Hot taps. Cold radiators.
Hot Water But No Heating? Common Causes Explained
If your hot water works but the heating does not, the heating controls may not be requesting heat, or a fault may be stopping heat reaching the radiators. Start with the thermostat, programmer, heating mode and radiator valves, then check system pressure against your manual. Persistent problems can involve a diverter valve, zone valve or circulation fault and need an engineer.
The boiler is doing something. The question is why that heat is not reaching your radiators.
Before you start
Combi boiler or hot-water cylinder?
With a combi: normal hot water at the taps tells you the boiler can produce heat. Check whether it responds when heating is requested.
With a cylinder: the water may have been heated earlier, or by an immersion heater. Hot taps do not prove the boiler is working now.
An electric shower can heat its own water, so check an ordinary hot tap instead.
Start with what you can see
Which radiators are cold?
A single cold radiator tells a different story from a whole house with no heating.
Every radiator
Start with heating mode, the timer and the room thermostat. Then look for a response on the boiler display.
Check the heating controls →One or two radiators
If the rest of the home warms up, look at the affected radiator’s valve setting and whether it has cold spots.
Look at radiator problems →One floor or zone
Check the controls for that part of the house. A separate heating zone may have its own thermostat and valve.
Understand zone valves →The checks you can do
Start here before booking a repair
You do not need to open the boiler, touch wiring or take a valve apart for any of these checks.
Check heating mode and the programmer
Make sure central heating is enabled. Summer mode or a hot-water-only setting can leave the taps working and every radiator cold. Check the programmer’s day, time and schedule, particularly after a power cut or clock change. If your controls offer a heating boost, use it according to the user guide.
Set the thermostat above the room temperature
A thermostat set to 19°C will not normally ask for heat when it reads 21°C. Raise the target a little above the displayed room temperature for the check, then restore your preferred setting afterwards.
Check for flat batteries, holiday mode or a connection warning. With smart heating, look at the room device as well as the app. If it shows a connection problem, our guide to a thermostat losing its boiler connection covers that separate fault. Do not open a receiver or bridge any wiring.
Open the radiator valves using their normal controls
Turn thermostatic radiator valves, or TRVs, up for the check. A frost setting will usually keep a radiator off unless the room becomes very cold. Smart radiator valves may also have their own schedules or batteries.
See whether every radiator is cold or only one or two. Do not force a stuck valve, remove its body or start adjusting unfamiliar pipework.
Read the system pressure
On a sealed heating system, compare the pressure gauge with the range in your boiler’s manual. Many domestic models specify around 1–1.5 bar when cold, but your model’s instructions take priority. Older open-vented systems may not have a pressure gauge or filling loop.
Only top up if the user manual permits it and you can confidently follow the instructions for that exact filling arrangement. Stop if you are unsure, see a leak or find the pressure unusually high. If it keeps falling, read about pressure loss that returns after topping up and arrange a check. Repeated filling is not a repair.
Observe the boiler with the hot taps closed
Many combis temporarily prioritise hot water while a tap or shower is running. Close the hot taps, request heating and allow time for the system to respond. Note whether the display recognises heating demand, the boiler fires, or an error appears.
A thermostat’s flame symbol alone does not prove the boiler has received the request. These observations help an engineer narrow down the fault.
Narrow down the problem
What is your boiler doing?
These clues help decide what to check next. They do not confirm a failed part.
| What you notice | What it could mean | Safe next step | Engineer? |
|---|---|---|---|
| All radiators cold; boiler does not respond to heating | Heating mode, schedule, thermostat communication or controls fault | Check settings and batteries; note display response | If correct settings do not restore heating |
| Combi heats taps but radiators stay cold after taps close | Heating demand fault, diverter valve or restricted circulation | Request heating and record what happens | Yes, if basic checks fail |
| One radiator cold; others warm | Local valve setting, trapped air or restricted flow | Check its normal valve controls and compare other radiators | If it stays cold or a valve is stuck |
| Only one floor or heating zone is cold | Zone controls, motorised valve or circulation imbalance | Check the affected zone’s thermostat and schedule | If the zone still does not heat |
| Boiler fires briefly, then stops; radiators stay cold | Circulation restriction, pump or temperature-control issue | Record the sequence and any fault code; avoid repeated resets | Yes, if the pattern persists |
| Hot cylinder water but no heating | Stored heat or immersion use may mask a boiler fault; zone controls may also be involved | Identify how the cylinder is heated using normal controls | If heating settings are correct |
Still no heating?
Get the fault checked properly
Our Croydon boiler repair and fault-finding service checks the boiler and the wider heating system.
When settings are correct
Could a valve be stopping the heating?
A diverter valve that does not switch back
On combis that use one, the diverter valve routes heated primary water towards the radiators or the hot-water heat exchanger. A sticking valve or faulty actuator can stop the heating circuit receiving heat after hot-water demand ends.
It is a possibility, not a conclusion you can draw from cold radiators alone. An engineer needs to check the heating demand and valve operation before recommending a replacement. Leave internal valve testing and repairs to a Gas Safe registered engineer.
A motorised zone valve on a cylinder system
Systems with a hot-water cylinder may use separate motorised valves for heating and hot water, or a three-port valve. The cylinder can heat while the radiator circuit remains shut. Some faults also prevent the controls signalling the boiler to fire for heating.
The arrangement matters. In a house with separate upstairs and downstairs zones, one failed valve or control can affect only part of the property. Do not open actuator covers or force valve levers.
Heat needs somewhere to go
What about the pump?
The heating pump moves water around the circuit. A pump fault, restriction or airlock can prevent enough heat reaching the radiators. Noise or a boiler that heats briefly before stopping gives useful clues, but neither identifies the failed part on its own.
On many combis, the pump also operates during hot-water production. Reliable hot water therefore makes a completely failed shared pump less likely, although it does not rule out circulation problems on the heating side.
Only part of the house is cold?
Look at the radiators before blaming the boiler
A radiator cold at the top can indicate trapped air. A cold lower section can suggest deposits or poor circulation. Neither pattern automatically means the whole system needs a power flush.
Bleeding may help trapped air when done with the heating off and radiators cool, following suitable instructions. It also lowers pressure on a sealed system. Avoid bleeding every radiator just because the heating will not start. If upstairs heats but downstairs stays cold, that guide looks more closely at uneven heating.
A message on the display
Write down the fault code
Photograph the code and note the exact boiler model. Worcester Bosch, Vaillant, Ideal and Baxi use different code systems, and meanings can change between models.
Our boiler error-code guide is a starting point. Follow the manufacturer’s user manual for your appliance.
Only reset where the manual allows it. If the code returns, stop resetting and arrange a check. No code does not rule out a fault with the controls or a valve.
When it needs a visit
Correct settings. Still cold.
If the controls are set correctly and heating still does not start, book a fault check. The same applies to pressure that keeps dropping, repeated lockouts or unusual noises.
Internal gas boiler work needs a Gas Safe registered engineer qualified for the appliance. Electrical controls and external heating components also need a suitably competent professional.
If you rent, report the fault to your landlord or managing agent. Mention if someone at home is especially vulnerable to the cold.
Smell gas or suspect carbon monoxide? Stop troubleshooting, get everyone into fresh air and call 0800 111 999 from outside. Avoid flames and electrical switches. Call 999 if someone is seriously unwell.
An MPS customer’s experience
A boiler fault checked at short notice
Marcello contacted MPS about a faulty boiler. In his review, he describes Ansar attending at short notice and identifying the problem.
The customer’s account describes fault diagnosis; it does not specify the component or confirm a hot-water-but-no-heating fault.“Ansar came round at short notice to diagnose and identify the issue with my faulty boiler.”
A few common questions
Hot water works.
Heating doesn’t.
There is no single part that explains every case. The type of system and how it responds matter.
Can the thermostat stop the heating while hot water still works?
Yes. On a combi, hot-water demand can operate independently of the room thermostat. A low target temperature, wrong schedule, flat batteries or communication fault can prevent heating while the taps still run hot.
Does this definitely mean the diverter valve is broken?
No. Heating settings, thermostat communication, radiator valves and circulation problems can produce similar symptoms. A diverter valve is one possibility on boilers fitted with one, and needs testing before replacement.
Is it normal for heating to pause when I run a bath?
Many combi boilers prioritise hot-water demand, so a temporary pause can be normal. Once the hot tap is closed, heating should resume if the controls are still requesting it. Radiators remaining cold afterwards needs investigation.
Why is only one radiator cold?
If the others heat normally, check the cold radiator’s ordinary valve setting first. A local valve problem, trapped air or restricted flow is more relevant than assuming the whole boiler has failed.
Do I need a new boiler?
Not necessarily. A settings issue may need no repair, and a failed control or valve may be repairable. The diagnosis, boiler condition and availability of parts should guide the decision.
Heating help in Croydon
Tell us what happens when you turn the heating on
MPS Heating & Plumbing LTD covers Croydon, Thornton Heath, Addiscombe, Norbury, Selhurst, South Croydon, East Croydon and West Croydon.
Call 0203 576 6769. Open Monday–Sunday, 8am–2am. Ask about appointment availability.