No Hot Water But Heating Works (and Vice Versa): Fixes by Boiler Type

How Heating and Hot Water Share the Same Boiler

In most UK homes, a single gas boiler provides both central heating and domestic hot water, but the two functions run on separate circuits inside the appliance. Because they share components at the heart of the boiler — the heat exchanger, pump, and controls — a single failed part can knock out one circuit while leaving the other running perfectly. That is why the symptom of heating works but no hot water (or the reverse) is so common and so specific.

The type of boiler you have matters a great deal here:

• Combi boiler: heats water on demand, no storage cylinder. A diverter valve switches the hot water between the heating circuit and your taps. Around 80% of new boilers sold in the UK are combis.
• System boiler: heats water and pumps it to radiators and a separate hot water storage cylinder. Uses motorised valves to direct flow.
• Regular (conventional/heat-only) boiler: works with both a feed-and-expansion tank in the loft and a hot water cylinder. Older installation type, still common in larger homes.

Identifying your boiler type before reading on will save you time, because the diverter valve — the most common cause of heating works but no hot water — only exists in combi boilers.

Scenario A: Heating Works But No Hot Water — Safe Checks First

This is one of the most reported fault patterns across UK boiler repair companies. Before you call anyone out, run through these homeowner-safe checks in order. Many are resolved without a single tool.

Check your programmer and timer settings first. Heating and hot water are often controlled independently on your programmer or smart thermostat app. It is surprisingly easy for the clocks changing, a power cut, or a drained backup battery to reset your schedule — leaving hot water switched off while heating carries on as normal. Open your controller or app and confirm that hot water is scheduled to come on at the times you expect.

Check boiler pressure. Low water pressure is one of the simplest yet most common reasons a combi or system boiler stops heating water. Most modern boilers need the pressure gauge to sit between 1 and 1.5 bar when cold. If yours reads below 1 bar, locate the filling loop — usually a flexible silver braided hose beneath the boiler with two small valves — open the valves slowly and watch the gauge rise to around 1.2–1.5 bar, then close them. Never exceed 2 bar.

Check for a fault code. Modern boilers display a fault or error code when something goes wrong. Note down any code or flashing light pattern and look it up in your boiler manual or on BOYLA. The code is often the fastest route to a diagnosis.

Try a reset — once or twice, patiently. Hold the reset button as described in your manual and let the boiler complete its full start-up sequence. Do not reset more than two or three times, as repeated resets can mask a more serious fault.

Check for a frozen condensate pipe in winter. Condensing boilers — which is most boilers made after 2004 — produce wastewater that exits through a plastic pipe on an outside wall. In cold UK winters this pipe can freeze solid. If your boiler has locked out during a cold snap, this is the first thing to check. You can safely thaw it by pouring warm (not boiling) water along the pipe or applying a hot water bottle. Once thawed, reset the boiler.

Scenario A: The Most Likely Engineer-Level Cause — Diverter Valve Fault (Combi Boilers)

If the safe checks above did not solve the problem on a combi boiler, the single most likely cause of heating working but no hot water is a faulty diverter valve.

The diverter valve is a motorised valve inside your combi boiler that acts as a traffic controller for hot water. When you turn on a tap or shower, it should swing fully over to send heat to the domestic hot water circuit, temporarily deprioritising the radiators. When it sticks, wears out, or the actuator motor fails, the valve stays directed toward the heating circuit — so your radiators stay warm but your taps run cold.

A diverter valve fault does not always mean a complete failure. A partially stuck valve may mean you get a brief burst of hot water that quickly runs cold, or that the water is only lukewarm. You may also notice your radiators warming up slightly while you are trying to run a hot tap — a tell-tale sign the valve is not switching over properly.

Diverter valves are specific to combi boilers. If you have a system or regular boiler with a hot water cylinder, you will not have one. Instead, suspect a faulty cylinder thermostat, a failed temperature sensor on the cylinder, or a motorised valve that is not opening to allow hot water flow to the cylinder coil. The boiler may be firing happily and heating the radiators, but if the cylinder thermostat contacts have failed, it never signals the motorised valve to divert flow to the cylinder — and the water inside stays cold.

All of these repairs require a Gas Safe registered engineer. Do not attempt to open the boiler casing or interfere with internal components.

Scenario B: Hot Water Works But No Heating — Safe Checks First

Cold radiators with perfectly hot taps is a different fault pattern, and it has its own checklist of homeowner-safe checks.

Turn up the thermostat. Set your room thermostat to 25°C temporarily. If the boiler fires and the radiators begin to warm, your thermostat was set too low or the timer schedule was not calling for heat. Smart thermostats can occasionally lose connection to the boiler hub after a power cut or router reboot — re-pairing the device often resolves this instantly.

Check boiler pressure. Although you may still get hot water with pressure slightly below normal, a significant pressure drop can stop the heating circuit from working while the domestic hot water side continues to fire. Top up via the filling loop if the gauge reads below 1 bar.

Check for stuck TRV valves. Thermostatic radiator valves that have sat closed all summer can seize in the shut position. Turn each TRV head fully open and give the pin underneath a gentle push if accessible. If every radiator in the house is cold, a seized pump or the diverter valve is more likely than multiple stuck TRVs.

Bleed your radiators. If some radiators have cold patches at the top, trapped air is blocking hot water flow. Use a radiator bleed key on each radiator, starting upstairs, until water dribbles out steadily. Remember to recheck boiler pressure afterwards, as bleeding releases a small amount of water.

Check the zone valve for the heating circuit. On system and regular boilers, a separate motorised valve controls heating and hot water zones. If the heating zone valve has failed in the closed position, the hot water zone will work fine but the radiators will stay cold.

Scenario B: Engineer-Level Causes — Diverter Valve and Circulation Pump

If the safe checks above have not resolved cold radiators on a combi boiler, the diverter valve is again the prime suspect — this time stuck on the hot water side rather than the heating side. Because it is permanently directing flow to your taps and shower, the radiator circuit cannot receive any heat.

The other major engineer-level cause for this scenario is a seized circulation pump. The pump is the heart of your central heating system, pushing hot water from the boiler around all the radiators in your home. When it fails — often after sitting idle over summer and seizing — the boiler fires up and heats the water, but it goes nowhere. Your taps may still get hot water because a combi boiler heats domestic water directly from the mains, which does not rely on the same pump. Radiators, however, stay cold.

Both diverter valve replacement and circulation pump replacement require a Gas Safe registered engineer. Do not attempt to access internal boiler components yourself.

Scenario C: No Heating and No Hot Water at All

When both heating and hot water fail at the same time, start with the absolute basics before assuming the worst.

• Check the power supply. Has the circuit breaker or RCD in your consumer unit (fuse box) tripped? Reset it and check whether the boiler display comes back on.
• Check your gas supply. Do other gas appliances — hob, gas fire — work normally? If not, contact your gas supplier. If gas is flowing elsewhere but not to the boiler, call a Gas Safe engineer.
• Check the pilot light (older boilers). Boilers more than about 20 years old may have a standing pilot light. If it has gone out, your boiler manual will describe how to relight it safely, though many older models require an engineer.
• Check for a frozen condensate pipe. If both functions fail during a cold snap and your boiler display shows a lockout code, this is the most likely culprit. Thaw the pipe as described above and reset.
• Check boiler pressure. If pressure has dropped below about 0.5 bar, many boilers will lock out entirely, cutting both heating and hot water. Top up via the filling loop.
• If the boiler display is completely dead or showing an unfamiliar code after all the above, the printed circuit board (PCB) — the boiler's brain — may have failed. PCB replacement is an engineer job only.

Scenario D: Hot Water Goes Hot and Cold — Why It Happens

The frustrating pattern of water that starts hot, turns cold, then goes hot again is almost exclusively a combi boiler issue and has several distinct causes.

Partially stuck diverter valve. If the valve does not switch fully to the hot water position, it may drift back toward the heating circuit mid-use. You get a burst of hot water, then cooling, then perhaps hot again as the valve shifts. Watch whether your radiators warm slightly while you are running a hot tap — a strong clue the valve is not fully diverting.

Blocked or scaled plate heat exchanger. The plate heat exchanger transfers heat from the boiler's primary circuit to incoming mains cold water. In hard-water areas — London, the South East, the Midlands — limescale builds up inside the narrow plates over time. Flow becomes restricted, the water overheats locally, the overheat thermostat trips, the boiler shuts down briefly, and you get cold water until it resets. This is the most common cause of hot water that goes hot then cold after a few minutes of good flow.

Faulty flow or temperature sensor. If a sensor gives inaccurate readings, the boiler may believe the water is already at the target temperature and switch off too early, leaving you with cold water at the tap shortly after turning it on.

Low system pressure. Pressure that repeatedly drops below 1 bar causes intermittent firing, which produces the hot-cold-hot cycle. If you are constantly topping up pressure, there is likely a leak somewhere in the system — this needs an engineer to locate and seal.

Simultaneous demand (not a fault). Combi boilers do not store hot water — they heat it on demand. If two showers and a tap are all running at once, even a well-maintained boiler may struggle to keep up. This is a capacity issue, not a component failure.

Before you blame the boiler, run the hot tap in your kitchen while the shower is running. If the kitchen tap stays hot throughout, the boiler is fine and the shower mixing valve or cartridge is the likely culprit.

What a Gas Safe Engineer Will Do — and What It Costs

For any repair that involves opening the boiler casing, accessing gas components, or replacing internal parts, you must use a Gas Safe registered engineer. You can verify any engineer's registration at the Gas Safe Register website.

When the engineer arrives, they will typically carry out a visual inspection, check system pressure, run a gas analysis, and use diagnostic software or fault codes to pinpoint the fault before quoting for any parts. Always ask for a written quote before authorising repairs.

Prices vary by region — expect the top end (or 20–30% more) in London and the South East, and less in the North and Scotland. All figures below are typical UK ranges and are provided as guidance only; actual quotes will depend on your boiler make and model, parts availability, and local labour rates.

Step by step

1. Check your programmer, timer, and smart app settings

   Confirm that hot water (and heating) are scheduled to come on at the right times. Clocks changing, power cuts, and flat backup batteries are common culprits for schedules being wiped or altered. If you use a smart thermostat, open the app and check that hot water has not been inadvertently switched off or set to a frost-only mode.

2. Check boiler pressure on the gauge

   Find the pressure gauge on the front of your boiler — it should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop (a silver braided hose beneath the boiler) to slowly top up the pressure to around 1.2 bar while watching the gauge. Close both filling loop valves once done. Never exceed 2 bar.

3. Note any fault code or warning light

   Write down any code or flashing light sequence shown on the boiler display. Look it up in your manual or on BOYLA. The code often tells you — and your engineer — exactly which component is reporting a problem.

4. Reset the boiler once or twice, patiently

   Locate the reset button (often marked with a flame symbol or labelled 'Reset'). Hold it for the time specified in your manual and allow the boiler to complete its full start-up sequence before testing. Repeat once more if it locks out again, but do not reset more than two or three times — persistent lockouts indicate an underlying fault that needs diagnosing.

5. Check for a frozen condensate pipe in cold weather

   Look for a white or grey plastic pipe (typically 21.5 mm or 32 mm diameter) exiting the boiler through an outside wall and running down to a drain. If the weather is at or below freezing and the boiler has locked out, gently pour warm (not boiling) water along the pipe or apply a hot water bottle to it. Once the blockage clears, reset the boiler.

6. Bleed your radiators if heating is cold

   Use a radiator bleed key on the bleed valve (small square fitting at the top corner of each radiator). Hold a cloth underneath, open the valve slowly, and close it as soon as water — not air — starts to dribble out. Work upstairs first, then downstairs. Check boiler pressure again afterwards and top up if needed.

7. Call a Gas Safe registered engineer

   If none of the above steps restore hot water or heating, the fault is likely an internal component — diverter valve, plate heat exchanger, circulation pump, motorised valve, cylinder thermostat, or PCB. Do not attempt to open the boiler casing or interfere with gas fittings. Book a Gas Safe registered engineer and share the fault code if you noted one — it will speed up diagnosis.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions