If your radiators are warm but the taps run cold, the boiler is usually still firing. The fault sits in whatever sends heat to your hot water. On a combi boiler the most common culprit is a stuck diverter valve. On a system with a cylinder it is more often a timer, a thermostat or an immersion heater. Some checks take two minutes and cost nothing. Anything inside the boiler is Gas Safe work.
This guide takes you through the checks in order, explains what each likely cause means, and tells you plainly where DIY stops.
TL;DR
- Heating on but no hot water usually points to the diverter valve (combi boilers) or to hot water timer, cylinder thermostat or immersion heater settings (boilers with a cylinder).
- Check first: power to the boiler, the hot water programme, the pressure gauge (around 1 bar) and any fault code on the display.
- Never open the boiler casing or touch gas parts. Only a Gas Safe registered engineer may work on gas appliances.
- A diverter valve, pressure that keeps dropping or a repeated fault code needs an engineer, not another reset.
- We offer a 60 minute emergency response for heating and hot water, with no call-out fee.
Table of Contents
- First, work out which type of system you have
- Why the heating works but the hot water does not
- A step-by-step check you can do in ten minutes
- What you must not do
- Comparing the likely causes
- Our perspective: what we see on these calls
- When to call an engineer, and what to expect
- Book help with no hot water
- FAQ
- Sources
First, work out which type of system you have
The cause depends on how your home makes hot water, so spend ten seconds on this before anything else.
Combi boiler. There is no hot water tank. The boiler heats water as you open a tap, at mains pressure. If you have no cylinder in an airing cupboard or loft and no cold water tank, you almost certainly have a combi. The Ideal Heating guide describes combi boilers as mains-pressure, while system boilers are mains-fed but store hot water in a cylinder and heat-only (regular) boilers are gravity-fed.
System or regular boiler with a hot water cylinder. The boiler heats water that is stored in a cylinder, usually in an airing cupboard. Here the hot water is controlled by a programmer or timer and a cylinder thermostat, and many cylinders also have an electric immersion heater. Citizens Advice notes that combi boilers have no separate hot water cylinder, so the checks differ between the two.
Knowing this saves you from a classic wrong turn: hunting for a cylinder thermostat on a combi, or blaming the diverter valve when the real problem is a timer set to off.
Why the heating works but the hot water does not
A boiler does two jobs: it heats water for the radiators and it heats water for the taps. When one job works and the other does not, the fault is rarely the burner itself. It is usually the part that decides where the heat goes, or the controls telling it what to do.
The diverter valve (combi boilers)
Inside a combi, the diverter valve switches the boiler's heat between the radiator circuit and the hot water. According to Viessmann UK, a faulty diverter valve is a common cause of working heating with no hot water, because it stops the heat reaching the taps.
Typical signs:
- Radiators heat up normally, but taps and shower stay cold or only lukewarm.
- The boiler runs and sounds normal, but nothing changes when you open a hot tap.
- The problem has crept in over days or weeks, sometimes with hot water that comes and goes.
The diverter valve sits inside the boiler, so it is not a homeowner repair. It needs a Gas Safe registered engineer to diagnose and replace it.
Hot water settings, timer or controls
Viessmann also lists incorrect hot water timer settings, or boiler controls that have been switched off or changed, as a cause. This is the cheapest problem to fix, and it often happens after the clocks change, a power cut, or someone nudging a dial while cleaning.
Look at:
- The hot water mode on the boiler. Many boilers have separate heating and hot water settings, and some have an eco or comfort mode that affects how quickly hot water arrives.
- The programmer or smart thermostat. Heating and hot water are often scheduled separately, and a schedule that only covers "heating" will leave the taps cold.
- The boiler's own temperature dial for hot water, which may have been turned right down.
Low boiler pressure
Southwark Council's no heating or hot water guidance tells residents to check that the boiler pressure gauge sits at roughly 1 bar. Pressure that is too low can lock a boiler out of firing for hot water, and low pressure that returns after you top it up often means a leak somewhere in the system.
If you have the filling loop and your boiler manual shows how to use it, repressurising is a normal homeowner task. Follow the manufacturer's instructions, not a video for a different model. If the pressure keeps falling, stop topping it up and book an engineer to find the leak.
A frozen condensate pipe
In cold weather, a frozen condensate pipe can make a condensing boiler shut down or lock out. Depending on the boiler, you may lose heating, hot water or both. Citizens Advice says that a frozen condensate pipe can be thawed gently with a hot water bottle or a cloth soaked in hot water, if it is safely accessible.
We have separate guides on how to thaw a frozen condensate pipe safely and on what to do about a blocked condensate pipe. One rule from both: never pour boiling water over the pipe. If you want to understand what a burst or frozen pipe can mean for your home insurance, this overview of frozen pipe damage coverage explains the principle, although its examples are American.
Cylinder systems: thermostat, programmer and immersion heater
With a cylinder, Citizens Advice advises checking that the programmer switches the hot water on one to two hours before you need it, and that the cylinder thermostat is set to at least 60 degrees Celsius. If the cylinder is cold when it should be hot, the Ideal Heating guide points to a boiler or timer problem. If an immersion heater is fitted, it can provide extra hot water while you wait for a repair.
A cylinder that stays cold while the radiators work can also mean a failed motorised valve or a faulty cylinder thermostat. Both are jobs for an engineer.
Other possibilities
An airlock, a failed circuit board, a blocked heat exchanger or a worn pump can also leave you with heating but no hot water. You cannot reliably tell these apart from the diverter valve without testing, which is why guessing and swapping parts tends to cost more than a proper diagnosis.
A step-by-step check you can do in ten minutes
Work through these in order. Stop as soon as hot water returns.
- Check the display and the power. If the boiler display is blank, test lights and sockets to rule out a power cut, and check the fuses in the fuse box, as Viessmann suggests.
- Check gas. Try another gas appliance, such as the hob. If that does not work either, the problem may be the supply rather than the boiler. Do not investigate a suspected gas problem yourself (see the safety section below).
- Read the display for a fault code. If a code or warning light is showing, Citizens Advice says to use the manufacturer's guide to interpret it. Our guide to common boiler fault codes gives a starting point, but your own manual is the authority for your model.
- Check the pressure gauge. Aim for about 1 bar, as per the Southwark guidance. If it is far below, repressurise following your manual.
- Check hot water settings. Make sure hot water is switched on, the schedule includes the current time and any temperature dial is turned up. On a cylinder system, confirm the programmer and the 60 degree thermostat setting.
- Try a reset. Citizens Advice lists resetting the boiler, following the manufacturer's instructions, as a safe check. Do it once. If the fault returns, do not keep resetting.
- Run the hot tap for a minute. A tap that runs lukewarm and then cold points to a different fault than a tap that never warms at all. Tell the engineer which it is.
- Consider the weather. If it has been freezing, think of the condensate pipe and read the thawing guidance before touching anything.
If you are renting, report the problem to your landlord at this point rather than working through the list, and keep a record of when you reported it. For council and housing association tenants in England, Citizens Advice gives the right to repair response time for heating and hot water as one working day between 31 October and 1 May, and three days from 1 May to 31 October. See their right to repair page for the detail.
What you must not do
The temptation with a cold shower is to take the cover off the boiler. Don't.
The Health and Safety Executive states that anyone employed to work on gas appliances in domestic premises must be Gas Safe registered and competent for that type of work. Its employer guidance adds that gas work by anyone else is illegal. That covers the diverter valve, the heat exchanger, the gas valve and anything else inside the casing.
Also:
- Do not block ventilation or cover flues. The HSE warns that blocked ventilation and flues can allow dangerous levels of carbon monoxide to build up.
- Do not use a blowtorch, hairdryer on high or boiling water on a frozen pipe.
- Do not keep topping up a boiler that keeps losing pressure.
- If you smell gas or suspect carbon monoxide, stop. Turn off the gas at the meter if you can do so safely, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999. Do not wait for a boiler engineer.
It is also worth knowing that the Gas Safe Register reports that roughly one in six homes inspected had unsafe gas appliances. An intermittent fault is a good prompt to get the whole boiler looked at. Our explainer on Gas Safe certification shows how to check an engineer's credentials and what the card should say.
Comparing the likely causes
| Symptom | Likely cause | Can you fix it? | Who to call |
|---|---|---|---|
| Combi, radiators hot, taps cold, no fault code | Stuck or failed diverter valve | No | Gas Safe engineer |
| Hot water off or schedule wrong | Timer, programmer or boiler setting | Yes | Nobody, unless settings will not hold |
| Pressure well below 1 bar | Low pressure, possible leak | Repressurise per manual | Engineer if it keeps dropping |
| Cold weather, boiler lockout | Frozen condensate pipe | Gentle thaw if accessible | Engineer if it will not clear |
| Cylinder cold, radiators hot | Cylinder thermostat, motorised valve or timer | Check settings only | Engineer for parts |
| Fault code on display | Varies by model | Read manual, reset once | Engineer if it returns |
Our perspective: what we see on these calls
Most people who ring us about no hot water have already done the sensible things. They have checked the pressure, looked at the timer and reset the boiler. When the heating still works and the taps do not, on a combi the diverter valve is the first thing we want to test, because that is the pattern the manufacturers describe too.
What we would like homeowners to take from this is simple. Settings and pressure are worth five minutes of your time. Repeated resets are not. A boiler that is trying to tell you something with a fault code is better served by a diagnosis than by a fourth restart, and a lockout on a cold morning can hide a frozen pipe or a pressure leak that will come back.
We work to clear, fixed pricing and tell you what the job involves before we begin. Our heating and boiler labour is published at £110 per hour, there is no call-out fee, and you pay only after the work is done successfully. You deal directly with the engineer who comes to your home, not a call centre. Where a boiler is beyond sensible repair, we say so and give a free on-site quote for a replacement rather than keep billing you for patches.
— Michael
When to call an engineer, and what to expect
Call a Gas Safe registered engineer when:
- you have a combi boiler, heating works, hot water does not, and the settings and pressure are fine;
- a fault code returns after one reset;
- pressure falls again within days of topping up;
- you can hear banging, gurgling or a pump running without heat;
- the boiler has not been serviced for more than a year. The Gas Safe Register says regular servicing by a registered engineer helps keep a boiler in good working order.
Landlords have an extra duty: a gas safety check is needed every 12 months from a Gas Safe registered engineer, as Babergh District Council sets out. If you are unsure what a service involves, read our boiler servicing guide and how to service a gas boiler safely to see what you can check yourself between visits.
In the meantime, keep warm sensibly. Citizens Advice cautions that portable electric heaters can be expensive to run, so use them sparingly, and if you have an immersion heater, switching it on can give you hot water while the boiler waits for repair.
Book help with no hot water
If the checks above have not brought the hot water back, you can book an emergency heating engineer with a 60 minute response time, 24 hours a day, including holidays. We charge no call-out fee and back our work with a no fix, no fee guarantee and a 12-month warranty. Boiler and heating labour is £110 per hour. Our customers have left over 300 reviews on Trustatrader.
If the diagnosis points to a boiler that is past repairing, we supply and fit new boilers from £1,300 one-off, and you can request a free new boiler quote first. For general work, see our plumbing services.
Other boiler faults that look similar are covered in our guides to a boiler that will not ignite and a boiler not responding to the thermostat.
FAQ
Why does my combi boiler heat the radiators but not the taps?
The usual cause is a diverter valve that is stuck or failing, so heat is not directed to the hot water side. Settings, low pressure and a frozen condensate pipe can produce similar symptoms, so check those first. If they are fine, you need a Gas Safe registered engineer.
Can I fix a diverter valve myself?
No. The valve is inside the boiler, and work on gas appliances in homes must be done by a Gas Safe registered engineer. Doing it yourself is illegal as well as risky, and it can invalidate a warranty.
What pressure should my boiler be at?
Check the gauge. Around 1 bar is the figure given in council guidance for a boiler when it is cold, but your manual gives the exact range for your model. If it is much lower, repressurise following the manufacturer's instructions. If it keeps dropping, there is probably a leak.
Is it an emergency if I have no hot water but heating?
Not usually a safety emergency, but it is urgent if you have young children, elderly or vulnerable people at home, or it is very cold. It becomes an emergency if you smell gas or suspect carbon monoxide: leave the property and call 0800 111 999.
Does a boiler with a cylinder have the same problem?
Not quite. With a cylinder, the first suspects are the hot water timer, the cylinder thermostat (at least 60 degrees Celsius) and the immersion heater, because hot water is stored rather than made on demand. A cold cylinder with hot radiators can also mean a failed motorised valve.
Sources
- My boiler is working but I have no hot water, Viessmann UK
- No hot water? Here's what to do, Ideal Heating
- Getting your broken boiler repaired or replaced, Citizens Advice
- No heating or hot water, Southwark Council
- Using the right to repair scheme, Citizens Advice
- Gas safety for home owners, HSE
- Gas safety for employers, HSE
- Gas safety in winter, Gas Safe Register
- The home boiler guide, Gas Safe Register
- Your responsibilities as a landlord, Babergh District Council