Vented and Unvented Hot Water Cylinders

vented vs. unvented hot water cylinders for your boilerBoth System and Regular boilers rely on cylinders to store hot water, but the type of cylinder you use — vented or unvented — is up to you.

The main difference between them is the way each cylinder is supplied with water.

An unvented hot water cylinder takes its water supply directly from the mains.

Whereas a vented cylinder is connected to a water tank (by a vent pipe, hence the name.)

This water tank is stored in the loft as it relies on gravity to pull the water down into the cylinder and through the pipes of the house.

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Vented Cylinders

In a vented cylinder, a store of water from the mains is kept in a tank in the loft. The natural pull of gravity carries this water, via a vent pipe, down to the hot water cylinder which is usually stashed away in an airing cupboard.

Once in the cylinder the water is heated and either stored for domestic use in your taps, shower or bath, or used to heat your radiators. As water is heated it expands but the presence of the vent pipe and tank in the loft provides an escape route for the excess.


  • A vented cylinder is the simplest and cheapest option to both install and maintain.
  • If, for some reason, your water supply is turned off at the mains you will still have access to a store of water.


  • The main drawback of a vented cylinder is that it relies on gravity. The height of the cold water tank dictates the pressure, i.e. the more height you can create between the tank and your taps/shower the more gravity will drive the flow of water. This can mean that taps upstairs are weaker than downstairs, or that you need to install an additional pump to get things moving.

Unvented Cylinders

In an unvented cylinder there is no need for a cold water tank in the loft. The cylinder is connected directly to the mains so you are essentially cutting out the middleman and the pressure of your water should be stronger.

The water is then heated within the cylinder but, unlike vented cylinders, there isn’t a vent pipe or tank to cope with excess water as it heats and expands. To remedy this unvented cylinders have inbuilt safety features like an expansion unit at the top or an air bubble that is inserted at installation.


  • They don’t need a water tank which means the system takes less space in your home and you can be more flexible about where you install it.
  • Unvented cylinders tend to provide stronger water pressure as they are directly supplied by the mains, but of course this depends on the strength of the mains in the first place.
  • No need for a tank of water that could freeze in winter or be contaminated, it is a completely sealed system.


  • They have more complicated technology so are more expensive to install and maintain.
  • An unvented cylinder relies on a supply of water from the mains, so if this is turned off for any reason you won’t have any hot water.
  • Unvented cylinders aren’t always compatible with modern power showers and mixers.

So What Should I Choose?

When making a decision between a vented or an unvented cylinder you need to consider a few key factors:

  • What existing heating system and mains pressure are you working with?
  • How big is your house?
  • How much will it cost to install and maintain?

What Existing Heating System and Mains Pressure are you Working With?

The older your home the more likely it is that you have a vented hot water cylinder and a Regular heating system. This is important to know as installing an unvented cylinder could create a higher water pressure than your old network of pipes and radiators can handle.

If your mains pressure is low this might not be a problem, but then you need to consider if introducing an unvented cylinder is the best option as it will provide water directly from the mains at the same low pressure. Some people with vented cylinders who have problems with low mains pressure will install an additional pump to boost the flow.

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How Big is your House?

  • Do you have lots of space, preferably in an upstairs space like a loft? If the answer is no then a vented cylinder is probably not for you. You will struggle to store the large cold water tank in a high enough position to deliver the gravity the system needs.
  • How many bathrooms do you have? If you have 2 or more then a vented cylinder is the better option. This is because the cold water tank provides lots of water at the same time with no effect on pressure. An unvented tank relies on the flow of the mains which, if directed to too many outlets at once, will split and weaken. This shouldn’t be an issue in homes with up to 2 bathrooms.

How Much Will it Cost to Install and Maintain?

  • The simpler and cheaper option to install is a vented cylinder. Unvented cylinders require specialist installation and ongoing maintenance to keep them working at optimum levels.
  • However, in terms of convenience the installation of an unvented cylinder is considerably less hassle as it can be placed anywhere in your home and, with no water tank needed, doesn’t take up as much physical space.

Ongoing Costs

Running costs will be affected by your cylinder’s heating method. Vented and unvented cylinders can be heated either directly or indirectly.

Indirect heating system: Most cylinders are indirectly heated. This means that the cylinder is connected to an external boiler by a heat exchanger. The boiler is usually powered by gas or solar which are considerably cheaper than electric.

Direct heating system: Some cylinders rely solely on an internal element like an immersion heater which is powered by electricity. This makes it the more expensive option but for homes with no access to gas it may be more convenient.

There is a potential problem with choosing an indirect heating system. There is a chance that your boiler could fail and leave you with no hot water. You can prepare for this by also having an immersion heater in your cylinder that you can switch on in hot water emergencies. Get quotes for unvented cylinders now.

What if I Don’t Have Space for ANY Hot Water Cylinder?

A Combi boiler is the ideal choice for smaller homes that are short on space and only have one bathroom. A Combi boiler is a single, compact unit that heats water directly from the mains, providing all your heat and hot water as and when you need it. It doesn’t store any water so, as you’d expect, doesn’t require a water tank in the loft or a hot water cylinder and no extra energy is used in keeping a surplus of hot water.

What Size Hot Water Cylinder Do I Need?

It’s important that your cylinder is, on the one hand, big enough to handle the hot water demands of your home, but not so big that it wastes energy heating and storing far more water than you need.

Cylinder capacity is usually measured in litres which, unfortunately, doesn’t mean a lot to most of us when we’re thinking in showers, bathing the dog or washing dishes. As well as the amount of water we use each day, we also need to think about busy periods like everyone’s early morning showers or trying to run the dishwasher, washing machine AND bathe the kids at the same time in the evening.

The simplest way to work out what you need is to base it on the number of bedrooms, bathrooms and showers. The capacity you need will vary slightly depending on whether you are directly or indirectly heating the water; this is because cylinders that are heated by boilers (indirectly) tend to reheat quicker.

The below figures are used by industry professionals as a guideline:

Bedrooms Showers / Bathrooms Indirect (litres) Direct (litres)
1 1 75/120 120/150
2 1 150 180
3 2 180 210
4 2 210/250 250/300
Larger Properties 300+ 300+

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Finally, most cylinders are made from thin copper and you should make sure your cylinder is well insulated to prevent heat loss. Insulated cylinders are available, which are coated with foamed polyurethane, or you can insulate your hot water tanks and pipes using a jacket and foam tubing.

Troubleshooting Common Problems

Is your hot water cylinder misbehaving? Here’s a quick checklist to rule out easy fixes:

No Hot Water

If powered by a gas boiler you need to check that the boiler is firing. If the pilot light is out and won’t reignite you need to contact a Gas Safe registered engineer.

If it is powered by electricity e.g. an immersion heater, you need to check that your power supply is functioning. It may be that the RCD has been tripped by another circuit in the house.

Not Enough Hot Water

It may be that temperatures outside have taken a dive and the system has to work harder to heat up the same amount of water as usual. First port of call is to check the temperature setting on your cylinder. If turning it up doesn’t help then there is a chance the cylinder is simply not big enough to meet demand.

Make sure the cylinder is not leaking as this could be reducing the efficiency of the system and indicates a fault that needs a qualified engineer.

Water is Too Hot

Assuming you’ve tried turning the temperature down already (!) it may be a problem with the temperature pressure relief valve not shutting off when it reaches the set temperature. This could be dangerous especially if steaming or boiling water is coming out of your taps so you will need to contact an engineer to repair or replace the valve. If you hear boiling inside the cylinder, you need to contact a professional engineer as soon as possible.

Discoloured or Smelly Water

If the problem is with both cold and hot water (or just cold), you can rule out the cylinder as the problem will be coming from the mains. But if the problem is only with hot water there are a couple of possibilities. Discoloured water or dodgy smells may be the result of the cylinder’s anode rod and water with a high concentration of sulphates. The cylinder may need to be flushed or the anode rod replaced.

Strange Noises

As the water in the cylinder is heated it will expand which can cause the odd creak or gurgle. Over time this may become more frequent as sediment like limescale builds up and creates blockages. To keep things flowing smoothly it’s best to prevent buildup with regular flushing and servicing by a professional.

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