Boiler runs but domestic hot water from an indirect tank or combi function is not hot enough.

Safety first: Hot water can cause scalding. Use caution when testing temperatures and adjusting mixing valves.

• Step 1 – Check DHW setpoint: On the boiler or indirect tank control, verify the domestic hot water temperature setpoint is correct (often 120–140°F, per local codes).
• Step 2 – Inspect mixing valve: If a thermostatic mixing valve is installed, confirm its setting and that its hot and cold inlets are not reversed. Adjust slightly higher if safe and allowed.
• Step 3 – Verify DHW sensor/aquastat: Ensure the DHW sensor is firmly inserted in the well on the indirect tank, or that the aquastat is properly strapped and insulated. Replace if it does not respond correctly.
• Step 4 – Check boiler firing during DHW call: When hot water is used, the boiler should fire to its DHW setpoint. If not, the priority or DHW call wiring may be incorrect.
• Step 5 – Descale heat exchanger (pro): On combi or indirect systems, scale buildup in the coil or plate heat exchanger can severely reduce output. A professional should perform a descaling procedure.

Note: Always follow local codes regarding maximum domestic hot water temperature to prevent scalding.