Hot Water but No Heating: Combi vs System Causes

When your hot water is working but the heating will not come on, it can be confusing, especially if the boiler pressure looks normal. The underlying causes are usually different for combi and system boilers, and understanding this can help you talk clearly to an engineer and avoid guesswork.

First step: confirm your boiler type

Before thinking about parts like valves or pumps, it helps to know which type of boiler you have. Combi boilers heat water directly from the mains and usually have no hot water cylinder, while system or regular boilers feed a separate hot water cylinder and heating circuit.

If you have a small, single wall-hung unit with no separate cylinder and you get instant hot water, you almost certainly have a combi. If you have a boiler plus a hot water cylinder, possibly in an airing cupboard, then you are dealing with a system or regular setup.

Once you know this, the pattern of “hot water but no heating” symptoms becomes much easier to map to likely components rather than just looking at pressure or resetting controls.

• Combi boilers: often diverter valve or flow sensing issues

• System/regular boilers: more likely motorised zone valves or control wiring problems

• Both types: potential pump faults or air in the primary heating circuit

Section A: combi boiler causes when hot water works but heating does not

With a combi boiler, the same heat exchanger is usually serving both hot water and central heating. Internal valves and sensors tell the boiler whether to prioritise your taps or your radiators. If hot water is working, you know the burner and at least part of the internals are functional.

Diverter valve behaviour on combis

The diverter valve is the main traffic controller in a combi. When you open a tap, it sends heat to the plate heat exchanger for hot water. When the heating is on, it sends heat to the radiators instead. If this valve sticks in the hot water position, your taps stay hot but radiators remain stone cold.

Sometimes the valve does move but not fully, so you may notice a short burst of warmth in a few radiators that quickly fades. The boiler may also cycle on and off as it struggles to push water through the heating side.

Internal plate heat exchanger and priority

Most combis have an internal plate heat exchanger that transfers heat from the boiler water to your mains cold water. When a hot tap is opened, the boiler gives priority to this plate over central heating. This is normal behaviour, but if the controls fail to hand back to heating after the tap closes, you end up with a permanent “hot water priority” fault.

You might notice that the boiler fires perfectly whenever a tap is run, yet when only the room thermostat is calling, the boiler either does not light or only fires briefly. That points more towards diverter control or internal flow routing rather than pressure alone.

Flow sensors and demand recognition

Combi boilers rely on flow sensors or flow switches to detect water movement. One sensor usually monitors hot water demand, while separate circuits and thermostats signal central heating demand. If the boiler only “sees” hot water flow and not heating demand, it may ignore the room thermostat entirely.

A failed or sticky flow sensor can give inconsistent behaviour, such as the boiler firing only on certain taps, or needing a large flow to start. This is something an engineer will test electrically rather than by guesswork.

Section B: system & regular boiler causes when only hot water works

With system and regular boilers, the boiler itself often has fewer moving parts than a combi. The clever work is usually done by motorised valves, wiring centres, pumps and external controls around the hot water cylinder and heating pipework.

Motorised zone valves and heating circuits

Most system/regular installations use motorised zone valves to split hot water and heating. One valve sends heat to the cylinder coil for your stored hot water, and another sends it to the radiators. If the hot water valve works but the central heating valve fails, you will get perfect cylinder heating and no radiator heat.

The little lever or motor on the valve head can stick, the internal microswitch can fail, or the valve motor can burn out. From the homeowner’s point of view, that often looks like “boiler fires when hot water is on, but does nothing when heating alone is selected”.

Wiring centre, demand signals and controls

System boilers rely on a central wiring box that links the programmer, room thermostat, cylinder thermostat, boiler and zone valves. If one of the control signals is missing, the boiler may never be told that the heating needs to run, even though the hot water side works fine.

Typical examples include a loose wire on the heating valve’s switched live, a failed room thermostat or a programmer channel that has died. The boiler itself is often fine, but it is simply not being asked to light for central heating.

Pumps, air locks and the primary circuit

If the pump is weak, seized, or full of air, the boiler might still heat the cylinder through a short, easy circuit, but struggle to circulate water through the longer radiator loop. You may hear the boiler fire briefly then shut down because hot water cannot leave the heat exchanger.

Air trapped in the primary circuit or a partially closed valve can give similar results, with the cylinder heating but radiators staying stone cold. An engineer will usually listen for pump noise, feel pipe temperatures and bleed air from key points to confirm this.

Symptom mapping: what you see and the likely component

The patterns below can help you describe the fault clearly, but they are not a substitute for a professional diagnosis:

• Boiler fires on hot water taps, dead on heating call: Combi diverter valve, system boiler zone valve, or heating demand wiring

• Radiators briefly warm then cool, hot water fine: Partially sticking diverter valve, weak pump, or air in heating circuit

• Only one heating zone works, other zone cold: Failed motor head or microswitch on that zone valve, or local control fault

• Boiler runs but some radiators never heat: Balancing issues, air in those rads, or partial blockage away from boiler

• Boiler cycles rapidly on heating but stable on hot water: Poor circulation, pump issue, or control signal intermittently dropping

Safety notes and what not to do

Working on internal boiler components or gas connections is not a safe DIY task. Modern boilers have sealed combustion chambers, complex control boards and safety interlocks that must only be handled by a Gas Safe registered engineer.

As a homeowner, there are a few important “do not” points to bear in mind before you try anything adventurous.

Things you should avoid

While it is fine to check programmer settings, thermostat batteries and radiator valves, you should stop there. Going further can introduce real risk or cause more damage.

In particular, you should not:

• Open the boiler casing or disturb any gas-carrying parts

• Repeatedly reset the boiler in the hope it will “clear itself”

• Force motorised valves or diverter valves by hand

• Bridge or bypass safety devices, thermostats or switches

• Attempt electrical testing without proper training and equipment

How an engineer actually diagnoses these faults

A good heating engineer will start by confirming the symptoms: what fires when, which zones heat, and how the boiler behaves under different demands. They will then isolate the problem using a mix of electrical and mechanical checks rather than guessing at parts.

On system and regular boilers, this usually means testing motorised valves with a multimeter, checking that call-for-heat signals reach the boiler, and verifying that the valve microswitches are sending power to the pump and boiler when they open. They may also measure pump current draw or briefly run the pump on a test lead to confirm it is actually moving water.

On combis, an engineer will look at diverter valve movement, flow sensor readings and internal temperatures, often using test modes on the boiler. They will check control voltages at the PCB, compare resistance values of sensors and ensure the pump and fan are drawing sensible current. This methodical approach avoids replacing the wrong component and gets your heating back on more quickly.

Next steps: repair now, prevent repeat faults

If you have hot water but no heating, and the obvious settings and schedules are correct, the fault is usually in valves, controls or circulation rather than simple pressure. That is where professional testing and safe access inside the boiler make a real difference.

For a thorough Boiler Repair diagnostic, book a visit with BeeExpress Heating & Renewables on 020 8970 7489. Once your system is back up and running, regular Boiler Servicing can help catch sticking valves, weak pumps and control issues early, reducing the chance of the same “hot water only” problem appearing again.