What should I do if my boiler provides heat but no hot water?

What this symptom usually points to

When a boiler still heats radiators or baseboards but your taps run cold, the boiler itself is not completely down. That is why this symptom is different from a no-heat call. In most homes, it means the part of the system that makes domestic hot water is not being activated, not circulating, or not transferring heat well enough.

The first clue is what type of boiler setup you have. A combi boiler makes hot water on demand, so a sudden loss of hot water often points to a diverter valve, flow sensor, or plate heat exchanger. A boiler paired with an indirect water heater is more likely to have a tank aquastat, circulator, or zone control problem. An older boiler with a tankless coil often loses hot water because the coil or mixing valve has scaled up over time.

What you can observe before service

If the house heats normally but hot water is only weak or briefly warm, scale buildup is more likely than a complete electrical failure. If turning on a hot faucet does not change the boiler display, flame status, or operating sound at all, the unit may not be getting a domestic hot water call from its sensor, flow switch, tank control, or valve. If hot water fails only at one shower or sink, the problem may be a fixture cartridge rather than the boiler.

Water near the equipment changes the priority of the call. A small amount near a floor drain can be simple discharge or condensation from nearby piping, but water near wiring, circulators, zone valves, or controls can damage electrical components that operate the domestic hot water side. Moisture on walls or ceilings, or water showing up in several rooms, can also point to a plumbing leak that is separate from the boiler but still needs prompt attention.

What a technician usually checks

A service visit for this symptom usually starts with confirming the boiler can fire and heat the central heating loop, then tracing why that heat is not reaching your domestic water. The technician typically tests for a hot water call, checks diverter or zone valve movement, verifies circulator operation to an indirect tank, measures sensor readings, and inspects the plate exchanger or tankless coil for scale restriction. On some systems, they will also verify mixing valve operation and check whether a control board is sending power when hot water is demanded.

What affects repair cost

Cost depends heavily on which hot-water-making method your boiler uses. Replacing an aquastat, flow sensor, or mixing valve is often less expensive than replacing a diverter assembly, control board, or circulator, and descaling a plate heat exchanger can cost less than replacing one if the part is still serviceable. Typical 2025-2026 repair ranges are often about $200-$500 for a sensor, aquastat, or mixing valve issue, $350-$900 for a circulator or zone valve repair, $400-$1,200 for a diverter valve or plate heat exchanger repair, and more if access is difficult or multiple failed parts are found.