Boiler not giving hot water

First, which boiler do you have?

A combi heats water as you use it. There's no hot water cylinder in the hot press. A system or regular boiler heats a cylinder, and the hot water you get at the tap comes from the stored water. The two fail in different ways, so the checks differ.

Combi boiler: no hot water but the heating works

• Diverter valve stuck. The valve that sends the boiler's heat to the taps instead of the radiators sticks on the heating side. The classic sign is hot water only when the heating is on, or lukewarm water that fades. An engineer's job.
• Flow sensor or turbine. The boiler doesn't register that a tap is open, so it never fires for hot water. Often scale or debris.
• Hot water temperature sensor. Vaillant's codes F.12 and F.13 cover a short-circuited domestic hot water or cylinder sensor, both listed as "a case for a Vaillant engineer" (Vaillant, read 29 October 2026).
• Scaled plate heat exchanger. In hard-water parts of Dublin, the small exchanger that heats the tap water scales up. Water comes through warm, then cold at a decent flow.
• Low mains flow. A combi needs enough mains flow to trigger. A part-closed stopcock or a blocked filter means a trickle of warm water. If the water is hot but runs out when a second tap opens, the boiler is undersized rather than faulty: our sister site has the flow rates in combi boilers for large houses.

Combi: no hot water unless the heating is on

That pattern is the diverter valve almost every time. With the heating on, the valve is already partly in the right position and some heat reaches the plate heat exchanger. With the heating off, nothing moves. It's a part, not a new boiler, on a boiler that's otherwise sound.

System or regular boiler with a cylinder

• Timer and thermostat. Hot water has its own programme on most controllers. A change of clocks, a power cut or flat batteries in a wireless control can leave it off.
• Cylinder thermostat. The strap-on thermostat on the cylinder tells the boiler when to heat it. If it fails, the cylinder never calls for heat.
• Motorised zone valve. The valve that opens the hot water circuit sticks or its motor burns out. You may hear it click without moving.
• Immersion as a backup. A cylinder usually has an electric immersion. If it gives hot water and the boiler doesn't, the boiler side has the fault, and you have hot water in the meantime.
• Airlock after work. If the system was drained recently, air in the cylinder coil stops it heating.

No heating and no hot water

Then the fault is the boiler itself or its supply, not the hot water side. Work through the checks any engineer would ask you about first:

1. Is there a fault code?Write it down. Ideal's L2 and F2, and Vaillant's F.28 and F.29, point to the boiler failing to light. Ideal's first step for L2 is to check the condensate pipe, then whether other gas appliances work (Ideal Heating).
2. Does the hob light?If no gas appliance works, the problem is the supply or the meter, not the boiler. If you have a prepayment meter, check the credit.
3. What does the pressure gauge say?Below 1 bar, top up to about 1 bar. See boiler losing pressure.
4. Is the condensate pipe frozen?In frost, the small plastic pipe outside can freeze and stop the boiler. Baxi's guidance is to warm the exposed length with a hot water bottle or warm water, never boiling water from the kettle (Baxi).
5. Reset once.If it locks out again straight away, stop and book a repair.

After a power cut

Most boilers restart on their own. If not: check the timer has kept its programme, that any wireless thermostat has power, and that the boiler's own switch wasn't tripped. Some controllers reset to a default programme with the hot water off.

What it costs to fix

Dublin engineers who publish a call-out or repair price charge €80 to €160, middle figure €115, parts extra (22 October 2026). A diverter valve, flow sensor or zone valve is a routine repair. If the engineer says the fault is the main heat exchanger on a boiler over 15 years old, read repair or replace before you decide. See boiler repair for every published price.

Questions people ask