Eco Happy (Solar Expert)

Published February 28, 2024

An open vent boiler, otherwise known as a heat-only or conventional boiler, needs a tank and a hot water cylinder to function. It can supply both heating and hot water to large homes with lots of radiators and multiple bathrooms.

In this guide, we take a look at exactly how these boilers work, their pros and cons, and much more to get you fully up to speed. If you’re considering buying and installing an open vent boiler, our team at Eco Happy is ready to help you make an informed decision!

How Do Open Vent Boilers Work?

An open vent boiler system consists of a cold water storage tank, a hot water cylinder, and the boiler unit itself. Cold water from the mains is stored in the cold water storage tank and then fed into the cylinder.

The water in the cylinder is heated by the boiler and stored, ready to be supplied to faucets and showerheads as needed. Similarly, when the central heating system is turned on, a pump circulates hot water to radiators via the cylinder.

To keep a consistent amount of water in your heating system, a feed and expansion tank (F&E tank) is installed along with the other components.

Open Vent Boiler Pros & Cons

Let’s take a look at the upsides and downsides of open vent boilers to give you a better idea of whether they’re the right choice for you.


Easy operation and maintenance

Conventional boilers are easy to operate, as they have a relatively small number of devices and fittings. This also means they can be easier to maintain as there are fewer complex components to worry about.

Suitable for large homes

Open vent boiler systems are ideal for larger properties with many radiators and more than one bathroom, as they can comfortably supply hot water to multiple outlets simultaneously.

Work with older heating systems

If your property has an older heating system, then a conventional boiler could be a good option, as they work well with older pipes and radiators.

Good for emergencies

As they store hot water in a cylinder, these boilers are useful in times of emergency if the mains water supply is disrupted.


Take up a lot of space

The boiler unit, cylinder, and storage tanks require a lot of space, so they’re not suitable for smaller homes or apartments.

Expensive installation

While the cost of installing this type of boiler can vary depending on the property’s existing heating system, the property size, and the service provider, they tend to be more expensive to install than other types of boilers. The average cost of installation ranges from around £1,700 to £4,000.

Plus, the units themselves are often more expensive than other boiler types. On average, conventional boilers cost £2,000 + VAT, not including installation costs.

Relatively low efficiency

Conventional boilers are prone to heat loss from the F&E tank, which means they generally have lower efficiency ratings than other boiler types such as combi boilers. The ultimate result is that you may end up with higher energy bills than you would with a combi boiler.

Contaminant potential

The F&E tank can accumulate debris fairly quickly, which can pose a contamination risk if the tank isn’t flushed regularly.

Are Open Vent Boilers Better Than Combi And System Boilers?

Combi boilers have a number of advantages compared to conventional boilers. For a start, they’re far more compact, requiring a minimal amount of space in the home. These units can be wall-mounted and are designed to fit inside kitchen cupboards.

They heat water directly from the mains supply and don’t rely on additional tanks to supply hot water, but rather provide it on demand.

However, this also means they struggle to meet the hot water needs of larger homes and aren’t as effective as conventional boilers at supplying multiple outlets simultaneously. Plus, older heating systems usually need to be upgraded to work effectively with combi boilers.

Although they also rely on a separate hot water cylinder, system boilers are more compact than conventional boilers. This is due to the fact that they don’t require an additional cold water tank.

Like conventional boilers, system boilers are a great choice for larger homes with many radiators and multiple bathrooms. The fact they use a cylinder means they can supply hot water to several taps and showers at the same time, with no noticeable drop in water pressure.

Ultimately, the size of your home and its hot water demands will likely be the deciding factor in which type of boiler you choose.

What Size Open Vent Boiler Do I Need?

The size, in terms of power output, that you’ll need from a conventional boiler depends on the number of radiators and bathrooms in your home. Conventional boilers are available with a wide range of central heating (CH) outputs from 12 kW up to 38 kW.

Here are some general guidelines:

  • 12 kW: Up to 10 radiators and one bathroom
  • 15-18 kW: Up to 15 radiators and 1-2 bathrooms
  • 25 kW: Up to 20 radiators and 2-3 bathrooms
  • 30-38 kW: Up to 25 radiators and 3+ bathrooms

Before you make your decision, we recommend consulting a qualified heating engineer who can advise you on the exact boiler size that’s right for your home. The last thing you want is to pay for a new boiler to be installed and then realise that it’s not powerful enough to satisfy your home’s hot water demands.

Compare Open Vent Boiler Installation Cost With Eco Happy

As mentioned, the additional components of a conventional boiler system can make them expensive to install. That’s why you need to compare installation quotes with Eco Happy.

Our online tool allows you to compare quotes from installers in your area in just 30 seconds. This means you can ensure you’re getting the best and most affordable deal on the market.


How often should open vent boilers be serviced?

As with all other boiler types, an open vent boiler should be serviced once a year by a Gas Safe registered engineer.

What are the warning signs that an open vent boiler may need repair?

Some of the signs that your open vent boiler needs repairs are:

  • Leaks
  • Low water pressure
  • Discoloured water
  • Fluctuating water temperature
  • The boiler making strange noises
  • Failure to supply heating and hot water

Do open vent boilers require any special ventilation?

No, aside from an open flue system, open vent boilers don’t require any special type of ventilation.

Final Thoughts

The main benefit of open vent boilers is that they can meet the hot water demands of larger homes with many radiators and multiple bathrooms. The main downside is the space they require and the overall cost of buying and installing one. If you live in a smaller home or an apartment, then a combi boiler is probably a better option.

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