No Hot Water from Your Boiler? Here’s Why!

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Last updated: October 31, 2023

Hot running water on tap – one of the joys of modern life and one we can tend to take for granted. So, when we go to run a bath and discover there’s no hot water, it can feel like a personal disaster!

Especially if you have a combi boiler which provides hot water on demand, you should quite rightly expect your water to also run heated. And even with a conventional or system boiler, you should be able to plan around a lovely hot water supply. However, if your heating system has left you and your water cold, it’s a good idea to know what to do about it.

We take a look at why you may not have that lovely hot water just when you wanted it, and what you can do about it.

General tips: What You Need to Check First

You will most likely discover that you have no hot water at the point you need it, which is both inconvenient and, particularly if you were about to step into the shower or bath, not the most pleasant experience. But, before calling out your plumber or heating engineer, there are some things you need to check.

A sudden loss of hot water from your boiler and heating system could be due to a range of reasons, many of which you can potentially rectify yourself. The type of boiler you have could also influence the reasons why you may not have any hot water and so knowing the difference means you can follow the right trail to the most likely cause.

A combi – or combination boiler – supplies both your heating and hot water via a single, compact and sealed unit. A system boiler, on the other hand has a separate cylinder or tank to store the hot water. With a combi boiler, you have hot water on demand as it can switch between heating or hot water, whereas with a regular/system boiler you have a finite supply of hot water until the tank is refilled and heated again.

So, if you find yourself sans hot water, here is what you need to check:

  • Your boiler

It may sound obvious, but is your boiler working? No hot water could be a simple sign that all is not quite right with the boiler so check to ensure it is working ok. Any problems should show on the boiler display, typically with one of its fault codes. To know what a particular error code means, check your boiler manual. It may need a boiler reset to resolve any boiler faults and get everything back on track but if you are unsure or the error code means something serious, then call out a gas safe engineer.

  • The pilot light

Especially if your boiler is an older model, the pilot light – which is either a flame or element that ignites the gas used to heat your water – may not function as well as it should, meaning you could get intermittent hot water when you run a tap or shower. So, if your hot water taps are running luke warm, then check to see if the boiler pilot light has gone out. If the pilot light won’t stay lit, then the likely problem is to do with the thermocouple, which opens up a gas valve to keep the pilot safely lit. You will need an engineer to replace the thermocouple.

  • Your gas, electric and water supplies

Your boiler – and your hot water – is only as good as the gas, electricity and water it is fed with, so if there’s nothing obviously wrong with the mechanics of your boiler, then check its supplies.

No display on your boiler could indicate a power cut so check your lights and fuse box to see if this is the case and there is an issue with the electrical power supply. If you suspect a gas supply issue, check other gas appliances such as your cooker to see if they are working. And finally, take a look at your water supply and if there is no water at all coming out of your taps, then there could be an issue there.

  • Your boiler settings

If there are no boiler error codes and the water, gas and electric supplies are all working fine, then staying with your boiler, check that it is set correctly on the control panel. It needs to be on water and heating, so if it is on ‘heating only’, it needs to be reset. And while you are at it, just check the thermostat for the water temperature as you want to ensure it is set at a reasonable level to give you the hot water you want.

  • The boiler timer

Your final ‘quick check’ before considering any other reason as to why you have no hot water is to look at the boiler timer. The timer can be affected by a host of things, from a power cut or batteries failing to the simple impact of the clocks changing. Any of these could mean your boiler is out of synch with your hot water expectations.

Cropped shot of male plumber repairing electric boiler

What to Check Next

If these checks draw a blank and all seem to be working as they should, but you still have no hot water from boiler to your taps, then there are other things you can check. But if you don’t have the right skills or knowledge of your boiler and central heating system, it may be worth calling out a gas safe engineer to check them for you:

Boiler Pressure

Low pressure in your combi boiler could be the reason why you suddenly have no hot water coming out of your taps. The correct water pressure for most domestic boilers is between 1bar and 1.5bar of pressure and this should be clearly shown on the boiler’s pressure gauge display. The pressure in your boiler can sometimes drop to below 1 bar, meaning it is under pressurised and this can have an impact on its ability to heat up the water supply flowing into your taps as well as central heating system.

You can re-pressurise your boiler yourself using its filling loop and following the instructions in its manual, or if you are in any doubt, call an engineer to check things out.

Water Leaks

A fall in boiler pressure can also be caused by a water leak somewhere in the system, with the inevitable knock-on effect with your lack of hot water. Check anywhere in your heating system that can spring a leak, including pipework and pipework joints as well as looking for signs of water such as damp patches on your ceiling, flaky paint or swollen woodwork.

Diverter Valve

If you still have no hot water but your central heating and radiators are working as they should, then turn your attention to your boiler’s diverter valve. This valve enables a combi boiler to switch between heating and hot water whenever you turn on your hot tap. If the diverter valve is showing a fault code or is stuck over in the central heating position, you could experience of loss of hot water. You will need a gas safe heating engineer to assess and fix your diverter valve if it is faulty.

Airlocks in Pipes

Air in your system can be a culprit for your lack of hot water, especially if you also get a spluttering when the tap is turned on. When air gets trapped in the water in your pipes, it can cause an airlock, which rises to the high points in the pipework to cause a partial or even full blockage that restricts the normal water flow.

If you suspect an airlock, you can do a DIY remedy which will hopefully push the air out and let that lovely hot water to flow. To do this, you need to securely attach a piece of hosepipe to the cold tap and then connect the other end to the fully opened hot tap. Now turn on the cold tap to let the water pressure force the air out. If this doesn’t resolve the issue, it is time to call an engineer.

You can also check out our detailed article on How To Prevent and Clear Airlock in Pipes.

Frozen Pipes

And finally, if the weather is cold outside and temperatures have dropped, check to see if you have any frozen pipes. The pipe that is most likely to affect your hot water flow is the condensate pipe, which is the waste pipe that takes condensation that builds up inside the boiler and drains it outside. If you discover a frozen pipe, it should be easy to fix by pouring hot, but not boiling water over the pipe until it thaws. Now fire up your boiler and hopefully that lovely heating and hot water will be back on track.

No Hot Water from a Regular/System Boiler

If you have a regular/system boiler and have worked through our checklist of reasons why you may not have hot water, then you need to check that there is water in your heater’s header tank.

Also known as the expansion tank or cistern, the header tank is a key component in an open vented central heating system you get with a regular boiler. The header tank is there to supply water to your central heating system and to also maintain the boiler’s level of water.

If on checking, you find that the header tank has run dry, then you could have a problem with the system’s ballcock getting stuck or seizing and so preventing the tank from refilling.

However, if this is not the case and you are still experiencing a lack of hot water when you need it, then it could be a simple fact that your boiler is not big enough to cope with your household’s hot water demands. If this is so, then upgrading to a new boiler that is the optimum size for your home may need to be considered so that you get hot water without running out.

No Hot Water from a Combi Boiler

As well as checking the other reasons we have listed as to why most boilers fail to supply you with hot water, if you have a combi, then low pressure could well be the cause.

Low pressure, which we have briefly looked at, is actually one of the main issues you may experience if you have a combi boiler or a sealed system. Combi boilers don’t have a separate hot water tank or cylinder which is typically kept the loft so low pressure can occasionally become a problem.

If the hot water pressure coming out of your taps drops all of a sudden, then check the pressure gauge on your boiler. It should be sitting between 1 and 1.5 bar, depending on its make and model; anything below 1 bar means your boiler is under-pressured and needs to be brought back into the normal zone.

You should be able to re-pressurise the boiler yourself, but make sure you follow the manufacturer’s instructions. But if you find you need to remove the boiler panel to get at the internal components, then leave alone and call out a gas safe heating engineer as you may invalidate your warranty.

Still Having Problems?

If, after taking into consideration the type of boiler you have and then working through all the possible reasons why you have no hot water, the issue remains, then getting a Gas Safe heating engineer to thoroughly check it over is essential. It may be that you need to upgrade your system or get a new boiler to ensure you and your family get all the hot water you need.

And to keep it that way, always keep your boiler regularly maintained, including the all-important annual service, so you don’t find yourself in hot bother when it comes to your heated water supply.


  1. No hot water – What to check – Vaillant
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