Andrews A12 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A12 fault code mean?
The A12 fault code signals that the water pressure inside the heating system has fallen below the minimum threshold the appliance needs to operate safely. When this happens, the boiler shuts itself down as a precaution — you will typically see the pressure gauge sitting below 1 bar and find that heating or hot water has stopped working. A correctly pressurised sealed system should sit between 1.0 and 1.5 bar when the heating is cold. Important note: Andrews Water Heaters primarily manufactures commercial and industrial appliances, and no Andrews-specific technical manual was available to confirm this code definitively. The A12 = low water pressure definition is drawn from how this code is consistently used across UK boiler and water heater ranges. Always cross-reference with your Andrews appliance manual or contact Andrews directly for model-specific confirmation.
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
- Natural pressure loss over time Common
A sealed heating system will very gradually lose tiny amounts of water through seals, radiator vents, and valve packings. This slow seepage is the most common reason pressure dips below 1 bar, especially in older systems or after a period without a top-up.
- Radiators bled without repressurising afterwards Common
Letting air out of radiators is good practice, but each bleed releases a small amount of water along with the air. If the system is not topped back up via the filling loop afterwards, the pressure gauge will read low and the boiler may fault.
- System leak Common
A dripping radiator valve, weeping pipe joint, or pinhole in a radiator can cause a steady or rapid pressure drop. Check all visible pipework, radiator tails, and under the boiler itself for damp patches or corrosion staining.
- Faulty or waterlogged expansion vessel Sometimes
The expansion vessel absorbs the increase in pressure when water heats up. If it becomes waterlogged or loses its charge, system pressure will yo-yo — often reading fine when cold but rising excessively when hot, causing the pressure relief valve to weep and pressure to drop once it cools again.
- Defective pressure sensor or wiring fault Sometimes
The pressure sensor (also called a pressure transducer) reads system pressure and sends that signal to the control board. If the sensor itself is faulty, or its wiring has come loose or corroded, the boiler may report low pressure even when the actual system pressure is acceptable.
How to fix it
- Check the pressure gauge reading DIY safe
Locate the pressure gauge on the appliance — it may be a dial gauge or a digital display. If it reads below 1 bar, low pressure is almost certainly the trigger for the A12 code. If it reads between 1 and 1.5 bar, the fault may be sensor-related rather than actual low pressure; proceed to reset first before investigating further.
- Top up system pressure using the filling loop DIY safe
Find the filling loop — usually a short braided hose with one or two valves connecting the cold mains to the heating system. Slowly open the valve(s) and watch the gauge rise. Stop when it reaches 1.2–1.5 bar. Close the valve(s) firmly. Never leave the filling loop open and unattended, and avoid going above 1.5 bar. If you are unsure where the filling loop is, check your Andrews appliance manual or look for the hose near the underside of the unit.
- Check for obvious leaks before resetting DIY safe
Before restarting the appliance, do a quick visual check of all accessible radiator valves, towel rail connections, and any exposed pipework for drips, damp patches, or limescale streaking that can indicate a slow weep. Check the area directly under the boiler or water heater too. If you spot an active drip, do not reset — call a Gas Safe registered engineer to locate and repair the leak first.
- Reset the appliance DIY safe
Once pressure is at the correct level and no leak is visible, press and hold the reset button for 3–5 seconds (a brief tap is often not enough on commercial-style units). The appliance should attempt to restart. If it fires up and runs normally, monitor the pressure over the next 24–48 hours to confirm it is holding steady.
- Monitor system pressure over the following days DIY safe
After a successful reset, keep an eye on the pressure gauge over the next few days. A healthy sealed system should hold pressure without repeated top-ups. If the gauge drops again within a short period, this points to an ongoing leak or a failing expansion vessel — do not keep repressurising and resetting as a workaround.
- Call a Gas Safe registered engineer if pressure keeps dropping or the fault returns Gas Safe engineer
Repeated pressure loss after repressurising is a reliable sign that something needs professional attention — whether that is tracing and repairing a hidden leak, recharging or replacing the expansion vessel, or testing and replacing a faulty pressure sensor. A Gas Safe registered engineer has the tools and expertise to diagnose the root cause safely. For Andrews commercial appliances, look for engineers who hold commercial gas qualifications (CIGA or equivalent) in addition to domestic Gas Safe registration, as the scope of work may differ.
Parts you may need
- Pressure sensor / transducer · from £35
- Expansion vessel (residential-scale) · from £55
- Expansion vessel (commercial-scale) · from £120
- Filling loop assembly · from £18
- Pressure relief valve · from £25
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–£400, depending on the underlying cause.
Frequently asked questions
Can I repressurise an Andrews water heater myself or does it need a Gas Safe engineer?
Topping up system pressure via the filling loop is a homeowner-safe task on most sealed heating systems — you are simply adding cold mains water, not touching any gas components. Follow the steps in the appliance manual, aim for 1.2–1.5 bar, and do not exceed 2 bar. However, if you are unsure which model you have, or the appliance is a larger commercial unit with a more complex filling arrangement, it is always sensible to check with Andrews directly or have an engineer carry out the first repressurise and show you the correct procedure.
Why does my boiler keep losing pressure and showing A12 even after I top it up?
If pressure drops again within hours or days of repressurising, there is almost always a leak somewhere in the system — even a very slow one — or the expansion vessel has failed. A waterlogged expansion vessel is a particularly common culprit: it can cause pressure to spike when hot, force the pressure relief valve to release water, and then leave the system under-pressurised once it cools. A Gas Safe engineer can test the expansion vessel charge and carry out a leak detection check. Recharging or replacing the expansion vessel typically costs £150–£300 including labour, which is well within the normal repair range for this fault.
Could the A12 code be a sensor fault rather than actual low pressure?
Yes. If your pressure gauge reads a normal 1–1.5 bar but the A12 fault is still showing, a faulty pressure sensor or a loose wiring connection to it is a likely explanation. The boiler's control board relies on the sensor's reading — if that reading is wrong, the board thinks the system is under-pressurised even when it is not. Diagnosing and replacing a pressure sensor is a job for a qualified engineer, as it involves accessing internal components.
Is a Gas Safe engineer required for Andrews commercial water heaters, or are different qualifications needed?
Andrews Water Heaters primarily makes commercial and industrial appliances. While Gas Safe registration is the baseline legal requirement for any gas work in the UK, commercial gas work often requires additional competencies — such as the CIGA (Commercial Industrial Gas) accreditation or specific appliance-type endorsements. When booking an engineer for an Andrews commercial unit, confirm they hold the appropriate commercial gas qualifications for the appliance type, not just a domestic boiler ticket.