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Common Hot Water Cylinder Problems

Nick Geary
By: Nick Geary
Updated: 6th July 2022

Unvented hot water cylinder problems

Vented or unvented hot water cylinder problems need to be resolved quickly and professionally. After all, the hot water cylinder plays an important role in making sure your home has enough hot water.

Hot water cylinders are normally very reliable but from time to time they might hit a bump in the road. We've put together a list of the most common hot water cylinder problems to help you find out what could be causing the issue.

Vented vs Unvented hot water cylinders

Blaze with hot water cylinder
Let's get started by finding out which hot water cylinder you have. There are two different types: vented and unvented. The difference between vented vs unvented cylinders essentially comes down to how they're fed water.

A vented cylinder needs a supply of water from a large tank of cold water in the loft. The water travels down a vent pipe to the hot water cylinder which is often found in an airing cupboard. As water expands when it's heated, the vent pipe also acts as an escape route for any excess water from the cylinder. You’ll need one of these if you have a regular boiler.

Unlike vented cylinders, unvented cylinders don't need a cold water tank in the loft because they're directly connected to the mains. While vented cylinders have a vent pipe to deal with any excess water, unvented cylinders don't. To deal with any excess water as it heats and expands, unvented cylinders have safety features built into them. This will either be an expansion unit at the top or an air bubble that gets added during the installation. Unvented cylinders need to be installed with a system boiler.

The only boiler type that doesn’t need to be installed alongside a cylinder is the combi boiler.

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Common hot water cylinder problems

No hot water

A good place to start is by taking a look at the thermostat on the hot water cylinder. Make sure it's set to the correct temperature and timings. If the temperature on the thermostat is accurate then we need to take a deeper look.

Should the system be powered by electricity (an immersion heater) then you need to check that your power supply is functioning. It might also be the case that the Residual Current Device (RCD) has been tripped by another circuit in the house.

Any hot water cylinder powered by a gas boiler relies on the boiler to warm the water up. In the event that it's failing to ignite then you'll be left with no hot water.

The first thing to check at this point is whether the boiler is displaying an error code. Modern boilers display error codes to help you easily determine the cause of the issue.

On occasion, you may be able to resolve the issue by resetting the boiler. However, many common problems that can lead to a property being left without hot water will need attention. These include:

  • Low boiler pressure
  • An airlock in the pipes
  • Frozen condensate pipe

Low boiler pressure

Boiler pressure is measured on the pressure gauge – which you'll find on the boiler. Ideally, the pressure should be between 1 and 2. However, do check the manufacturer's instructions for your boiler before attempting to increase the pressure. If it turns out that the boiler pressure is too low then it will need to be increased.

This is something that you can do yourself. Simply follow the steps in our guide to increasing boiler pressure. Always contact a professional if you're unsure.

An airlock in the pipes

Too much air in the heating system can block hot water from making its way around the pipes. There are two ways to remove the airlock which you can find in our guide to Airlock in Pipes: Here's How to Clear It.

Frozen condensate pipe

This is most likely during a period of freezing weather. Modern condensing boilers (any installed since April 2005) have a condensate pipe which sends waste acidic water away from the boiler and down a drain. Some are outside, susceptible to the elements, which can lead to freezing. To get the boiler going again, it will need to know How to Thaw a Frozen Condensate Pipe.

Still having trouble finding the cause of this hot water cylinder problem? Then you'll need to contact a Gas Safe registered engineer to fix this issue.

Not enough hot water

This is a common problem for hot water cylinders fitted with twin immersion heaters. Twin immersion heaters are made up of two heaters: one at the top and one at the bottom. The one at the top of the cylinder heats water at the top while the other heats all of the water. An issue with the supply of electricity to the bottom heater would lead to you not getting enough hot water. Issues can also be caused by a faulty part or limescale build-up.

During colder months, the system might have to work even harder to heat up the same amount of water as it usually would. The first thing you should check is that the temperature setting on your cylinder is correct. If that doesn't do the trick then your cylinder might not be big enough to meet the demand.

It's also worth checking that the cylinder isn't leaking which could greatly reduce the efficiency of the system. If you spot even the tiniest leak, you'll need to contact a fully qualified engineer.

No water at all

In the case that you're not getting any water at all, hot or cold, then there could be an issue with the filter. The most likely cause of a fault with the filter is that it's blocked, which you'll need an engineer to resolve.

Otherwise, it could be an issue with your supplier. Check with neighbours to see if they're getting water.

Water is too hot

Before looking into this issue, make sure the temperature of the water in the cylinder has been turned down.

It's likely that there could be an issue with the temperature pressure relief valve not shutting off when it reaches the set temperature. This fault can be particularly dangerous because steaming or boiling water could start coming out of your taps; a heating engineer will need to be contacted to repair or replace the valve.

You will also need a heating engineer if there's any sign of boiling from within the cylinder.

Water is discoloured or smelly

The best way to check if the smell is coming from your hot water cylinder is to run a cold tap and see if it has an odour. If not, then the issue is coming from the warm water in the cylinder. Smelly or discoloured cold water will mean that the issue lies with the mains water supply.

Hot water that's discoloured or has a dodgy smell can be the result of the cylinder's anode rod (which helps to prevent corrosion) and water with a high concentration of sulphates. To resolve this, the cylinder may need to be flushed, have its anode replaced or be completely replaced.

Flushing the cylinder will help to get rid of the smell-causing sulphates but the smell is likely to remain.

Leaky cylinder or pipes

Blink boiler falling
Even the slightest sign of leak should be dealt with seriously. Don't ignore even a tiny leak as it will only get worse over time. Some possible causes of a leak from the cylinder include:

  • Too much pressure
  • Loose connections
  • Cracks in the tank
  • Corrosion (most likely for old cylinders)

At the very least, you'll need a heating engineer to repair the fault. If it can't be repaired or your cylinder is so old that it isn't worth repairing then a replacement will be needed.

Strange noises

A hot water cylinder will make some form of noise as it works to heat the hot water. However, if you notice popping, screeching or sizzling sounds then there could be a problem.

A popping noise is fairly common noise and is the result of debris building up at the bottom of the tank. This build-up traps water which then makes a popping noise as it escapes when the water heats up. If you live in a hard water area then your cylinder will be more susceptible. To resolve this issue, a fully-qualified heating engineer will flush out the debris and this can be done annually.

Screeching can be one of the more worrying sounds a cylinder can make. Fortunately, the fix is relatively straightforward. It's likely to be the case that the inlet control valve isn't fully open, which is preventing water from making its way through. So, it needs to be opened up.

Finally, cold water leaking onto a hot burner will make a sizzling sound. To fix this one, you'll need a heating engineer to repair the leaking area.

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How to prevent hot water cylinder problems

The best way to prevent hot water cylinder problems is to arrange an annual service by a suitably qualified heating engineer. Having your cylinder serviced once a year will keep it in peak condition for longer.

After a successful cylinder service, you can be confident that your system is running reliably, efficiently and safely.

During a hot water cylinder service, the engineer will inspect the cylinder and take appropriate action where needed. Amongst other things, you can expect them to:

  • Test the expansion relief valve
  • Check the temperature
  • Look for signs of leaking
  • Top up pressure (if needed)
  • Inspect the electrical components

At the end of the service, you'll be handed a service report. You should hold onto this as proof of service, especially if the cylinder is still under warranty.

A hot water cylinder service will take around an hour and typically cost between £60 and £80. It's a good idea to have your boiler serviced at the same time, which can take costs up to £120, but will give you the peace of mind that your entire heating system is reliable and efficient.

Unvented hot water cylinders should be serviced by a G3 qualified engineer.

Is it time to replace your cylinder?

Getting tired of venter or unvented hot water cylinder problems? If you live in a property with 1-2 bathrooms and a fairly moderate demand for hot water then you may be able to remove your hot water cylinder altogether. This is thanks to combi boilers which provide hot water on demand.

By swapping a regular or system boiler for a combi, you'll be investing in a single cost-effective and compact heating system. Combi boilers don't need any additional cylinder or tanks in the loft (necessary for a regular boiler) so you'd be freeing up some space around your home.

Not only that but the boiler would only be heating the water you're using so energy wastage could be reduced. And any reduction in your energy usage is good news for your heating bills.

You can get free quotes for a combi boiler right here on Boiler Guide.

Get quotes for a new hot water cylinder

Looking for a new unvented hot water cylinder? Then you can get free quotes by visiting our sister website unventedcylinder.com.

Take a few moments to complete our simple online form and you'll get free quotes for the installation from up to 3 professional installers. Once you have your quotes, you'll be in a position to compare them and be confident that you're getting the most competitive price possible.

Nick Geary

About the author

Nick Geary

Nick has a wide range of experience writing about conventional and renewable heating solutions, and is always increasing his knowledge by researching any new heating technologies on the horizon.

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