What is a heat only boiler?

A heat only boiler is also known as a regular or conventional boiler. These are both terms that you may be more familiar with. The system usually has two tanks situated in the loft, a  hot water cylinder  and the boiler.

This type of boiler is better suited to businesses or large homes that have a considerable demand for water and require multiple taps and showers to use hot water at the same time.

How does a heat only boiler work?

As mentioned above, a heat only boiler requires two tanks in the loft and a hot water cylinder to run effectively.

The first tank is the cold water tank. It fills with water from the mains supply and this water is heated in the hot water cylinder to supply your taps and shower heads.

The second tank, sometimes referred to as the feed and expansion tank, will bring in cold water to maintain the water level within the central heating system. When radiators are heated, some water may evaporate or leak, and therefore be lost from the system. The feed and expansion tank ensures that this water is replaced. This tank may also be used for additional storage should your system heat up too much water.

So how does your heat only boiler work from start to finish?

Step 1:  The cold water tank in the loft will fill with water from the mains supply. This water will go down to the hot water tank where it is stored until you turn on a shower or hot water tap.

Step 2:  The feed and expansion tank brings in cold water that feeds down to the boiler. The boiler will ignite and the heat exchanger inside the boiler will warm the water up.

Step 3:  A pump will transport the water to the hot water cylinder where it’s kept until your heating is due to come on. When your heating does come on, the water will circulate around your radiators.

Heat only boilers require an expansion vessel. This is because water expands when it is heated up. This can cause a build up of pressure that may otherwise damage your pipes and pumps. The vessel will control the expansion of water.

There are many benefits to having a heat only boiler. The main benefit is that it can heat large amounts of water for future use and supply hot water to multiple outlets at the same time. This is great for busy households that have a high demand for hot water. Unlike in a combi system, you’ll be able to run multiple appliances and taps at the same time with minimal effect on the water pressure. This is why combi boilers may be better suited to small households and  heat only gas boilers  may be better for large homes. Read on to find out which size heat only boiler you need.  

Do I have an open vent or sealed boiler system?

You can easily check which kind of system your home is currently running.

An open vent system will include a feed and expansion tank, so check your loft to see how many water tanks you have up there. If you have two, your system is open vent. The device that makes your system open vent is a pipe that will run from the hot water cylinder to the feed and expansion tank. It is this pipe that releases any excess pressure caused by water expansion.

A sealed system doesn’t have a feed and expansion tank. Instead, it will have an  expansion vessel  (though this may be situated inside the boiler). If you can’t find your expansion vessel, do not remove the casing of the boiler to check. Only a Gas Safe registered engineer is allowed to do this. Excess water needs a place to go and this is the purpose of the expansion vessel. When the system has cooled back down, the water can be released back into the system.

While an open vent system can automatically bring water into the system via the water mains, a sealed system requires a filling loop which the user can turn on and off when extra water is required. You’ll know if your system requires additional water by the pressure gauge on the front of your boiler. It should read between 1 and 1.5 bar.

The three types of gas boilers -  combi, heat only and  system  - can all work on a sealed system. Just a heat only boiler can run on an open vent system.  

What size heat only boiler do I need?

Now you know how a heat only boiler works and whether you have an open vent or sealed system, but how do you know which size boiler you need? You could take a look at the kilowatts size of your current boiler. It’s likely that a straight swap will be sufficient.

As a general rule, a small or medium home with up to 10 radiators could use a 24-30 kW boiler. A larger home with up to 20 radiators may need a larger 35-42 kW boiler. However, boiler size is dependent on many other factors too. You should also think about the number of people living in your home, the type of building it is (terraced, semi-detached, etc.), how well insulated it is and how old it is. For example, if you live in a terraced house, you may find that it holds heat much better due to being surrounded by other properties either side. This could mean you need a smaller boiler.

For more information on boiler sizes, take a look at our  boiler size guide  to help you decide.