Applicable Systems
Baxi Heating Boiler
Symptoms
The domestic hot water temperature fluctuates. The outlet temperature sometimes exceeds 65C and at other times drops below 45C. The user reports inconsistent hot water delivery.
Possible Causes
- Plate heat exchanger partially blocked by limescale, reducing heat transfer
- Diverter valve sticking, sending hot water to the central heating instead of the plate heat exchanger
- Flow sensor giving erratic readings, causing the burner to modulate incorrectly
- Thermistor temperature sensor drifting, providing incorrect feedback to the controller
Troubleshooting Steps
- Step 1: Descale the plate heat exchanger using a chemical descaler pump
Circulate descaling solution for 30 minutes, then flush with clean water - Step 2: Test the diverter valve by switching between heating and hot water modes
If the valve does not switch, replace the diverter valve cartridge - Step 3: Measure the flow sensor output during hot water draw
Compare to the manual specification; replace if erratic - Step 4: Check the thermistor resistance at known temperatures using a calibration bath
Replace if outside the tolerance band of +/-5%
How to Reset
Reset: The temperature stabilizes after the root cause is corrected. No manual reset is required.
Common Replacement Parts
- Plate heat exchanger: OEM-specified, 30-50 plates (Match to boiler model)
- Diverter valve cartridge: OEM kit, includes seals (Replace without draining the system)
- Flow sensor: Hall-effect or turbine type (Match to boiler model)
Frequently Asked Questions
- Q: How do I descale a plate heat exchanger?
Use a chemical descaling pump with a 5% citric acid solution. Circulate for 30 minutes, then flush with 3 system volumes of clean water.
- Q: What is the ideal hot water temperature?
Set to 50-55C for safety. Temperatures above 60C risk scalding; below 45C risks Legionella growth in the pipework.
- Q: How often should I descale?
Annually in hard water areas. Every 2-3 years in soft water areas. Install a magnetic scale inhibitor for prevention.
Source: OEM Installation Manual
Category: Home Boiler