Boiler cycles on and off but domestic hot water from the Laars combi boiler is not hot enough or fluctuates.

Safety first: Domestic hot water can scald. Be cautious when testing temperatures.

• Check DHW setpoint: On the Laars combi control, verify the domestic hot water temperature setting is appropriate (often 120°F/49°C).
• Verify flow rate: Very low or very high flow can cause instability. Try adjusting the faucet to a moderate flow and see if temperature stabilizes.
• Clean inlet filters: Some Laars combi units have cold water inlet strainers; clean them to ensure proper flow.
• Observe boiler response: When a hot water tap is opened, the boiler should switch to DHW mode and modulate. If it does not, the flow sensor or DHW sensor may be faulty.

When to call a professional: Descaling the DHW heat exchanger and replacing sensors or mixing valves should be done by a technician.