If a boiler provides space heating but no hot water, or the reverse, that difference helps narrow the fault. Abnormal pressure, a missing demand signal, diverter valve, sensor, flow issue, or ignition fault are possibilities depending on the system. Keep checks to pressure, controls, and visible fault messages; do not open the combustion chamber or adjust gas components.
See whether hot water alone fails or heating fails too
If radiators work but taps stay cold, the fault is more likely in hot-water demand sensing or diversion. If space heating is also unavailable, first check power, system pressure, thermostat status, and any fault code.
Read pressure and fault messages without removing panels
Compare the gauge with the manufacturer range when the system is cold. Very low pressure or a persistent fault code can prevent operation. Do not repeatedly top up a boiler that keeps losing pressure without finding the reason.
Notice whether opening a tap creates a hot-water demand
On combi boilers, opening a tap usually creates an icon, display change, or ignition attempt. If nothing changes, a flow sensor may not be detecting demand; if it tries and stops, the fault lies later in the sequence. That observation is useful without dismantling the unit.
Gas, combustion, and internal components are service work
A gas odor requires leaving and calling from outside. Without an odor, a diverter valve, sensor, control board, or combustion fault still needs professional diagnosis when external controls and pressure are normal. Do not bypass lockouts or reset indefinitely.
Sources
- U.S. Department of Energy — Energy Saver — Practical information on home heating, cooling, thermostats, maintenance, and efficiency.