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What is a heat-only boiler an how does one work?

System boilers: everything you need to know

Heat-only boilers are also known as open-vent, regular or conventional boilers. This traditional type of boiler provides central heating and a hot water supply that’s ready when you need it.

A conventional boiler is the best option for larger properties with low water pressure. You’ll need to have room in the loft for a feed and expansion tank and a cold water cistern – regular boilers also require a hot water storage cylinder, which is usually kept in an airing cupboard.

What is a heat-only boiler?

A heat-only boiler provides central heating and hot water, which is kept in a storage cylinder. Unlike other types, these boilers can operate with low water pressure thanks to the feed and expansion tank in the loft – the hot water cylinder also allows them to heat your home and supply multiple bathrooms at the same time.

If the water pressure (or flow rate) of your mains supply is low, then a regular boiler could be the right option for you. Heat-only boilers are also ideal for larger households as they can cope with high demand for hot water from different parts of a home and still offer consistent central heating.

The main point to consider with this traditional style of boiler is the requirement for space in your home: as well as the boiler itself, you’ll need three separate tanks (two in the loft and one in an airing cupboard).

Conventional boilers vs combi boilers

Conventional boilers were the first type to be made available for domestic customers, whilst combi boilers are the most modern kind. The main differences between them are:

Conventional boilersCombi boilers
Store hot water in a storage cylinderHeat water on demand
Require separate feed and expansion, cold water, and hot water tanks All-in-one boiler with no separate tanks
Fed by a tank in the loftTake water directly from the mains supply
Can supply hot water to multiple bathrooms and central heating at the same time May only supply one hot water outlet or central heating at any given time depending on size.
Suitable for homes with low water pressure or flow rate Not suitable for homes with low water pressure or flow rate

Regular boilers vs system boilers

Regular boilers and system boilers are similar in that they both store hot water in a storage cylinder rather than supplying on demand. The table below compares these two type of boiler:

Regular boilersSystem boilers
Store hot water in a storage cylinder Store hot water in a storage cylinder
Require separate feed and expansion, cold water, and hot water tanks Require a separate hot water tank
Fed by a tank in the loft Take water directly from the mains supply
Can supply hot water to multiple bathrooms and central heating at the same time Can supply hot water to multiple bathrooms and central heating at the same time
Suitable for homes with low water pressure or flow rate Not suitable for homes with low water pressure or flow rate

How does a conventional boiler work?

We’ve mentioned the various cisterns and tanks that an open-vent boiler relies on, but how does all of this come together to provide heating and hot water?

A regular boiler works using the following process:

1. Cold water travels from the mains supply to the cold water cistern, which typically sits in the loft next to (or above) the feed and expansion tank.

2. From here, the water passes into the feed and expansion tank.

3. The boiler itself is supplied with water from the feed and expansion tank. This ensures that there is always the right amount of water in the boiler, accounting for the expansion and evaporation of water as it is heated.

4. Once the cold water moves down into the boiler, it passes over a heat exchanger – heat is transferred to the water, raising it to the temperature set via the heating control.

5. This heated water is then pumped around the home via radiators to provide central heating at the desired temperature.

6. In the same way. the water in your cylinder is heated to reach 60-65°C where it sits until it’s used.

7. When you turn on a shower or hot water tap, the water in the cylinder is pushed out by water pressure from the cold water cistern in the loft, this pushes the water through the tap.

With the additional tanks in the loft, there are a few more stages involved in the workings of a conventional boiler by comparison to the other types.

These additional processes enable a heat-only boiler to supply multiple bathrooms and central heating simultaneously, even if the flow rate of the house’s mains water supply is low.

Pros and cons of heat-only boilers

This guide has already touched on some of the pros and cons of conventional boilers. Here, we’ve summarised the key considerations to help you work out if this type of boiler is the right choice based on your circumstances.

What are the advantages of a conventional boiler?

Although conventional boilers are the oldest and most traditional type, they still offer some advantages that make them ideal for certain houses:

  • They can accommodate low water pressure – the main reason to choose a heat-only boiler is if the water pressure or flow rate of the mains supply to your house is low.
  • Back-up hot water supply – the hot water storage cylinder that accompanies a regular boiler can be fitted with an immersion heater to offer a back-up hot water supply to keep you going if your boiler is ever on the blink.
  • Ideal for households with high hot water demand – if you live in a large household where more than one shower, bath, or hot tap is likely to be in use at any one time, then a heat-only boiler is the perfect solution. Unlike combi boilers, this type of boiler can supply multiple hot water outlets and central heating simultaneously.
  • Compatible with old radiators – other boiler types such as system boilers transport water at high pressures, which can cause old-style radiators to leak or malfunction. Conventional boilers were designed to work in older properties, so this won’t be an issue.

Considerations before deciding on a regular boiler

In order to check a regular boiler is right for you, consider the following points:

  • They take up a lot of space – in addition to the boiler itself, you’ll need room for cold water and feed and expansion tanks in the loft, plus a suitable space for a hot water storage tank (usually kept in an airing cupboard near the boiler itself).
  • Water needs heating ahead of time – combi boilers provide hot water on demand. By contrast, you’ll need to set a regular boiler to heat up water before you plan to use it.
  • You can run out of hot water – once the hot water in the storage cylinder is used up, you’ll then need to heat up some more water before you can have another hot shower. This issue can be addressed by installing a larger hot water storage tank.
  • More complicated installation and repair – with so many working parts, a regular boiler takes longer and costs more to install. Equally, if your boiler has a fault, a repair specialist will need to identify which individual part of the system is the issue, making repairs more complicated and potentially more expensive.

How do I know if I need a regular boiler due to low water pressure?

The main reason that people tend to choose a regular boiler is that their water pressure is too low to support a system or combi boiler. Follow these simple steps to work out whether this applies to your home:

1. Grab a timer and a large measuring jug

2. Place the jug underneath your shower or tap and turn it on full blast for six seconds.

3. After six seconds, turn the shower or tap off.

4. Check the amount of water that you’ve collected in litres – multiply this number by 10 to work out the flow rate in litres per minute.

If you find that your water pressure or flow rate is less than 10 litres per minute, then a regular boiler would be the most suitable option for your home.

What size heat-only boiler do I need?

Generally speaking, the more bathrooms, hot water outlets, and radiators your property has, the larger your heat-only boiler will need to be to cope with the demand.

The table below summarises the ideal size of heat-only boiler for different properties:

Number of radiatorsHeat-only boiler size (kW)

You may want to consider a larger conventional boiler if your home has a small number of radiators but several bathrooms to supply. For example, if you have 12 radiators and 3 bathrooms that will require hot water at the same time, a 25kW open-vent boiler might be more suitable than an 18kW one.

How much is a regular boiler?

A regular boiler on its own can cost anywhere up to £2000 depending on its size.

When you receive a quote for a new heat-only boiler, this will typically include the cost of buying and installing the appliance. The total price will depend on factors like what type of boiler you’re replacing and whether you need a new cold water cistern, feed and expansion tank, or hot water storage cylinder.

Below, we’ve provided some rough price estimates for the full cost of installing a new regular boiler based on different scenarios:

  • Replacing an existing heat-only boiler with a new one, where no new tanks or cisterns are required is one of the cheaper installation options.
  • Replacing a regular boiler with a new one, in addition to a new hot water storage tank may cost slightly more due to the additional labour and parts required.
  • Replacing a combi boiler with a conventional boiler plus all three tanks can cost up to £5500 and is one of the more expensive regular boiler installations.

The actual price you pay to install a new heat-only boiler can also be affected by how difficult it is to remove your old boiler and where you live. Boiler installation specialists in London and areas in the south of the UK often charge more than those elsewhere.

How long should a conventional boiler last?

You would usually expect a conventional boiler to last for around 10 to 15 years after it has been installed. Certain parts of the system may need replacing before this point, but you may also find that other components will last for much longer.

Top tips for increasing the lifespan of an open-vent boiler include:

  • Get an annual boiler service to ensure everything is in good working order.
  • Contact a boiler repair specialist if you notice your heating or hot water supply is not working as expected.

High-quality regular boilers from Glow-worm

Glow-worm manufactures a complete range of modern gas boilers that provide reliable, consistent central heating and hot water. We make a range of different heat-only boilers – take a look at our regular boilers page for more information

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Interested to learn more about the other types of boiler? You may want to take a look at these posts: