Check the boiler first
Look at the boiler display or indicator lights. Most modern boilers show a fault code when there is a problem. Note the code and look it up in your boiler manual or search the code with your boiler brand name. Common fault codes relate to low pressure, ignition failure, or a sensor fault.
Resetting the boiler (usually a reset button on the front panel) sometimes clears a temporary fault. If the same code returns within an hour, there is an underlying issue that needs a plumber or gas engineer.
Check your boiler pressure
Most combi and system boilers need a water pressure of 1 to 1.5 bar to operate. The pressure gauge is on the front of the boiler — if the needle is in the red zone (below 0.5 bar or above 3 bar) the boiler will lock out and stop firing.
To repressurise: locate the filling loop (a flexible braided hose under the boiler), open both valves slowly until the gauge reads 1 to 1.5 bar, then close both valves firmly. If the pressure drops again within a few days, you have a leak somewhere in the system that needs to be found and fixed.
Check the timer and programmer
Your boiler programmer or smart thermostat may have switched to a different schedule, or a power cut may have reset it. Check that hot water is enabled for the current time period. In older systems with a gravity-fed cylinder, there may be a separate hot water timer on the cylinder or hot press — check this independently of the boiler.
If you have a smart thermostat (Hive, Nest, Honeywell), open the app and check that hot water is not accidentally set to off or scheduled for a different time.
If you have a hot water cylinder
Homes with a system boiler or back boiler store hot water in a cylinder, usually in the hot press. If the boiler is working but the hot water is cold, the cylinder thermostat may have failed, or the motorised valve (which directs hot water to the cylinder) may be stuck.
You can test whether the immersion heater is working as a temporary backup — switch it on and wait 45 minutes. If hot water returns, the boiler or its controls are the issue, not the cylinder itself.
When to call a plumber or gas engineer
Call a plumber if the boiler shows a fault code that does not clear after a reset, if system pressure drops repeatedly, if you smell gas near the boiler (in which case call Gas Networks Ireland on 1800 20 50 50 first), or if the boiler makes banging, gurgling, or whistling sounds.
Do not attempt to open the boiler casing yourself. Gas appliances must be serviced by an RGII-registered gas engineer. Annual boiler servicing (€80 to €120) is the best way to prevent no-hot-water emergencies.
