Alpha Red A Red B Alternating Fault Code: Causes, Fixes & Repair Costs
What does the Alpha Red A Red B Alternating fault code mean?
When the A and B indicator lights on your Alpha boiler flash alternately — each one lighting up in turn — the boiler is telling you that system pressure has dropped too low for safe operation. You will typically see the pressure gauge sitting below 1 bar, outside the green zone. The boiler shuts down to protect itself until pressure is restored to the correct level (between 1.0 and 1.5 bar). This is one of the most straightforward fault indicators on Alpha boilers and, in many cases, can be resolved by the homeowner without any tools. Note: do not confuse this with a red A light flashing alongside a steady yellow B light, which signals a burner ignition failure — that is a separate fault requiring an engineer.
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
- Gradual pressure loss over time Common
Sealed central heating systems lose tiny amounts of water through microscopic seepage at fittings, valve packings, and pump seals over months and years. This is the most common reason pressure drifts below 1 bar, and a single top-up usually resolves it temporarily. If it needs topping up more than two or three times a year, the underlying cause needs investigating.
- Recently bled radiators Common
Bleeding a radiator releases air but also lets a small amount of water out of the system, which can push the pressure just below the acceptable threshold. If you or a plumber recently bled radiators and the Red A / Red B code appeared shortly afterwards, this is almost certainly the reason.
- Failing or waterlogged expansion vessel Sometimes
The expansion vessel contains a pressurised air chamber separated by a rubber diaphragm. As heating water expands and contracts, the vessel absorbs the change. If the diaphragm perishes or the air charge is lost, the vessel can no longer do its job — pressure drops when the system cools and rises sharply when it heats up. Repeated low-pressure faults that keep coming back are a strong indicator of this problem.
- Weeping pressure relief valve (PRV) Sometimes
The PRV is a safety device that opens to release water if pressure climbs too high. Over time the valve seat can corrode or scale up, causing it to weep small amounts of water continuously. You may notice a damp patch or a thin discharge pipe dripping outside the property. A weeping PRV will cause the system to slowly but persistently lose pressure.
- A leak elsewhere in the heating system Sometimes
A joint, radiator valve, or section of pipework may be leaking — sometimes so slowly that no obvious wet patch is visible until you look carefully. A sudden or rapid pressure drop is a strong indicator of a leak rather than ordinary seepage. Check radiator valve tails, visible pipework joints, and the area beneath the boiler for signs of moisture.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge (either a dial or a digital reading on the display). If it reads below 1 bar the low-pressure diagnosis is confirmed. Make a note of the reading before you do anything else — this gives you a baseline.
- Locate the filling loop and repressurise the system DIY safe
The filling loop on most Alpha boilers is a short braided flexible hose with one or two valves, usually found beneath the boiler casing or inside a cupboard on the pipework. Slowly open the valve(s) by turning a quarter-turn and watch the gauge rise. Stop when the needle reaches 1.2–1.5 bar (in the green zone). Close the valve(s) fully. Do not overfill — if the pressure goes above 1.5 bar, bleed a small amount of water from a radiator bleed valve until it drops back into range.
- Reset the boiler DIY safe
Once pressure is back in the green zone, reset the boiler by rotating the selector dial to the RESET position. Hold it there for approximately 30 seconds, then return the dial to your normal heating or hot water setting. The alternating lights should stop and the boiler should fire up normally.
- Monitor the pressure over the next few days DIY safe
Check the gauge daily for four or five days. If pressure holds steady above 1 bar, the issue was simply gradual seepage or a recent radiator bleed. If it drops again within a week or two, there is an underlying problem that needs professional investigation — do not simply keep topping up without finding the cause.
- Inspect visible pipework, radiator valves, and underneath the boiler for leaks DIY safe
Run your hand along accessible pipework and around the base of radiators to feel for damp. Look for mineral staining (white or brown deposits) around joints, which indicates that water has been escaping and evaporating. Check the condensate pipe and its connections too. Report any findings to the engineer.
- Call a Gas Safe registered engineer to diagnose recurring pressure loss Gas Safe engineer
If the pressure drops repeatedly, or if you found any signs of a leak during your inspection, you need a Gas Safe registered engineer. They will pressure-test the system, check the expansion vessel pre-charge pressure, inspect the PRV, and trace any hidden leaks. Do not continue to top up the system without finding the root cause — persistent low pressure can eventually damage the pump and heat exchanger.
Parts you may need
- Expansion vessel (replacement unit) · from £55
- Expansion vessel Schrader valve · from £5
- Pressure relief valve (PRV) · from £20
- Filling loop assembly · from £15
- System pressure gauge · from £18
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 £80–£320, depending on the underlying cause.
Frequently asked questions
Can I repressurise my Alpha boiler myself to fix the Red A Red B fault?
Yes — repressurising is explicitly described in Alpha's own user manuals as a homeowner task. Locate the filling loop beneath the boiler, open the valve(s) until the gauge reaches 1.0–1.5 bar, then close the valve and reset the boiler. If you are comfortable following those steps there is no need to call an engineer for a one-off pressure drop. However, if the pressure falls again within a short time, that points to a leak or a failing expansion vessel and you should get a Gas Safe engineer to investigate rather than keep topping up.
Why does my Alpha boiler keep losing pressure even after I top it up?
Recurring pressure loss usually means one of three things: a slow leak somewhere on the system (often at a radiator valve, a compression fitting, or the boiler's own connections), a weeping pressure relief valve that is releasing water through the discharge pipe, or a waterlogged expansion vessel that can no longer compensate for normal pressure fluctuations. A Gas Safe engineer can pressure-test the system, recharge or replace the expansion vessel, and replace a faulty PRV. Continuing to top up without fixing the root cause can mask a worsening leak and risk longer-term damage to the pump.
How much does it cost to fix the cause of low boiler pressure in the UK?
If a single top-up solves the problem, the fix is free. For a professional repair, costs depend on what is wrong: replacing or recharging an expansion vessel typically runs £150–£250 including labour; replacing a pressure relief valve is usually £80–£180; tracing and repairing a system leak generally costs £150–£350 depending on where the leak is and how accessible the pipework is. Very large jobs such as a corroded heat exchanger can cost considerably more, but those are rarely the cause of straightforward low-pressure faults — mention any concerns about big-ticket components to your engineer before they start work.
What is the correct pressure for an Alpha boiler when cold?
Alpha recommends a cold-system pressure of 1.0–1.5 bar, shown in the green zone on the pressure gauge. When the system is fully up to operating temperature the pressure will rise slightly — typically to around 1.5–2.0 bar — which is normal. If the gauge reads above 2.5 bar when cold, or if the PRV discharge pipe is dripping, the system may be overpressurised and you should call an engineer rather than adding more water.