Ravenheat CSI28 Fault Code: Causes, Fixes & Repair Costs
What does the Ravenheat CSI28 fault code mean?
Fault code 28 on the Ravenheat CSI 120 range is the low-water fault: the pressure switch that guards against the boiler firing without enough water has tripped, or the conditions it monitors are wrong. Ravenheat's fault-finding for this code covers the pump, incorrect system pressure and the low-water pressure switch itself needing replacement. In everyday terms there are three possibilities: the system genuinely is low on water (the most common and the one you can fix yourself), the pump is not creating the flow the switch expects, or the switch has failed and is reporting low water that is not real.
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
- System pressure genuinely low Common
Sealed systems slowly lose pressure through tiny weeps, radiator bleeding and valve seepage. Once the gauge drops toward 0.5 bar the low-water protection trips. Topping up to around 1.2 bar via the filling loop resolves this case — though a fast, repeated drop means a leak that needs finding.
- Pump not circulating properly Common
A seized rotor, air-locked pump or failing pump motor does not produce the flow/pressure differential the protection expects, so the boiler refuses to fire even with the gauge reading normal. A humming pump that never gets going is the classic sign.
- Failed low-water pressure switch Sometimes
The switch itself can stick or fail, reporting low water permanently. Ravenheat lists replacement of the switch as the remedy when pressure and pump both check out.
- Blockage or airlock in the primary circuit Sometimes
Sludge or trapped air restricting the primary flow can mimic pump failure. A system flush or bleeding resolves it once identified.
How to fix it
- Check the pressure gauge first DIY safe
If it reads below about 1 bar cold, top up slowly to around 1.2 bar using the filling loop, close the loop properly, and reset. This clears the most common cause of code 28 outright.
- Watch how quickly pressure falls afterwards DIY safe
If you are topping up more than every few weeks, there is a leak — check visible pipework, radiator valves and the boiler area for damp, and tell your engineer rather than endlessly refilling.
- Listen for the pump DIY safe
On a call for heat, a healthy pump gives a soft hum and the flow pipe warms steadily. Silence, loud humming or rattling suggests the pump — note it for the engineer.
- Engineer: test the pump and the pressure switch Gas Safe engineer
If pressure is correct and the fault persists, a Gas Safe registered engineer will free or replace the pump, test the low-water pressure switch, and check for primary-circuit blockages. Replacing the switch is the documented remedy when it has failed.
Parts you may need
- Low-water pressure switch · from £40
- Circulation pump · from £120
- Filling loop assembly · from £20
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 £100–£300, depending on the underlying cause.
Frequently asked questions
What does code 28 mean on my Ravenheat CSI?
The boiler's low-water protection has tripped. Either the system pressure is genuinely low (check the gauge — topping up to about 1.2 bar may clear it), the pump is not circulating properly, or the low-water pressure switch itself has failed and needs replacing.
I topped up the pressure but code 28 came back — why?
Two likely reasons: the system is losing pressure again through a leak, or the fault was never really pressure — a struggling pump or a failed switch produces the same code with a healthy gauge. Either way it now needs a Gas Safe registered engineer to test the pump and switch.
Is it safe to keep topping up the system?
Occasional topping up is normal; frequent topping up is not — constantly adding fresh, oxygenated water accelerates corrosion and hides a leak. If you need the filling loop more than every few weeks, have the leak found rather than managing the symptom.