, ask your engineer to consider this possibility.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge — it should read between 1 and 2 bar when the system is cold. If it reads below 1 bar, low pressure is the likely cause of the ER04. If the gauge reads 0 or is clearly faulty, note this for the engineer.
- Top up system pressure using the filling loop if pressure is low DIY safe
Locate the filling loop (usually a braided silver hose with one or two valves beneath the boiler). Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches approximately 1.5 bar, then close the valve(s) firmly. Never exceed 2 bar. If you are unsure where the filling loop is or how to operate it, check your boiler manual or call an engineer.
- Reset the boiler DIY safe
Once pressure is at the correct level, attempt a reset. Most Biasi boilers have a reset button on the front fascia — press and hold it for around three seconds as described in your model's manual. Alternatively, switch the boiler off at the mains, wait ten seconds, and switch it back on. Limit resets to two or three attempts; repeated resetting without resolving the underlying fault can cause further damage.
- Bleed your radiators if pressure keeps dropping DIY safe
If the pressure returns to normal after topping up but then drops again within a day or two, trapped air or a slow leak may be involved. Bleeding the radiators — using a radiator key to release trapped air from the bleed valve at the top of each radiator — can help eliminate an airlock as a cause. Remember to re-check and top up pressure again after bleeding.
- Check the gas supply is on DIY safe
Confirm that the gas isolation valve near the boiler is in the open position (handle inline with the pipe) and that other gas appliances in the property are working. This helps rule out a gas supply interruption as a contributing factor.
- Call a Gas Safe registered engineer to investigate further Gas Safe engineer
If the fault persists after topping up pressure and resetting, or if pressure keeps dropping, a Gas Safe engineer must carry out a full diagnosis. They will check the circulation pump (speed setting, seals, and condition), inspect the heat exchanger for leaks or cracks, test the pressure switch and flow sensor for accuracy, and — where model-specific ER04 definitions include the fan — inspect and test the fan assembly. Do not continue running the boiler or repeatedly resetting it at this stage.
Parts you may need
- Circulation pump (e.g. Grundfos UP series) · from £80
- Pump seals / O-ring kit · from £15
- Pressure switch · from £25
- Pressure gauge · from £20
- Fan assembly (model-specific) · from £90
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Biasi ER04 myself by topping up the pressure?
If low pressure is the cause, topping up via the filling loop is a safe DIY task and may clear the fault immediately. However, if the pressure drops again within a few days, there is a leak or another underlying fault that requires a Gas Safe registered engineer. Repeatedly topping up without finding the root cause is not a long-term fix.
How often should system pressure drop on a Biasi boiler?
A small pressure loss over several months (perhaps 0.1–0.2 bar) is normal as systems naturally lose a little water over time. If you are needing to top up more than once a month, that points to an active leak — either in the pipework, radiators, or internally within the boiler — which should be investigated by an engineer.
Why does my Biasi boiler show ER04 even though the pressure looks fine?
A normal-looking pressure reading doesn't always rule out a circulation problem. The fault can also be caused by a faulty pump, an airlock preventing water from flowing properly, or a defective pressure switch giving a false reading. On certain Biasi models, ER04 can also relate to the fan rather than circulation — worth mentioning to your engineer if pressure-related checks don't resolve the code.
Is it worth repairing a Biasi boiler showing ER04, or should I replace it?
For most common causes — pump, pressure switch, or seals — repair costs typically fall in the £120–£380 range and are usually worth doing. If the heat exchanger is cracked, replacement parts and labour can push costs above £500, at which point many engineers and industry guidance suggest that a new boiler may be more economical, particularly if the boiler is over ten years old.