Combi Boiler Hot-Water Faults
No Hot Water But Heating Works In Croydon?
Radiators warming up but the taps are cold? The boiler may not be switching into hot-water mode properly.
Quick Answer
What This Usually Means
If you have no hot water but the heating works, the boiler is not necessarily completely dead. The radiators may still warm up while a fault prevents the boiler from producing hot water correctly.
Common symptoms include cold or lukewarm water, water that goes hot and then cold, or a boiler that does not respond when a hot tap is opened.
We regularly diagnose these faults on combi boilers throughout Croydon, East Croydon, Addiscombe, Thornton Heath, Norbury, Selhurst, South Croydon and West Croydon.
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Jump Straight To Your Hot-Water Problem
Select the problem that best matches what your boiler is doing. You will be taken directly to the most useful section.
Understanding The Fault
Why Is My Heating Working But There Is No Hot Water?
With a combi boiler, heating and hot water are controlled in different ways. If your central heating is working but there is no hot water, the problem is often isolated to the boiler’s hot-water components rather than the heating circuit itself. Your radiators can still heat up because the boiler is able to fire and send water around the heating circuit.
But when you turn on a hot tap, the boiler has to detect that demand and switch priority over to hot water. On some systems, heating controls or thermostat communication faults can also interfere with how the boiler responds during demand changes.
That switch-over is where the fault often sits. It might be the diverter valve sticking, the flow sensor not detecting water movement, a thermistor giving the wrong reading, or the plate heat exchanger being restricted by scale and sludge.
London hard water does not help. Over time, scale can build up inside plate heat exchangers, especially on older combi boilers in tight cupboards where servicing has been missed or rushed.
If the boiler has not had a proper recent check, our boiler servicing checks for Croydon homes can help spot wear, scale signs, pressure issues and safety problems before they turn into repeat breakdowns.
Hot-Water Demand
Why Does My Boiler Fire Up For Heating But Not Hot Water?
Some homeowners describe this as “the boiler is on but no hot water is coming through the taps”. Even though the boiler appears to be working, a fault on the hot-water side can prevent heat reaching the domestic water supply.
If the boiler fires normally when the heating comes on but does nothing when a hot tap is opened, the issue is usually on the hot-water side of the boiler rather than the heating circuit.
Many homeowners assume the boiler cannot be at fault because the radiators are hot. In reality, a combi boiler can still heat the property while failing to produce hot water.
Common causes include a faulty flow sensor, a sticking diverter valve, a blocked plate heat exchanger, temperature-sensor faults or problems detecting hot-water demand.
Some boilers show no reaction when a hot tap is opened. Others fire briefly before shutting down again. Both symptoms can indicate that the boiler is not switching correctly into hot-water mode.
Likely Boiler Components
Common Reasons For No Hot Water When The Heating Still Works
The most common causes of heating working but no hot water include diverter valve faults, blocked plate heat exchangers, flow-sensor problems and hot-water temperature-sensor issues.
Could A Faulty Diverter Valve Be Causing The Problem?
A faulty diverter valve is one of the most common reasons a combi boiler has heating but no hot water. We diagnose these issues regularly as part of our boiler repair service in Croydon.
The diverter valve directs heat either to the radiators or to your hot-water supply. When you open a hot tap, the valve should move and prioritise domestic hot water.
If the valve sticks, becomes worn or fails internally, the boiler may continue heating the radiators while little or no heat reaches the taps.
Signs of a diverter valve problem include heating working normally, cold hot-water taps, radiators warming during hot-water use, or the boiler firing without increasing water temperature.
| Possible Fault | What It Usually Looks Like |
|---|---|
| Diverter valve sticking | The boiler fires for heating, but does not properly switch over when a hot tap is opened. Sometimes the radiators warm when you are trying to run hot water. |
| Blocked plate heat exchanger | Hot water may start warm, then go cold, or only work at a slow tap flow. Scale and sludge can restrict heat transfer inside the boiler. |
| Flow sensor fault | The boiler may not realise a hot tap has been opened. You might hear nothing from the boiler when you turn the tap on. |
| Thermistor or temperature-sensor issue | The boiler may read the water temperature wrongly, causing unstable hot water or the burner to cut in and out. |
| Low pressure or system fault | Some boilers stop hot-water performance when the pressure is too low or there is another fault code showing on the display. |
The Boiler Is Still Partly Working
The Boiler Is Working, Just Not In The Way You Need
A lot of people say, “The boiler can’t be broken because the heating works.” Fair thought. But on a combi boiler, heating working does not prove the hot-water side is healthy.
We had one job near East Croydon where the radiators were fine every evening, but the shower would go hot, cold, hot, then cold again. The customer thought it was the shower valve. It was not. The boiler was struggling to transfer heat properly when the tap demand changed.
On another call in Norbury, the boiler fired perfectly for heating but did almost nothing when the kitchen tap opened. That pointed more toward the boiler not detecting hot-water demand, not a general heating failure.
The pattern matters. Hot then cold is different from no response at all. Lukewarm water is different again. Some homeowners still get heating upstairs but notice hot water takes far longer than normal to arrive, especially once flow rates and heat transfer start deteriorating inside the boiler.
Match The Symptom
What Is Your Hot Water Doing?
Hot Water Goes Hot Then Cold
This can happen when the plate heat exchanger is restricted, the boiler is overheating then cutting back, or the hot-water temperature readings are unstable. If the issue becomes intermittent, the boiler may also begin locking itself out during demand changes, especially when sensors or circulation problems worsen over time. It needs proper checking, not just turning the temperature dial up.
Boiler Not Detecting Hot-Water Demand
If the boiler does not react when you open a hot tap, it may not be detecting hot-water demand correctly. This is often caused by a faulty flow sensor, wiring issue or hot-water control fault inside the boiler. In many cases the boiler simply does not realise that a hot tap has been opened.
Radiators Warm During Hot Water
If radiators start warming while you are running a hot tap, the diverter valve may not be sending heat where it should. This is a common combi boiler fault.
Safe Initial Checks
What To Check Before Calling A Gas Safe Engineer
Before booking a boiler repair, there are a few simple checks that may help identify whether the problem is with the hot-water settings, boiler pressure, a local water-supply issue or a fault inside the boiler itself.
Start simple. Check the boiler display for an error or boiler fault code. Even intermittent hot-water problems can sometimes leave fault history or warning codes behind. Check the pressure gauge if your boiler has one. Make sure the hot-water temperature has not been turned right down by accident. Try more than one tap, because one faulty tap or shower can confuse the diagnosis.
If every hot tap has the same problem, and the heating still works, the issue is more likely around the boiler’s hot-water controls, diverter valve, flow sensor or heat exchanger. If only one shower is affected, that might be a shower valve or local plumbing issue instead.
If you are in Croydon and the taps have suddenly gone cold while the heating still comes on, call 0203 576 6769. We will ask what the boiler does when the tap opens before we tell you what is likely.
Still No Hot Water?
Tried The Safe Checks And Still Stuck?
Talk directly to a Gas Safe engineer in Croydon for free telephone advice. If a repair is needed, ask about today’s availability.
Clear Boiler Diagnosis
Hot-Water Fault Help Without Guesswork
No Guesswork
We check the boiler response at the tap, hot-water flow, temperature behaviour, fault codes, pressure, visible leaks and whether heat is being sent the wrong way through the system.
Straight Advice
If it is a small part or control issue, we say so. If scale, sludge or a blocked plate heat exchanger is likely, we explain what that means before any work goes ahead.
Local Experience
MPS Heating & Plumbing LTD is Gas Safe Registered — ID 662170, working across Croydon on Worcester Bosch, Vaillant, Ideal and Baxi combi boilers.
When A Repair Is Needed
When It Needs An Engineer
If the heating works but there is no hot water, the repair cost depends on what has failed. A sensor fault is different from a sticking diverter valve, and a scaled-up plate heat exchanger is different again.
We do not invent a price before checking the boiler. We diagnose the fault, explain what we have found, and quote before repair work starts. If the boiler needs repair rather than a service, we will tell you clearly.
Customer Feedback
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Book A Hot-Water Fault Check
No Hot Water But The Heating Still Comes On?
If you are in Croydon, East Croydon, Thornton Heath, Norbury, Addiscombe, Selhurst, South Croydon or West Croydon, MPS can check why the boiler is not switching into hot-water mode properly.
When you call, we will ask whether the boiler fires at the tap, whether the water goes hot then cold, whether the radiators warm up during hot-water use, and whether any fault code is showing.
Common Questions
No Hot Water But Heating Works FAQs
Why do I have heating but no hot water?
On a combi boiler, the heating side can still work even if the hot-water side has a fault. Common causes include a sticking diverter valve, a faulty flow sensor, a blocked plate heat exchanger, or a temperature-sensor issue.
Could it be the diverter valve?
Yes. A diverter valve sends heat either to the radiators or to the hot-water side. If it sticks or fails, the boiler may heat the radiators but not send heat properly to the taps.
Why does my hot water go hot then cold?
Hot water that keeps cycling hot and cold can point to restricted flow, scale inside the plate heat exchanger, unstable temperature readings, or the boiler cutting back because it cannot transfer heat properly.
Can London hard water cause this?
It can contribute. Hard water can scale up parts of the hot-water side over time, especially the plate heat exchanger on older combi boilers. Sludge from the heating system can also make the issue worse.
Why does my boiler not react when I turn on a hot tap?
This often points towards a flow-sensor fault, hot-water control issue or the boiler failing to detect hot-water demand correctly.
Can a blocked plate heat exchanger cause no hot water?
Yes. A blocked plate heat exchanger can restrict heat transfer, causing poor hot-water performance, fluctuating temperatures or complete hot-water loss.
Why do my radiators get hot when I run a hot tap?
This is often a sign of a diverter valve fault. The valve may be allowing heat to travel towards the heating circuit instead of prioritising hot water.
Why is my boiler on but there is no hot water?
If the boiler is on but there is no hot water, common causes include a faulty diverter valve, blocked plate heat exchanger, flow-sensor fault or a problem detecting hot-water demand.
What should I check before booking a repair?
Check whether all hot taps are affected, whether the boiler display shows an error code, whether the pressure is low, and whether the boiler reacts when you turn a hot tap on. Do not remove the boiler casing or try to work on gas-side parts.