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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing).
Published: July 24, 2019. Updated: January 23, 2024

What is an Open Vent Boiler? | A Detailed Guide

The Open vent boiler – also known as regular or conventional – is a very popular type of boiler in the UK. Parts include a cold water storage tank in the attic, and a hot water cylinder typically in the airing cupboard.

There are slightly different variations of this setup available, but this is the most common. Because of their popularity, manufacturers such as Viessmann continue to provide some of the industry’s best open vent boilers like the Viessmann Vitodens 100-W.

They have strengths and weaknesses, but their effectiveness depends largely on the type of property you live in.

For this article, we’ll break down what Open Vent boilers are, their benefits, and if they have any flaws. By the end, you’ll have the information you’ll need to ensure your next boiler choice will be the right one.

To find out more about what types of boiler are available, take a look at some of our other blogs:

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What is an Open Vent boiler?

Open vent boilers use a storage tank usually placed in the attic to store water ready to be heated. Some setups also have a hot water cylinder, but they’re not necessarily included in every single installation. Open vent boilers supply heating via your radiators and other heat emitters by being pumped through the whole heating circuit.

The storage tank takes its water supply from the mains. If there is a cylinder in your setup, this is where the hot water will be stored and ready to be used. The advantage over combi boilers is that when it’s used, you won’t experience any drop in temperature or pressure. Hot water will be ready and waiting for you.

Open Vent Boilers can be installed without a hot water cylinder, but 90% of Open Vent Boiler installations will have this kind of setup:open vent boiler diagram

The open vent boiler supplies heating via radiators and is a pumped into the heating circuit through radiators and other heat emitters. When combined with a gravity cylinder hot water system, you can achieve the following results:

  • Timed and temperature heating
  • Timed and temperature hot water

As well as this, fitting a boost pump on the hot water side will increase flow rate for multiple applications like your baths and showers.

For those who already have an Open Vent boiler, installations for a replacement can generally be made easier.

How are they different to combi and system boilers?

Although they take up space, being able to supply hot water across several bathrooms makes them ideal for larger households. They’re a good option for larger families that come with a high demand for hot water. Think about this playing out in the morning when everyone’s getting ready for the day and using the taps/showers at the same time.

You’ll need a strong enough water supply that open vent boilers can definitely provide.

Combi Boilers and System Boilers are slightly different to open vent boilers because they use a pressurised system. Open vent boilers can be ideal for larger households, with the feed and expansion tank placed high in the property’s attic. It uses the gravity from this position to supply your hot water, but there are best practices to adhere to, like including a minimum distance of 4 feet between the bottom of the storage tank and the highest radiator that sits below it.

If your heating system is too old, a new boiler replacement is typically the right course of action. Radiators and pipes installed in the 80s and 90s, comes with a higher chance of failure. Rather than throw lots of money at constant repairs, an entire replacement might actually save you the most money in the long run.

Best Open Vent Boiler on the market

Viessmann Vitodens 100-W Open Vent

Viessmann Vitodens 100-W

Viessmann’s entry in the ‘open vent boiler race’ uses high quality components like the modulating MatriX cylinder burner and stainless steel Inox-Radial heat exchanger. With a material density close to surgical steel, this heat exchanger drastically improves the lifespan of the boiler by making it far more resistant to corrosion.

As a wall-mounted condensing boiler, it offers surprisingly compact dimensions, with a depth of only 285mm. Because of this, the Viessmann Vitodens 100-W is a perfectly suitable option for integration into your kitchen cupboard, and is ideal if your space is limited.

  • Excellent corrosion resistance thanks to stainless steel heat exchanger
  • Modulation ratio of 1:3
  • Compatible with solar panel technology
  • Room thermostatic and weather compensated control unit options
  • Depth of only 285mm for installation into standard kitchen wall units
  • Boilers in this range offer outputs of 13kw, 16kw, 19kw, 26kw and 35kw
  • 10 year warranty on the Inox-Radial heat exchanger
  • 5 year warranty as standard (Upgraded to 10 years if you book through Boilerhut*).

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Vaillant ecoTEC plus regular

vaillant ecotec plus regular open vent boiler

The high performance, control and efficiency of this boiler make it another excellent choice for open vent boilers. They’re compact enough to fit in most kitchen cupboards, and also offer a wide range of control options. It also responds well to energy usage; if it doesn’t need as much power, it can drop to 16% so your energy bills are reduced even more.

  • 5 year warranty as standard
  • Dimensions of 375mm x 600mm x 320mm
  • Compatible with solar panel technology
  • ErP A rated for heating efficiency
  • Control options available from simply boiler timer to smartphone app
  • Outputs available range from 12kw to 15kw, 18kw, 24kw, 35kw

Worcester Bosch Greenstar Ri Boiler (9kw to 24kw)

worcester bosch greenstar ri open vent boiler

This compact wall-mounted boiler is ideal for smaller or medium sized homes. These fit comfortably into most kitchen cupboards, making it a good choice for those looking to save on space.

  • Easy-to-use control knob on front of boiler
  • Frost protection to help prevent freezing
  • Compatible with solar panels
  • Suitable for central heating systems that include a hot water cylinder
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What type of property is an Open Vent boiler most suitable for?

If you’re getting a new open vent boiler, you’ll need to consider the property type and size it’s installed in.

A basic guide to remember is that when the house size goes up, so does the demand for DMH (Domestic Hot Water). So if you live in a large property, you’re going to want a more powerful boiler in order to meet your demands.

When talking about larger properties, consider how often the water supply will be needed across multiple outlets. Compared with other boiler choices, an open vent boiler may be your best option.

Hot water is provided instantly when you need it because of the setup’s cold water storage tank and hot water cylinder. They can be used across multiple bathrooms at once.

However, even though Open Vent boilers are more suited for larger properties, there are still other options. Storage Combi boilers are a reliable alternative, capable of producing enough water with their built in storage tank. Depending on your mains water pressure, a storage combi boiler such as the Viessmann 111-W could easily give you about 20 litres per minute.

Many open vent boilers such as the Viessmann Vitodens 100-W are also compatible with solar thermal technology – an excellent way to harness environmentally-friendly energy.

What are the flaws of an open vent boiler?

A non-sealed system used by your open vent boiler will still have a few weaknesses to consider. Air can get sucked into the system which causes a buildup of magnetite sludge, affecting the way your central heating system operates.

Other points of failure to anticipate include faulty zone valves, pumps, old and outdated mechanical controllers, and various other component failures.

What other types of boiler are there?

Combi Boiler

They’re short for combination boiler, and unlike open vent boilers, don’t require any cold water storage tanks or hot water cylinders. The water that comes from the mains supply is heated by the boiler when required by the user.

By only heating water when needed, annual energy bills can be reduced. This makes it a popular choice for homeowners today.

System Boiler

System boilers are a little bit different to an open vent/conventional boiler. They don’t require a cold water storage tank as they get their water supply directly from the mains to heat the system which is then stored in the hot water cylinder.

Installations for a system boiler can be quite straightforward – especially if you’re going from an open vent – because of the lack of a cold water storage tank. This makes system boilers an ideal choice if you have any plans for an attic conversion.

Storage Combi Boiler

In simple terms, a storage combi boiler produces pressurised hot water similar to a system boiler with a hot water cylinder. The cylinder itself is housed inside the boiler unit itself, and by storing the heated water this way whenever it’s needed, comes with some great space-saving benefits.

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Why Viessmann and Boilerhut?

Because we’re a premium Viessmann installer, Boilerhut can offer services that make your life a great deal easier.

We understand that a boiler can break down when you least expect it – you want it sorted out as quickly as possible. As a result, we make ourselves available for contact throughout the week from 9am to 9pm, and 11am to 5pm on the weekends. This way, you won’t be waiting weeks for an installation to begin.

By entering your postcode into the box below, our quoting system can give you a quick online price estimate to determine which boiler is best in minutes. Upon receiving this breakdown, you’ll be able to speak to one of our heating experts who can discuss your options with you further

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