Baxi H.01-.21 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi H.01-.21 fault code mean?
The H.01-.21 code is a temporary fault that appears when your Baxi boiler detects the flow temperature climbing too rapidly during domestic hot water (DHW) operation. The boiler interprets this as a sign that water is not circulating properly — or that a temperature sensor is misbehaving — and shuts down as a precaution. Unlike a full lockout 'E' code, H.01 codes are classified as temporary faults, meaning the boiler may recover on its own once the underlying problem is resolved. However, it will not resume normal operation until that issue is addressed. This code sits within the broader H.01 family: H.01-.00 relates to a loss of communication between the gas valve and boiler board; H.01-.14 flags a maximum flow temperature being exceeded or poor circulation; H.01-.18 points to absent or insufficient water circulation or a sensor fault; and H.01-.21 specifically identifies the flow temperature rising too quickly during DHW mode, again pointing to circulation problems or a sensor error.
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
- Poor or absent water circulation Common
If water cannot move freely around the system, heat builds up rapidly inside the boiler rather than being carried away — exactly what this fault code detects. Common culprits include a stuck or seized circulation pump, air locks in the pipework, a heavily blocked system filter, closed isolation valves, or sludge accumulation restricting flow through the heat exchanger.
- Faulty or failing circulation pump Common
The pump is responsible for driving hot water around your heating system and through the DHW circuit. A pump that is wearing out, running at reduced speed, or has seized entirely will cause heat to stagnate near the boiler's heat exchanger, triggering a rapid temperature rise that the boiler's safety logic flags as H.01-.21.
- Faulty flow temperature sensor (NTC thermistor) Sometimes
Baxi boilers rely on NTC thermistor sensors to continuously measure flow water temperature. If one of these sensors develops a fault — whether through component failure, a loose or corroded connector, or damaged wiring — it can report an inaccurate or erratic reading that mimics a real overheating event, causing the boiler to throw this code even when circulation is actually fine.
- System sludge or scale restricting the heat exchanger Sometimes
Over time, corrosion debris and limescale can partially block the internal passages of the heat exchanger. This restriction reduces the volume of water flowing through the boiler at any given moment, causing localised hot spots and a rapid temperature spike in DHW mode — enough to trigger the H.01-.21 protection response.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Baxi's front panel — on most models it is marked with a flame or reset symbol. Press and hold it for around 3–5 seconds until the boiler attempts to restart. Only do this once or twice. If the fault code reappears shortly after, further resets will not help and could mask a worsening problem; move on to the next steps instead.
- Check system pressure DIY safe
With the heating off and the system cooled down, look at the pressure gauge on the boiler's front panel. A healthy cold reading is between 1.0 and 1.5 bar. If the gauge shows below 1.0 bar, locate the filling loop — usually a short braided silver hose with two inline valves beneath the boiler — and open both valves slowly, watching the gauge, until pressure reaches around 1.2 bar. Close both valves fully, then try resetting the boiler. Low pressure alone can sometimes impair circulation enough to trigger this fault.
- Check that all system valves are open DIY safe
Walk around your home and confirm that the lockshield and thermostatic radiator valves (TRVs) on your radiators are not fully closed, and that any isolation valves on the pipework near the boiler are in the open position (handle in line with the pipe). A surprising number of circulation faults trace back to a valve that was accidentally left shut after maintenance.
- Inspect and clean the system filter DIY safe
If your system has an inline magnetic filter (common on Baxi installations), check whether it is overdue a clean. A heavily loaded filter can severely restrict flow. If you are confident handling the filter according to the manufacturer's instructions and there is an isolation valve on either side, you can remove and rinse it. If you are unsure, leave this for the engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault returns after the above checks, the root cause almost certainly requires professional diagnosis. An engineer will test the NTC thermistor sensors with a multimeter, check wiring harness continuity, assess pump operation and speed settings, and inspect the heat exchanger for sludge or scale restriction. They may also carry out a power flush if significant system debris is found. Do not attempt to work on the pump, sensors, PCB, or any gas components yourself — this must be handled by a Gas Safe registered engineer.
Parts you may need
- NTC flow temperature thermistor sensor · from £25
- Central heating circulation pump (e.g. Grundfos UPS2 or equivalent) · from £85
- Magnetic system filter (e.g. Adey MagnaClean) · from £55
- Pump head or cartridge (if pump body is retained) · from £45
- 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Will the H.01-.21 fault clear itself on a Baxi boiler?
Because H.01 codes are classified as temporary faults rather than hard lockouts, the boiler can in theory recover on its own if the cause is transient — for example, a brief air lock that clears itself. In practice, however, if the code keeps returning or the boiler will not fire at all, something more persistent such as a failing pump, a blocked filter, or a faulty sensor is usually to blame. A single reset is worth trying, but repeated resets without fixing the root cause will not help and may delay a proper repair.
Can I fix a Baxi H.01-.21 fault myself?
A few preliminary checks are safe for homeowners: topping up system pressure via the filling loop, checking that radiator and isolation valves are open, and resetting the boiler once or twice. Beyond that, the most likely causes — a worn circulation pump, a faulty NTC sensor, or a sludge-restricted heat exchanger — need a Gas Safe registered engineer. Working on pumps, sensors, or the PCB without the correct qualifications is unsafe and will invalidate your Baxi warranty.
How much does it cost to fix a Baxi H.01-.21 fault in the UK?
For the majority of homeowners, the repair bill falls somewhere between £120 and £380. A sensor replacement with labour typically comes to around £120–£200, while a circulation pump swap usually runs £200–£350 all-in. If the system needs a power flush to clear sludge causing restricted circulation, expect to add £350–£500 on top of any parts. In rarer cases where the PCB itself has been damaged by overheating, costs can climb to £400–£600, but this is not the typical outcome for this fault code.
Does the H.01-.21 code affect heating as well as hot water on a Baxi boiler?
H.01-.21 is specifically triggered in DHW (domestic hot water) mode, but because the underlying causes — poor circulation, a failing pump, or system sludge — affect the entire hydraulic circuit, you may well notice your central heating performing poorly at the same time. If the pump is failing or the system is badly sludged, both functions will be impaired. An engineer attending for this fault will normally assess the whole system, not just the hot water side.