The house feels cold, the radiators aren't warming up, and yet the hot tap is working perfectly. That combination confuses a lot of homeowners because it looks like the boiler is half-broken. In many cases, that's exactly what's happened.
This is a very common version of hot water but no heating, especially in homes with a combi boiler. The boiler can still heat water for taps and showers while something in the central heating side stops the radiators from getting hot. The good news is that a few checks are safe to do at home, and they can save time before booking an engineer.
Table of Contents
- Why You Have Hot Water But Your Radiators Are Cold
- Simple DIY Checks You Can Do in Five Minutes
- Identifying Common Mechanical Boiler Faults
- How to Attempt a Safe Fix and When to Stop
- Calling a Gas Safe Engineer What to Expect
- Prevent Future Heating Failures with Regular Servicing
Why You Have Hot Water But Your Radiators Are Cold
When taps run hot but radiators stay cold, the boiler usually isn't completely dead. It's still producing heat. The problem is that the heat isn't being sent around the heating circuit properly.
In many UK homes, that points straight at a combi boiler. A combi handles domestic hot water and central heating from one appliance. That means one internal fault can leave the shower working while the radiators do nothing.
Industry guidance links this fault pattern strongly with combi boilers and regularly points to the diverter valve as a main suspect. One trade reference summarised in BestHeating's guide on hot water with no central heating says diverter valve failure is among the top three combi boiler faults in the UK, particularly in boilers older than 8 to 10 years.
Two separate jobs inside one boiler
A combi boiler does two different jobs:
- Hot water mode heats water for taps and showers.
- Heating mode sends heated water through radiators and pipework.
If the hot water side still responds but the heating side doesn't, that narrows the fault. It often means the burner still fires, but something inside the system isn't switching, circulating, or opening as it should.
Practical rule: If the boiler makes hot water normally, the fault is often in the heating controls, pressure, circulation, or internal switching parts, not in the gas supply itself.
What tends to cause it
A few faults show up again and again:
- Low pressure can stop the heating circuit from working properly.
- A stuck diverter valve can keep heat on the hot water side.
- Pump or motorised valve issues can stop circulation.
- Thermostat or control problems can mean the boiler never gets a proper call for heat.
- Sludge or trapped air can leave parts of the system blocked or slow.
That's why the best approach is simple. Check settings and pressure first. Then look for signs that point to an internal mechanical fault.
Simple DIY Checks You Can Do in Five Minutes
Most homeowners shouldn't start by taking panels off a boiler. The first checks are much simpler than that, and they often find the problem quickly.
A useful reality check comes from BOXT's guide to no hot water or heating, which says the pressure gauge should ideally sit between 1 and 1.5 bar when the system is cold. The same guide also reports that in a 2024 survey of more than 2,000 respondents, 17% had recently experienced no hot water or heating.
Check the thermostat first
This sounds obvious, but it gets missed all the time. The thermostat might be turned down, timed off, stuck in holiday mode, or not calling for heating.
Run through this in order:
- Turn the room thermostat up so it's clearly above room temperature.
- Check the heating schedule on the programmer or smart control.
- Make sure only heating is being tested. If someone has just run a hot tap, wait a moment and then try the heating again.
- Listen for response. A click at the thermostat or boiler can tell a lot.
If the boiler doesn't react at all when the thermostat is turned up, the problem may be with controls rather than the boiler's main heating parts.
Look at the boiler pressure gauge
Low pressure is one of the first things to check because it's common and visible. On most systems, the gauge should sit around 1 to 1.5 bar when cold, as noted in the BOXT guidance above.
A quick reading guide helps:
| Gauge reading | What it usually means |
|---|---|
| Around 1 to 1.5 bar | Pressure is likely normal when cold |
| Below 1 bar | Pressure may be too low for proper heating operation |
| Very high reading | Leave it alone and get advice before adjusting anything |
If the pressure is low, topping it up may solve the issue. Homeowners who need help finding the valve arrangement can use this guide to a boiler filling loop.
If the pressure keeps dropping after topping up, that's no longer a simple reset job. It points to a leak, expansion issue, or another fault that needs proper diagnosis.
Read the boiler display properly
Modern boilers usually try to tell the user what's wrong. The display may show a fault code, a warning light, or a symbol that indicates low pressure, ignition lockout, or circulation trouble.
Check for these signs:
- A flashing fault code on the front panel
- Reset prompt shown on screen
- Heating demand symbol missing when the thermostat is up
- Pressure warning or red-zone indication on the gauge
Don't guess at fault codes. Read the manual if it's available. If the code points to gas, combustion, or an unknown lockout, that's the point to stop and call an engineer.
Identifying Common Mechanical Boiler Faults
If the quick checks don't fix it, the boiler may have an internal fault or the heating circuit may be struggling to circulate water. The reason the pattern matters more than the brand name becomes clear at this point.
Diverter valve problems
In a combi boiler, the diverter valve directs heated water to either the taps or the radiators. When it sticks or fails, the boiler may keep serving hot water while ignoring the heating circuit.
This is why it's such a common suspect for hot water but no heating. Trade guidance covered by BestHeating identifies diverter valve failure as a top-three combi boiler fault, especially in appliances older than 8 to 10 years, as noted earlier.
Typical clues include:
- Hot taps work normally
- Radiators stay cold even though the boiler fires
- Heating may work briefly, then stop
- The problem is worse after using hot water
A homeowner shouldn't try to strip or free a diverter valve. That's engineer territory.
Pump and valve issues
The boiler can create heat, but the system still needs to move that heat through the house. That job depends on the central heating pump and, on some systems, motorised valves.
If the pump isn't circulating properly, the radiators won't warm up as expected. If a motorised valve doesn't open, hot water may never reach the heating side.
Look for patterns rather than trying to identify parts by sight:
- Boiler fires, but heat doesn't spread
- Only one section of the home warms up
- Pipework near the boiler gets hot, but radiators don't
- Heating cuts in and out without a clear pattern
A working burner doesn't guarantee a working heating system. Heat has to be switched and circulated, not just produced.
These faults can sound simple when described online, but replacing pumps, heads, or internal actuators isn't a DIY job on a gas boiler.
Sludge and trapped air
Not every fault is inside the boiler casing. Some happen in the wider central heating system.
UK repair guidance notes that sludge and trapped air can stop the heating circuit from circulating properly even when hot water still runs. Sludge tends to settle in radiators and pipework. Air collects in high points and leaves radiators patchy or cold.
Common signs include:
- Radiators cold at the top. Air is often the reason.
- Radiators warm at the top but cold at the bottom. Sludge is often involved.
- One radiator cold while others heat. That may be a local blockage or balancing issue.
- Several radiators barely warming. Circulation may be poor across the whole system.
Thermostat and control faults
Controls don't always fail completely. Sometimes they fail just enough to confuse things.
The thermostat may show as on, but the signal isn't reaching the boiler. The programmer may power up, but not switch the heating channel. Wireless controls can also lose connection.
That usually looks like this:
| Symptom | Likely area to investigate |
|---|---|
| Boiler never reacts to heating demand | Thermostat, programmer, receiver |
| Hot water works on demand, heating schedule doesn't | Heating controls |
| Boiler shows no heating symbol when thermostat is raised | Control signal issue |
At this stage, the useful task for a homeowner is observation. The useful task for an engineer is testing.
How to Attempt a Safe Fix and When to Stop
Two homeowner fixes are usually reasonable. Repressurising the boiler and bleeding a radiator. Both can help when the fault is low pressure or trapped air. Neither should turn into a guessing exercise.
Repressurising the boiler
If the pressure gauge is sitting low, the system may need topping up through the filling loop. The exact layout varies by boiler, but the safe approach is consistent.
- Turn the heating off and let the system cool.
- Find the pressure gauge and confirm it's low.
- Locate the filling loop. This is often a braided hose or built-in filling connection.
- Open the filling valves slowly.
- Watch the gauge the whole time.
- Stop when the pressure reaches the normal cold range shown in the manual.
- Close both valves fully.
Once that's done, test the heating again.
Stop immediately: If the pressure rises too fast, goes unusually high, or the filling loop arrangement isn't clear, leave it and book an engineer.
Bleeding a radiator
If a radiator is cold at the top and warm lower down, trapped air may be the issue.
A safe method looks like this:
- Turn the heating off and allow the radiator to cool.
- Use a radiator key at the bleed valve.
- Hold a cloth underneath because a little water may follow the air.
- Open the valve slightly, not fully.
- Close it as soon as water runs steadily.
After bleeding, check the boiler pressure again. Releasing air can lower it.
Stop if black water pours out heavily, the bleed valve won't close properly, or the system still won't heat after a couple of sensible checks. Repeated topping up, repeated bleeding, and repeated resetting usually mean the fault sits deeper than air or pressure.
Calling a Gas Safe Engineer What to Expect
Once the simple checks are done, a qualified engineer is the right next step. Gas boiler work isn't an area for trial and error. Anyone working on the appliance must be properly registered, and homeowners can read more about that in this guide on why Gas Safe registered engineers matter.
The phone call goes better when the fault is described clearly. A useful summary is short: the home has hot water but no heating, the thermostat has been checked, the pressure has been checked, and any visible fault code has been noted. That gives the engineer a workable starting point before arrival.
A few things usually happen at the visit. The engineer will check controls, system pressure, circulation, and whether the boiler is switching correctly into heating mode. If needed, they'll test components such as the pump, diverter valve, or motorised valve rather than replacing parts blindly.
The cost question is where many guides become misleading. A table of exact repair prices sounds helpful, but repair cost and time vary by boiler make, part access, location, and whether the fault turns out to be in the appliance or the wider system. For that reason, this article won't invent figures.
| Fault | Typical Repair Time | Estimated Cost (2026) |
|---|---|---|
| Diverter valve fault | Varies by boiler and access | Varies |
| Heating pump fault | Varies by system layout | Varies |
| Motorised valve fault | Varies | Varies |
| Sludge or circulation issue | Varies widely | Varies |
The useful trade-off is simple. Paying for accurate diagnosis first is usually better than asking for a specific part to be changed based on guesswork.
Prevent Future Heating Failures with Regular Servicing
A lot of heating failures don't arrive without warning. Parts often get noisy, slow, sticky, or erratic before they stop working altogether. Annual servicing gives an engineer a chance to spot those signs before the house goes cold.
That matters with exactly the kinds of faults covered above. Pumps can weaken. Diverter valves can start sticking. Controls can become unreliable. Sludge problems often show up in system performance before they become a complete loss of heating.
What servicing helps catch early
- Worn moving parts that still work, but not consistently
- Pressure problems that suggest a developing issue
- Circulation faults that point to sludge or poor flow
- Control issues that can be tested before they fail at the worst moment
Regular servicing won't prevent every breakdown, but it does cut down the number of surprises and makes faults easier to deal with before they become urgent.
For homeowners who tend to forget service dates, the simplest fix is a reminder system. Booking an annual boiler service reminder keeps the task from drifting into next year, which is how many avoidable heating problems start.
Service That Boiler helps homeowners and landlords remember boiler servicing before it gets missed. It's a free reminder service that keeps the next due date organised and makes annual upkeep easier to stay on top of. Set a reminder in seconds at Service That Boiler.