NOBLE and KNIGHT combi boilers use a stainless plate heat exchanger to produce DHW on demand. Hot boiler water on one side, cold incoming domestic water on the other, separated by stainless plates. Heat transfers across the plates; DHW exits at setpoint temperature.
How it fails
Hard water scales the DHW side of the plates. As scale accumulates:
- DHW flow rate drops (smaller channel cross-section)
- DHW outlet temperature drops (less heat transfer)
- Boiler high-fires longer to maintain setpoint
- Eventually E29 high-limit lockouts during DHW calls
Descaling (preferred — annual on hard water)
- Isolate DHW side — close inlet and outlet shutoffs (most combis ship with these)
- Connect descaling pump via the service tees on the DHW side
- Circulate descaling solution (vinegar or commercial descaler like Rydlyme/CLR) for 30-90 minutes
- Flush with fresh water until pH neutral
- Reopen shutoffs; verify DHW flow restored
Cost: contractor descaling service typically $150-250 add-on to annual service.
Hard water schedule
- Soft water (< 5 gpg): every 3-4 years
- Moderate (5-10 gpg): every 2 years
- Hard (10+ gpg): annually OR install whole-home water softener
- Very hard (15+ gpg): softener not optional
When descaling won\'t help
If scale has bridged across the plates or pinhole leaks have formed (cross-contamination between boiler-side and DHW-side), the exchanger needs replacement:
- Symptoms: boiler-side water visibly fouling DHW (chemicals taste in hot water)
- OR: visible water leak from DHW side onto boiler floor
- OR: descaling fails to restore flow/temperature
Replacement
- Power off; gas off
- Drain unit
- Disconnect boiler-side and DHW-side piping at the exchanger
- Remove the exchanger from its mounting
- Install new exchanger with new gaskets/O-rings
- Refill and re-pressurize; commission with combustion analysis
Cost
- Plate exchanger (OEM): $300-600 depending on combi platform
- Contractor labor: $300-500
- Total replacement: $600-1,100
Bottom line
Annual descaling on hard water is dramatically cheaper than waiting for replacement. The plate exchanger is the most-common combi failure point in hard-water areas without softeners. Add a whole-home softener and the plate exchanger lasts 15-20+ years.