SEO title: Why Is My Heating Working but I’ve Got No Hot Water?
Meta description: Heating working but no hot water? Learn the common causes, what to check first and when a diverter valve, controls or cylinder fault needs an engineer.
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Supporting keywords: no hot water but heating works, combi boiler no hot water, diverter valve fault, boiler hot water fault, hot water cylinder not heating, boiler repair Weston super Mare
Why Is My Heating Working but I’ve Got No Hot Water?
TLDR
If your radiators heat normally but your taps stay cold, the boiler clearly has power and is producing heat. On a combi boiler, a faulty diverter valve becomes one of the main suspects. On a system or regular boiler with a hot water cylinder, the problem might sit with the programmer, cylinder thermostat, motorised valve or cylinder controls. Vaillant UK
Spoiler
Heating working normally narrows the fault down quite nicely.
The boiler has not completely died. The question becomes why the heat is reaching your radiators but failing to reach your taps or hot water cylinder.
The short answer
With a combi boiler, heating working alongside cold or lukewarm taps often points towards the domestic hot water side of the boiler. Vaillant and Baxi both identify the diverter valve as a common suspect because the valve directs boiler heat between radiators and hot water. On systems with a separate cylinder, controls such as the hot water timer, cylinder thermostat and motorised valve need checking. Baxi
What you need to know
- First identify whether you have a combi boiler or a hot water cylinder.
- If you have a combi, a diverter valve fault sits high on the list.
- With a cylinder, check the hot water programme before blaming the boiler.
- A fault code, low boiler pressure or supply issue still deserves checking.
- Internal boiler and motorised valve fault finding belongs with a competent engineer.
Fun fact
A combi boiler gives priority to hot water when you open a tap.
The diverter valve sends the boiler’s heat towards your tap rather than the radiators. Worcester Bosch describes a combi as heating mains water directly when you turn on a tap or shower, without storing hot water in a cylinder. Worcester Bosch
So if the radiators are lovely and warm but the shower resembles the Bristol Channel in February, something in the hot water side deserves attention.
Start with one important question: what type of heating system do you have?
This changes the diagnosis.
Combi boiler
No hot water cylinder.
The boiler heats your tap water on demand.
Open a hot tap and the boiler should recognise the demand, fire and send heat towards the domestic hot water circuit. Worcester Bosch
System or regular boiler
You normally have a separate hot water cylinder.
The boiler heats water which passes through the cylinder coil. A programmer, cylinder thermostat and motorised valve often control when the cylinder receives heat. Drayton explains that the cylinder thermostat switches the boiler’s heat supply to the cylinder on and off according to stored water temperature. Drayton Controls
Same symptom.
Different plumbing.
This is why “no hot water” is not a diagnosis.
If you have a combi boiler, check what happens when you open a hot tap
Open one properly.
Then look at the boiler display.
Does the boiler recognise a hot water demand?
Does a tap symbol appear?
Does the burner fire?
Does nothing happen at all?
This is useful information.
If the boiler does not react when you open a hot tap, the engineer starts thinking about how the appliance detects hot water demand.
If the boiler fires but the water stays cold or lukewarm, the investigation moves in another direction.
You do not need to understand the internal parts.
You only need to notice what happens.
Could the diverter valve be faulty?
Yes.
On a combi boiler, this is one of the classic faults where central heating works but hot water does not.
The diverter valve directs heat between two jobs:
- Heating the radiators.
- Heating your tap water.
Vaillant states that lukewarm or cold hot water alongside normally working radiators is a recognised sign of diverter valve trouble. Baxi gives the same diagnosis when heating works but hot water does not. Baxi
The valve might stick.
The actuator might fail.
Another component controlling the valve might cause the same symptom.
This needs proper fault finding.
Do not order a diverter valve because Google gave you confidence at 10:30 on Sunday evening.
The symptom points towards the area.
An engineer still needs to confirm the failed part.
What if the hot water is warm rather than completely cold?
That clue matters too.
A faulty diverter valve does not always fail completely.
Vaillant notes that a problem with the valve might result in lukewarm water rather than total hot water loss. Vaillant UK
Other faults also produce poor hot water temperature, so “slightly warm” does not automatically equal diverter valve.
Tell the engineer exactly what happens.
For example:
Cold at first.
Gets warm.
Never gets properly hot.
Temperature swings hot and cold.
Only one tap affected.
Every hot tap affected.
Those details save time.
Could low boiler pressure cause no hot water?
Yes, depending on the boiler.
Baxi and British Gas both advise checking boiler pressure when hot water stops working. Low system pressure might stop a modern boiler operating normally or trigger a fault code. Baxi
Look at the gauge.
For many sealed systems, roughly 1 to 1.5 bar cold is common, though your boiler manual gives the correct range.
If pressure is low and your manufacturer’s instructions explain homeowner repressurisation, follow those instructions.
If you are topping the boiler up every few days, stop treating the filling loop like part of your morning routine.
The pressure loss needs investigating.
Check the boiler display before resetting anything
Look for a fault code.
Take a photo.
Then check the manual.
Vaillant recommends checking the displayed error code because different codes point towards different boiler faults. Vaillant UK
A reset following the manufacturer’s instructions might clear a temporary fault.
Repeated resetting means the underlying problem still exists.
Pressing reset seventeen times does not become engineering through persistence.
What if you have a hot water cylinder?
The diagnosis changes.
If your central heating works but the cylinder stays cold, the boiler itself might be perfectly capable of producing heat.
The problem might lie between the boiler and the cylinder.
Common areas include:
- Hot water programmer settings
- Cylinder thermostat
- Motorised valve
- Wiring or controls
- Circulation through the cylinder coil
- A cylinder related fault
Drayton states that hot water heating will not operate if the programmer has switched hot water off. Its cylinder thermostat controls whether the cylinder calls for more heat. Drayton Controls
That makes the programmer a beautifully cheap place to start.
Check the programmer or timer
Make sure hot water is switched on.
Especially after:
- A power cut
- Clock changes
- Somebody pressing buttons
- Holiday settings
- A replacement controller
Vaillant specifically identifies timer settings as one cause of unexpected hot water loss on stored hot water systems. Baxi gives similar advice. Vaillant UK
Turn the hot water programme on or use the boost function where appropriate.
Then see whether the boiler responds.
If nothing changes, note that.
Could the cylinder thermostat be faulty?
Yes.
The cylinder thermostat monitors stored hot water temperature and requests more heat when required.
Drayton explains that the thermostat switches the heat supply from the boiler according to the cylinder temperature. Drayton Controls
If the thermostat or associated controls fail, the boiler might happily heat your radiators while receiving no valid request to heat the cylinder.
Do not start removing covers or testing mains voltage unless qualified.
A thermostat stuck to the side of a cylinder might look wonderfully harmless.
The wiring attached to heating controls disagrees.
Could the motorised valve be faulty?
Yes.
Many cylinder systems use a motorised valve to direct heated water towards the radiators, hot water cylinder or both.
Resideo describes its three port valve as controlling flow between the radiator and hot water cylinder circuits. Drayton gives its mid-position valve the operating modes “heating, hot water or a combination of both”. Resideo
A valve stuck in the heating position creates a fairly logical result.
Warm radiators.
Cold cylinder.
The boiler is producing heat perfectly well.
The heat is being sent to the wrong place.
Fault finding around motorised valves involves mains electricity and heating controls, so leave diagnosis and replacement to somebody competent.
A quick symptom guide
What you noticeWhere the investigation might startCombi, heating fine, all hot taps coldHot water demand, diverter valve, boiler DHW componentsCombi, hot water only lukewarmDiverter valve or domestic hot water faultCylinder system, heating fine, cylinder coldProgrammer, cylinder thermostat, motorised valveHot water stopped after power cutTimer or controller settingsBoiler shows a fault codeCheck manufacturer code firstLow boiler pressureRepressurisation or underlying pressure faultOnly one tap has a problemLocal tap, valve or pipework rather than boiler
This is fault finding direction.
Not a shopping list for replacement parts.
What if only one tap or shower has no hot water?
Now I become much less interested in the boiler.
If every other hot tap in the house works normally, the boiler is clearly producing hot water.
The fault is likely local to the affected outlet.
Possible areas include the tap, shower valve, local isolation, filter or pipework.
That distinction saves quite a lot of pointless boiler investigation.
Before booking someone, test:
Kitchen hot tap.
Bathroom basin.
Bath.
Shower.
Tell the engineer which ones work.
What if no water comes from the hot taps at all?
Different symptom.
“No hot water” often means:
Water flows normally, but stays cold.
If hardly any water comes from the hot outlet, the investigation needs to include the water supply and hot water pipework rather than temperature alone.
Vaillant notes that supply interruptions and pipework problems also cause hot water problems. Vaillant UK
Check whether cold taps work normally.
If the whole property has lost water, contact the water supplier rather than asking the boiler to solve a problem outside the house.
What you should safely check yourself
Before calling an engineer:
- Confirm whether you have a combi or a cylinder.
- Check every hot tap.
- Look at the boiler display.
- Photograph any fault code.
- Check boiler pressure.
- Check hot water timer settings.
- If you have a cylinder, select hot water boost and note whether the boiler responds.
- Check whether the problem started after a power cut.
- Follow your boiler manual if a single reset is appropriate.
That is plenty.
No boiler casing needs removing.
What needs a professional?
Call a heating engineer where:
- A combi recognises demand but produces no hot water
- A diverter valve fault is suspected
- Boiler pressure or fault codes keep returning
- A motorised valve or cylinder thermostat needs electrical testing
- The boiler needs opening
- Internal pumps, sensors, valves or heat exchangers need checking
- The problem persists after the basic homeowner checks
HSE states that domestic gas work must be undertaken by a Gas Safe registered engineer competent for the work involved. HSE
If you suspect the appliance is unsafe, switch the appliance off and leave it alone until checked. If you smell gas, call 0800 111 999. HSE
Common mistakes
Assuming the boiler is dead
Your heating works.
That already proves quite a lot of the boiler is doing its job.
Diagnose the hot water side first.
Ordering a diverter valve immediately
Yes, diverter valves cause this symptom.
No, that does not prove yours has failed.
Confirm the fault.
Ignoring the programmer on cylinder systems
Before paying somebody to dismantle anything, make sure the hot water channel is switched on.
Technology remains undefeated by the occasional wrong button.
Resetting repeatedly
One manufacturer-approved reset is sensible troubleshooting.
Daily resets are a fault report.
Comparing a combi with your neighbour’s cylinder system
Different system.
Different controls.
Different faults.
“Dave next door said his was the valve” is information.
Loose information.
What I would recommend
Start by identifying the system.
Combi?
Open a hot tap and watch the boiler.
Does it recognise demand?
Does it fire?
Does the water stay completely cold or become lukewarm?
Cylinder?
Check the hot water programme and thermostat setting.
Request hot water and see whether the boiler responds.
If the heating works normally but the hot water side still refuses to play along, get the system properly tested.
You already know the boiler produces heat.
Now the job is finding out why that heat is not reaching the hot water.
Heating works but no hot water in Weston super Mare?
Kesselmann Heating & Plumbing provides boiler fault finding and repairs across Weston super Mare, Clevedon, Portishead, Nailsea, Bristol and surrounding North Somerset. The current Kesselmann website lists boiler repairs alongside heating and hot water fault finding. Kesselmann Heating And Plumbing
For combi boiler faults:
Kesselmann boiler repair in Weston super Mare
For problems involving an unvented hot water cylinder, Kesselmann also provides cylinder installation, servicing and repair with G3-qualified engineers. Kesselmann Heating And Plumbing
Kesselmann unvented cylinder services
Need a Gas Safe engineer in Weston-super-Mare?
Frequently asked questions
Why do my radiators work but my taps are cold?
On a combi boiler, the diverter valve is one recognised cause because the valve switches boiler heat between central heating and domestic hot water. Controls, pressure and other internal components also produce similar symptoms. Vaillant UK
Does heating working mean my boiler is fine?
Not completely. Heating operation shows the appliance has power and is able to produce heat, but the hot water side still contains separate controls and components. A combi diverter valve or a cylinder system's hot water controls might still fail.
How do I know whether my diverter valve has failed?
Cold or lukewarm hot water alongside correctly working radiators is a recognised sign. Diagnosis still needs an engineer because other faults create similar symptoms. Vaillant advises professional repair rather than homeowner work on the diverter valve.
Why did my hot water stop after a power cut?
A power cut might reset the boiler clock, programmer or timer. Check the current time and hot water programme before assuming a component has failed.
Why does my hot water work from some taps but not others?
If most hot outlets work normally, the boiler is producing hot water. The fault is more likely around the individual tap, shower or local pipework.
Why is my cylinder cold when the radiators are hot?
A stored hot water system might have a programmer, cylinder thermostat or motorised valve fault. Those controls determine when heated water flows through the cylinder circuit.
