No Hot Water From Your Combi Boiler? What to Check First
If your combi boiler has stopped giving hot water it is frustrating, but a few simple checks can often narrow down the cause before you call an engineer. With gas appliances, safety always comes first, so it is important to know what you can check yourself and what must be left to a professional.
Safety first with combi boilers
Combi boilers are gas appliances, so never remove the boiler casing or attempt to work on internal components. Anything involving gas, combustion, or dismantling parts must be handled by a Gas Safe registered engineer.
If you ever smell gas, hear a hissing sound from pipework, or suspect a leak, turn off the gas at the meter if it is safe to do so, ventilate the area, avoid switches and naked flames, and call the National Gas Emergency line immediately. Do not try to restart the boiler until the system has been checked.
What to Check Before Tackling Hot Water Problems
Before looking at hot-water-specific issues, spend a couple of minutes on these basic checks. They can quickly tell you whether it is a simple user issue or something that needs an engineer.
Do you have heating? Note whether radiators still get hot, or if both heating and hot water have failed.
Check the pressure gauge: On most combis, the cold pressure should be around 1.0 to 1.5 bar. Below 1.0 bar often causes problems.
Look for error codes or flashing lights: The display may show a fault code that points to a specific problem.
Confirm electricity and gas supply: Make sure other gas appliances work and the boiler’s fused spur or socket is switched on.
If your pressure is low or there is a fault code showing, you may find it helpful to read dedicated guides on repressurising a combi boiler and interpreting boiler error codes. Those topics are closely related, but here we will focus on the hot water side of things.
Heating works but no hot water from your combi boiler
If your radiators heat up normally but you get no hot water from the taps or shower, the fault is usually in one of the hot-water-specific components. These tend to control how the boiler prioritises hot water over heating.
Diverter valve issues and clear symptom patterns
The diverter valve directs heat either to your radiators or to your hot water. When it sticks or fails, you will often notice very specific behaviour from your hot water taps.
Common diverter valve symptoms include hot radiators but stone-cold taps, hot water only when the heating is off, or a short burst of hot water that quickly goes lukewarm. Sometimes you might also hear clicking or mechanical noises from the boiler when taps are opened.
What you can do now: Try running a hot tap with the heating completely off for a few minutes, then try again with the heating fully on. Note whether your tap temperature changes depending on the heating. This information is helpful for an engineer’s diagnosis.
Engineer needed: Diverter valve repair or replacement is an internal boiler job and must be carried out by a Gas Safe engineer. Do not attempt to take off the casing or adjust valves inside the boiler.
Plate heat exchanger and limescale in hard-water areas
Most combi boilers use a plate heat exchanger to transfer heat from the boiler water to your tap water. In hard-water areas, limescale can build up inside this component over time, restricting flow and heat transfer.
Typical signs are hot water that runs hot, then quickly cools, or taps that never quite get as hot as they used to even when you run them slowly. You may also notice that some outlets, such as a shower, are worse than others if the flow rate is higher there.
What you can do now: Try running the hot water more slowly and see if the temperature improves. Note whether the boiler seems to cycle on and off while the tap is open. If you live in a known hard-water area, mention this when you call.
Engineer needed: Cleaning or replacing a scaled plate heat exchanger, and fitting any scale reducers, must be done by an engineer. They can also check overall water quality and advise on long-term protection.
Flow sensor and demand recognition problems
The flow sensor tells your boiler that a hot tap has been opened. If it fails or sticks, the boiler may not “realise” there is a demand for hot water, even though the heating still works.
This usually shows as completely cold water from the taps, with the boiler not firing at all when you turn on hot water, even though it fires correctly for heating. In some cases, the boiler may start and stop repeatedly while the tap runs.
What you can do now: Try a couple of different hot taps and see if any of them trigger the boiler. Watch the boiler display or indicator lights when you open and close taps. Make a note of any flickering symbols or codes.
Engineer needed: Diagnosing and replacing flow sensors is internal work and must be done by a professional. They will check wiring, controls, and sensor operation using the boiler’s service menus.
No heating and no hot water from a combi boiler
If both heating and hot water have stopped, the fault is more likely related to pressure, safety devices, or the boiler’s controls. Some of these have simple checks you can do, but do not push past anything you are unsure about.
Low system pressure checks
Combi boilers shut down if the system water pressure falls too low, to protect internal components. This often results in no heating and no hot water.
Look at the pressure gauge on the front of the boiler or underneath. When the system is cool, it should typically sit around 1.0 to 1.5 bar. If it is close to zero, the boiler may lock out and show a low-pressure symbol or fault code.
What you can do now: If you are comfortable and have done it before, you may be able to repressurise using the filling loop following the manufacturer’s instructions. If not, leave the pressure as it is and turn the boiler off until it is checked.
Engineer needed: If pressure keeps dropping, or you are unsure how to top it up safely, an engineer should investigate for leaks, expansion vessel issues, or faulty safety valves.
Controls, thermostat settings and simple resets
Sometimes the fault lies in the boiler controls rather than the boiler itself. Incorrect programmer settings, room thermostats set too low, or flat batteries can all stop your hot water or heating coming on.
Check that the boiler is in the correct mode for both heating and hot water, that any separate hot water settings are not accidentally off, and that room thermostats and wireless controls have power. Check that any holiday or eco modes are not restricting operation.
If your manual states it is safe to do so, you can also try a simple boiler reset by pressing the reset button or turning the selector to reset briefly, then back to the normal setting. Make sure you only reset once or twice; repeated resets point to a real fault that needs attention.
Frozen or blocked condensate pipe in cold weather
In winter, condensing boilers can stop working if the external condensate pipe freezes or becomes blocked. This can cause both heating and hot water to fail, sometimes with a specific condensate-related error code.
Signs include gurgling noises, the boiler trying and failing to start, and the fault appearing during or immediately after a cold snap. The pipe is often a small plastic pipe running from the boiler to an outside drain.
What you can do now: If you can safely access the external pipe, you may be able to gently pour warm (not boiling) water over it to defrost it, then reset the boiler. Never use kettles of boiling water, open flames, or tools that could damage the pipe.
Engineer needed: If the fault keeps returning or the pipe layout is poor, an engineer can reroute, insulate, or upsize the condensate pipe to reduce the risk of future freezing.
Hot water symptom checklist: what you can do vs engineer jobs
It can help to think in terms of what is safe and sensible for you to handle, and what should always be left to a professional. As a quick reference:
You can usually check: pressure gauge readings, thermostat and programmer settings, whether the boiler fires when taps are opened, and whether any visible external pipes (like condensate) are blocked or frozen.
Engineer required for: diverter valve and plate heat exchanger work, internal flow sensors, gas supply issues, repeated low-pressure faults, and any work involving boiler casings or combustion.
Taking notes on symptoms, timings, and any error codes will speed up diagnosis when an engineer visits, often saving time and labour costs.
When to book a Gas Safe engineer
If you have worked through the basic checks and still have no reliable hot water, it is time to arrange a professional diagnostic. Persistent problems such as hot water going warm then cold, hot water only when the heating is off, or the boiler not firing for taps almost always point to internal components that need expert attention.
For an expert, Gas Safe approach to finding and fixing the fault, contact Robert Hill and Company (Electrical Engineers) Ltd on 01419562245 for reliable boiler repairs. We can test hot-water-specific parts, check for scale and sensor issues, and get your combi boiler delivering steady hot water again as efficiently as possible.
