Why Is My Boiler Working for Hot Water but Not Heating?
If your boiler makes hot water but your radiators stay cold, the cause is usually the heating controls (programmer, thermostat or radiator valves), low boiler pressure, trapped air, or a faulty diverter or motorised valve. Work through the quick checks below before calling an engineer - several you can fix yourself in minutes.
Hot water but no heating is one of the most common boiler problems, and in most homes it comes down to a handful of causes - some simple, some needing a Gas Safe engineer. This guide walks through the quick checks to try first, the usual culprits, and when to call in a professional. Assett Plumbing diagnoses and fixes heating faults across Northamptonshire, including an emergency plumber in Northampton, Kettering and Wellingborough.
Quick Checks: What to Do First
- Check that the heating is switched on at the programmer or timer - set to 'on' or within a heating period, not just hot water.
- Turn the room thermostat up above the current room temperature, and replace its batteries if it's wireless.
- Check that your radiator valves (TRVs) are open - turned up, not stuck or set low.
- Check the boiler pressure is between 1 and 1.5 bar on the gauge.
- Bleed radiators with cold tops to release trapped air.
- In winter, check for a frozen condensate pipe (a gurgling boiler that cuts out).
If all of those are fine and you still have hot water but no heating, the fault is likely internal - a diverter valve, motorised valve, pump or circuit board - and that needs a Gas Safe engineer.
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Common Causes of Hot Water but No Heating
Heating turned off at the programmer or timer
The simplest cause: the heating is switched off or outside its timed period, even though hot water is on. Check the programmer is set to 'on' (or 'constant') and that the current time falls within a heating slot - after a power cut, the timer can reset to the wrong time.
Faulty or flat room thermostat
If the thermostat isn't calling for heat, the radiators stay cold. Turn it up above the room temperature; on a wireless thermostat, replace the batteries and check it's still paired to the boiler. If it doesn't respond at all, it may need replacing.
Thermostatic radiator valves (TRVs) closed or stuck
If only some radiators are cold, the TRV (the numbered valve on the side) may be turned down or stuck closed. Turn it fully up; if it still won't warm, the pin underneath can seize - gently freeing it often fixes it.
Low boiler pressure
If the pressure gauge reads below about 1 bar, the boiler may run hot water but not fire the heating. Top it up using the filling loop: turn the boiler off, open both valves slowly until the gauge reaches 1.2 bar, then close them and restart. If the pressure keeps dropping, you may leak and should call an engineer.
Airlocks or trapped air in the radiators
Trapped air stops hot water circulating, leaving radiators cold at the top. Bleeding them usually clears it - see our guide on how to bleed a radiator. Close the valve as soon as water appears.
Frozen condensate pipe (winter)
In cold weather, the condensate pipe - the plastic pipe running outside from your boiler - can freeze and block, making the boiler cut out or gurgle. Thaw it by pouring warm (not boiling) water over the frozen section, then reset the boiler. If it keeps freezing, it may need re-routing or lagging.
Stuck or faulty diverter valve
In a combi boiler, the diverter valve switches the boiler's heat between your hot taps and your radiators. If it sticks on hot water, the heating stops. Freeing or replacing a diverter valve is a job for a Gas Safe engineer.
Faulty motorised (zone) valve
System and regular boilers use a motorised valve (often a mid-position valve) to send hot water to the radiators or the cylinder. If it sticks on hot water, the heating stops. You may hear it buzzing or find it stuck - replacement is an engineer's job.
Broken circulating pump
The pump moves hot water around your radiators. If it fails, you can have hot water but cold radiators. A grinding or banging noise is a common sign - see why your boiler is making a noise. A failed pump needs replacing by a professional.
Faulty PCB (circuit board)
The PCB is the boiler's 'brain'. If it fails or throws a fault, it can run hot water but not call for heating, often with an error code on the display. If your boiler is locking out or showing a code, note the code and have it checked - a PCB is one of the costlier repairs.
If You Have a Combi Boiler
On a combi boiler, hot water but no heating very often points to a stuck diverter valve - it's sending the boiler's heat only to your taps, not your radiators. It can also be the heating controls or low pressure. If the quick checks don't fix it, the diverter valve needs a Gas Safe engineer. New to boiler types? See our guide to what a combi boiler is.
How to Avoid It Happening Again
Most of these faults are caught early by an annual boiler service - an engineer spots a tired pump, sticking valve or pressure problem before it leaves you cold. Bleeding radiators once a year and keeping the pressure topped up also helps.
When to Call an Engineer
If the quick checks haven't restored your heating, the fault is likely internal - a diverter or motorised valve, pump or PCB - and needs a Gas Safe engineer. Assett Plumbing offers boiler repair across Northamptonshire with no call-out fee, and a 24/7 emergency plumber when you're left with no heat in winter - including Northampton, Kettering, Corby and the wider Northamptonshire area.
Got the opposite problem? If your heating works but you have no hot water, see our separate guide.
Frequently Asked Questions
Why is my boiler giving me hot water but no heating?
Usually, it's the heating controls (programmer, thermostat or TRVs), low boiler pressure, trapped air in the radiators, or a faulty diverter or motorised valve. Work through the quick checks first; if the heating still won't come on, it's likely an internal valve, pump or PCB fault that needs a Gas Safe engineer.
Why are my radiators cold but I still have hot water?
If only some radiators are cold, it's usually trapped air (bleed them) or a closed TRV. If all of them are cold, check the programmer, room thermostat and boiler pressure first, then suspect the diverter or motorised valve.
Can a diverter valve cause no heating?
Yes. On a combi boiler, a stuck diverter valve can send heat only to your hot taps and not your radiators. It needs a Gas Safe engineer to free or replace it.
Can low boiler pressure stop the heating?
Yes. If the pressure gauge reads below about 1 bar, the boiler may run hot water but not fire the heating. Top it up to 1-1.5 bar via the filling loop; if it keeps dropping, you may leak.
Why is my heating not working in winter but hot water is fine?
A common winter cause is a frozen condensate pipe, which makes the boiler cut out. Thaw it with warm (not boiling) water and reset the boiler. The usual control and valve causes still apply too.
Is hot water but no heating an emergency?
Not usually - you still have hot water - but in very cold weather, or for vulnerable or elderly people, no heating should be sorted quickly. An emergency plumber can restore heat the same day.
What's the most common cause of hot water but no heating?
The heating controls (it's switched off or the thermostat/timer isn't calling for heat) and low boiler pressure are the two most common - and both are quick to check yourself before calling anyone
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Heating still off after the quick checks?
If your hot water works but the radiators stay cold — and the programmer, thermostat, TRVs and pressure all check out — it’s usually a diverter valve, motorised valve or pump that needs a Gas Safe engineer. Assett Plumbing will find the fault and get your heating back on, often the same day. Family-run across Northampton, Kettering, Wellingborough and Northamptonshire, with no call-out fee and a clear written quote first.
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