Hot Water Cylinders Explained: the differences between vented vs unvented and direct vs indirect

Hot Water Cylinders Explained: the differences between vented vs unvented and direct vs indirect

If you’ve made your way onto this post it’s likely you’ve been researching Hot Water Cylinders because you’re in the market to buy one and want to get your head around how the hot water cylinder works and the different types - i.e. vented, unvented, direct and indirect. Now you’re probably also wondering why there isn’t just one type to make life simple (it would make life a lot easier but sadly… life isn’t easy!). However friends… worry not! For I have picked the brains of our resident plumbing and heating specialists Ally and Paul to get the gist on what the most common hot water heaters are, how they work and why there’s so many different ones on offer.


In the UK there’s generally 2 main ways in which cold water is fed into your hot water cylinder and this determines whether the cylinder is classified as vented or unvented. It’s pretty straightforward once you get your head around the terminology, but for argument's sake I have listed the 2 most common ways below.


This is common in UK properties, especially older homes, you’ll usually find a big fresh water feeder tank up high in the loft that uses the power of gravity to distribute water throughout the property.

  • Advantages of a vented cylinder: Fairly simple to set up and they generally require less maintenance. Another added bonus is that if the mains fail you’ll still have water in your property (provided the storage tank in the loft was full beforehand).
  • Disadvantages of a vented cylinder: Because they rely solely on gravity to dictate the water pressure, the height at which you can fit your feeder tank will dictate the pressure of the water flow in the property, also you need to factor in the space that the feeder tank will take up in your loft, especially if loft space is limited. Other downsides include the off chance that the feeder tank could freeze in winter (if your loft space is liable to be susceptible to freezing conditions then you might want to factor this in).

Diagram of a vented hot water system


The water supply is fed directly into the cylinder from the mains so the water pressure is dictated by the water pressure from the mains supply as opposed to gravity fed from a water tank in the loft.

  • Advantages of an unvented cylinder: Hot and cold water are even pressures, showers are easier to balance, you don’t have to worry about the effects of a gravity fed system causing slow speed of water flow and you don’t have to factor in accommodating a big and bulky feeder tank in your loft space. Another bonus is that as it isn’t reliant on a feeder tank in the loft you can place this cylinder virtually anywhere that is convenient to you (it’s a preferred choice especially if you have limited options of where it can be placed, no loft space etc).
  • Disadvantages of an unvented cylinder: You may find that your pre-existing mixer taps may perform differently if you’ve say moved from a vented cylinder system to an unvented cylinder system (example: the pressure of the water flow has changed and the hot water might not come through as fast as it did before). This sort of thing can sometimes be rectified with a new mixer tap suited to your new hot water system. Check with a plumbing or heating professional to see what might be relevant for you.

Diagram of an unvented hot water system



When we talk about direct vs indirect with regards to hot water cylinders we are actually referring to how the water is heated for distribution around your property. Read on to see the main differences between direct and indirect hot water cylinder types.


This method is fairly straight forward, powered by electricity you heat the water in the cylinder using immersion heating elements inside the cylinder tank (think of it like a giant electric kettle and you’re boiling water to make a giant cup of tea).

  • Advantages of a direct cylinder: The main advantage of this is that as it is run by electricity there is no dependence on a boiler to heat your water. An added bonus is that immersion elements are generally convenient and relatively easy to install and run.
  • Disadvantages of a direct cylinder: Electricity costs can err on the more expensive side when it comes to heating your water especially if you have a large property / lots of people using the hot water (depending on a variety of factors like your provider, tariff, rates etc).

Diagram of a direct hot water cylinder



The main provider for heating the water inside the cylinder comes from an external source ie. oil or gas boiler. This heated water is pumped around the cylinders internal coil (heat exchanger) from the said heatsource. This indirectly heats the water inside the cylinder which comes out of your taps .The coil inside the cylinder acts in the same way as an immersion element would.

  • Advantages of an indirect cylinder: Indirect water heaters tend to be the more efficient option for many homes, even if they require a storage tank. An indirect water heater, if used in conjunction with an economical boiler and well-insulated tank, can be the more cost effective and financially savvy means of supplying your property with hot water.
  • Disadvantages of an indirect cylinder: If heating your hot water is totally reliant on an external source like your boiler and that boiler should fail… you might end up without any hot water while you get that issue sorted. Not such a major issue if you have an electric shower and no need to run a bath, but if you’ve a large household and are frequent users of hot water then you may find this a bit of an issue should something go wrong with your boiler etc.

Diagram of an indirect hot water cylinder


Curious about what water heaters you can get on our site? At Buy Plumbing Online we are proud to offer a wide range of OSO hot water cylinders. Why OSO we hear you ask? Through our experience in the industry we have found that OSO cylinders have several advantages like a clean line appearance with concealed pipework within the top of the cylinder meaning they’re professionally presented units with A-rated options. And a significant stand out feature is that they are readily available, competitively priced and can be delivered quickly in around 2-3 working days!

Needing other parts relating to your hot water system besides the cylinder itself then we’ve got you covered. Head on over to the online store where you’ll find all manner of expansion vessels, immersion elements, heating controls and lots more!

Are you in the market for a new hot water cylinder and aren’t sure on what might be best for your situation? If you find yourself still scratching your head give us a phone on 01862 808040 and chat with our resident plumbing and heating experts directly. Ally and Paul will be able to answer your technical queries and give you free advice relating to all things plumbing and heating, however if you’re keen to see the hot water cylinders we have available online just click here.

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