In This Guide
What does it mean when you have heating but no hot water?
A combi boiler heats your central heating and your hot water taps from the same gas burner, but it switches between the two using a part called the diverter valve. When you turn on a hot tap, the diverter valve redirects the hot water flow from the radiators to the plate heat exchanger, which transfers heat to the cold mains water on its way to the tap. If the boiler can run the radiators but cannot send hot water to the taps, the diverter or one of the components on the hot water side has failed while the heating side stays healthy.
The fault is specific to combi boilers and to system boilers feeding a hot water cylinder via a similar valve. It does not affect older gravity-fed systems with a separate hot water tank in the airing cupboard, where no hot water normally points to the cylinder thermostat or immersion heater instead.
What causes a boiler to have heating but no hot water?
There are six common causes of a combi boiler running the heating but failing to deliver hot water, ranging from a closed isolation valve under the kitchen sink at the simple end through to a stuck diverter valve or clogged plate heat exchanger that needs an engineer to replace. The table below maps each cause to the symptom you are most likely to see at the tap, and whether the fix is safe to attempt yourself.
Heating but no hot water causes table
Symptom at the tap | Likely cause | Can you fix it yourself? |
Cold water only from hot taps, heating runs fine | Faulty diverter valve stuck in heating position | No, needs a Gas Safe engineer |
Hot water lukewarm rather than hot, cold-then-warm pattern | Limescale build-up on the plate heat exchanger | No, needs a flush or part replacement |
Boiler fires briefly when hot tap opened, then cuts out | Faulty DHW flow sensor or microswitch | No, needs a Gas Safe engineer |
No hot water with an E321 or similar fault code shown | Faulty DHW NTC thermistor sensor | No, needs a Gas Safe engineer |
Pressure gauge reading below 1.0 bar | Low system pressure preventing the boiler firing for DHW | Yes, repressurise through the filling loop |
No hot water at one tap only, other taps fine | Closed isolation valve or tap cartridge fault at that sink | Yes, check the local isolation valve |
If you are unsure at any point, switch the boiler off and book a Gas Safe registered engineer. Checking system pressure, looking at isolation valves, and pressing the reset button are safe to do yourself. Replacing components, opening the boiler casing, or working on the diverter valve or heat exchanger is regulated heating work that must be carried out by a Gas Safe engineer under UK law, and unauthorised work can void your manufacturer’s warranty.
Can you fix no hot water yourself?
For most causes, no. Diverter valves, heat exchangers and DHW sensors all sit inside the boiler casing and replacing them is regulated heating work. The four homeowner checks below take less than ten minutes and will tell you whether the fault is something simple or something that needs an engineer.
- Check the pressure gauge on the front of the boiler. If it reads below 1.0 bar, the system is too low to deliver hot water and needs repressurising through the filling loop. The same fault that causes a Baxi E119, Vaillant F22 or Worcester EL229 code can leave you with running radiators but no hot water.
- Try the reset button on the front panel. Hold it for three to five seconds. If the boiler clears a fault code and hot water returns, the problem may have been a one-off glitch. If the fault returns within minutes, book a boiler repair rather than resetting again.
- Check the isolation valves under the boiler. There should be at least four, one for the cold mains in, one for the hot water out, and two for the heating flow and return. Make sure the cold mains and hot water valves are fully open. A valve closed during recent plumbing work is a more common cause than people realise.
- If you only have no hot water at one tap, check the local isolation valve under that sink. The fault may be a closed valve or a tap cartridge issue, not the boiler at all.
If none of these resolves the fault, stop and call a Gas Safe registered engineer. Continued resetting can mask a developing problem and may shorten the life of the diverter valve actuator.
How much does it cost to fix no hot water?
Our boiler callouts for no hot water start from £120, covering attendance from one of our Gas Safe engineers and a full diagnosis. The final cost depends on what the diagnosis reveals: an isolation valve fix or pressure top-up sits at the lower end of the range, a diverter valve replacement is mid-range, a plate heat exchanger replacement is higher, and a faulty hot water sensor or microswitch is priced separately.
See our current rates for the full price list, including weekend, evening and emergency call-out variations. Every quote we issue includes VAT and all unavoidable charges, and we confirm the final figure with you before any work begins. If the boiler is past its economic repair point, we will tell you that during the diagnosis rather than after a part has been ordered.
When no hot water means a deeper system problem
A combi boiler that loses hot water repeatedly, or that delivers hot water that runs lukewarm and then cold before recovering, almost always has system-wide contamination rather than a single failed part. Sludge and limescale in the central heating water gradually coat the inside of the plate heat exchanger, restricting flow and stopping the boiler from transferring heat to the tap water efficiently. The hot water side gets the symptoms first because the plate heat exchanger has tighter clearances than the main heat exchanger.
In these cases, a powerflush of the whole central heating system is often more cost-effective than swapping out the heat exchanger alone, because it addresses the underlying cause rather than the symptom. A magnetic system filter fitted afterwards captures any future debris before it reaches the boiler, and significantly extends the working life of every internal component.
In rare cases, a no hot water fault is caused by a hidden leak elsewhere in the system that drops the pressure as soon as the boiler tries to fire. If repressurising the system does not hold for more than a few hours, a trace and access investigation is the right next step before any internal boiler work is booked.
How to prevent losing hot water
The most effective way to prevent a recurring no hot water fault is an annual service from a Gas Safe registered engineer, who will check the diverter valve operation, inspect the plate heat exchanger for early signs of scale, and flush the hot water side if needed. A service catches the slow build-up before it becomes a full lockout.
Hard water is the single biggest preventable contributor to plate heat exchanger failure in London and the South East. A magnetic system filter and a scale reducer on the cold mains, fitted during a routine boiler service, can double the working life of the heat exchanger and the diverter valve.
Between services, make a note of the pressure gauge reading once a month, and run a hot tap for thirty seconds. If the hot water takes longer to arrive than it used to, or if it cycles between hot and cool while running, the plate heat exchanger is starting to scale up and the boiler will benefit from a flush before the fault becomes a complete lockout.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer the same day if your boiler is running the heating but giving you no hot water. Unlike pressure faults, this is not a code that clears on its own, and the diverter valve, heat exchanger and DHW sensor faults that cause it all get worse rather than better with continued use.
Call out as a priority if anyone in the household is vulnerable or relies on hot water for medical reasons, if a baby or young child needs hot water for bathing, if the fault has appeared in winter when cold weather is forecast, or if it appears alongside another fault code on the display. Combined fault codes may need emergency boiler repair rather than a routine appointment.
Frequently Asked Questions
Is no hot water an emergency?
In the UK, no hot water on its own is usually classed as an urgent repair rather than a life-threatening emergency, but it should be fixed within twenty-four to forty-eight hours, especially in winter or where vulnerable people, young children or anyone with medical needs is affected. Most Gas Safe engineers including ourselves treat a complete loss of hot water as a same-day callout rather than a routine appointment.
Why is there no hot water in my shower but the heating works?
If only the shower is affected and the kitchen and bathroom taps run hot normally, the fault is in the shower itself rather than the boiler. The common causes are a failed thermostatic shower cartridge, a blocked shower hose or head, or a closed isolation valve behind the shower wall. If hot water is failing at every tap as well as the shower, the cause is at the boiler and you should follow the diagnosis steps above.
Why is my Vaillant, Baxi or Worcester boiler giving heating but no hot water?
The four common causes (diverter valve, plate heat exchanger, DHW sensor, low pressure) apply across all major UK boiler brands. Vaillant ecoTec models are particularly known for diverter valve issues after about seven years. Baxi Duo-tec and EcoBlue boilers most commonly show plate heat exchanger scaling in hard water areas. Worcester Greenstar boilers often present as a DHW sensor or microswitch fault. The diagnosis pathway is the same regardless of brand.
How long does it take to fix no hot water?
Most no hot water faults are diagnosed and repaired in a single visit lasting between forty-five minutes and two hours, including a full system test once the repair is complete. Diverter valve replacements and plate heat exchanger swaps are the more involved jobs and can occasionally need a return visit for parts. The engineer will give you an honest timescale before any work begins.
Am I entitled to compensation if my landlord will not fix no hot water?
If you rent your home, your landlord is legally required to keep the hot water system in working order under the Landlord and Tenant Act 1985. After reporting the fault in writing, you can usually expect a repair within twenty-four hours in winter and within a few days at other times of year. If the landlord does not act, you may be able to claim a reduction in rent or compensation through the local council’s environmental health team or the small claims court. Keep a written record of every report and response from the day the fault appeared.
How quickly can a Gas Safe engineer attend a no hot water fault?
0800 Homefix attends no hot water callouts across London and the South East seven days a week, 365 days a year. Most callouts are scheduled the same day or the next working day. Emergency callouts, where the fault affects vulnerable household members or has appeared during cold weather, are prioritised and usually attended within a few hours.
Closing
A combi boiler running the heating but failing to deliver hot water is one of the more common faults we attend in London and the South East, and in most cases a Gas Safe engineer can diagnose the cause inside the first half hour of the visit. The simpler causes are fixed the same day. The deeper causes, like a scaled plate heat exchanger or a system that needs a powerflush, are still cheaper to address now than to ignore until the boiler stops running entirely.
If your boiler has heating but no hot water and you are in London or the South East, our Gas Safe registered engineers will diagnose the fault and quote for the fix before any work begins. Our callouts start from £120, with full pricing on our rates page. Call us today for same-day attendance.
Spotted Any of These Warning Signs?
Our leak detection specialists cover London and the South East using acoustic detection, thermal imaging and moisture meters — no unnecessary damage, full written report, available 7 days a week.
Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
