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What is a system boiler?

If you want to find out more about system boilers, you’re in the right place. The many boiler models on the market for anyone looking to install a new central heating system may seem confusing, but we can help you work out which is best for your property.

In this blog, we’ll take a close look at system boilers, exploring the types of property they’re best suited to, their key benefits and taking a look at how they work. We’ll also explain which alternatives would be your best bet if a system boiler isn’t right for you. 

What is a System Boiler?

A system boiler works together with a hot water cylinder to provide central heating and hot water to our homes. The boiler heats water directly from the mains to warm up our radiators, and fills the cylinder with boiling water that’s used for our sinks, showers and more.

To boil the water required to keep us warm and comfortable, system boilers use the mains as a heat source — or sometimes bottled gas or oil. System boilers are best suited to larger homes with several radiators and bathrooms, which always create more demand for hot water.

For a family of four or more, a system boiler could be the most efficient and convenient way to heat your home and your water.

How Does a System Boiler Work?

Let’s take a closer look at how exactly system boilers provide hot water and central heating for our homes.

In a gas-powered system boiler, the process starts with the flame heating up water in an enclosed hot water system. The hot water is pumped around the radiators to give out heat, as well as the hot water cylinder, where it warms up the water inside. 

Electric boilers work in exactly the same way, but heat up the circulated water using an electric heat exchanger instead of a gas flame.

The water in the system is used over and over, being pumped through the coil in the hot water cylinder and the radiator system. The water that comes out of the tap is from the cylinder — it never goes near the boiler itself.

Does a System Boiler Need to Be On for Hot Water?

Neither the radiator system nor the hot water system is in operation 24/7. They turn on and off automatically with thermostats and timers. If there’s no demand for hot water or hot radiators, the system will stop boiling water. If there’s only demand for one of them, a valve will stop water flowing to the other one, so you save energy.

Most households set their system boilers to start warming up the water in the cylinder in the small hours of the morning. By the time the inhabitants wake up and need washes, baths and showers, there’s a cylinder full of piping hot water to draw from. Once the hot water is used up, however, the process of heating the cylinder will have to start all over again, which is why you’ll have to wait if you want more hot water.

System boilers normally use mains water pressure to make hot water flow from the tap. Cold water enters the cylinder at the bottom, and hot water is drawn from the top. If you use water faster than it can re-heat (i.e. if you run a bath or two), the water will go cold and you’ll need to wait for it to reheat before you get more hot water.

What are the Advantages of a System Boiler?

The system boiler has several advantages over combi boilers and heat only boilers. Let’s look at them individually.

System Boilers’ Advantages Over Combi Boilers

Like system boilers, combi boilers use circulating boiling water to both power radiators and heat water, but they don’t use a hot water cylinder. Although the hot tap and shower water is heated up incredibly quickly, there are several reasons a system boiler might suit your home better than a combi:

  • If several people in a household all want hot water at the same time (for example a kitchen sink, a bath and a shower), with a system boiler they’ll all be able to get it without a drop in flow or temperature.

  • Electric immersion heaters can be fitted to the cylinder to heat water. These can be used to take advantage of cheap overnight electricity or electricity from solar panels. They can also act as a backup should the boiler break down or need servicing.

  • If you have greater need for hot water, you can install a bigger cylinder with a system boiler, whereas combi boilers have a limited flow of hot water — and that’s the most you’ll ever get.

System Boilers’ Advantages Over Heat Only Boilers

The heat only boiler (also called a conventional, traditional or regular boiler) is similar to the system boiler in that it stores its hot water in a cylinder. The biggest difference is that it requires another tank to store cold water, which in turn feeds the hot water and acts as an overflow for any excess hot water and to relieve pressure in the system. 

Because it’s gravity-fed, this cold water tank is normally in the loft, and the system is not under pressure. There will often be a second cold water tank up there to maintain the levels in the radiators, too.

Here are some reasons why a system boiler may be a better choice for your home than a heat only boiler:

  • Because there’s no need for loft tanks, system boilers take up much less space. That’s ideal if you have a converted loft or are limited for storage (or, of course, if you don’t have a loft).

  • If you’re installing a new central heating system from scratch, a system boiler is generally cheaper than heat only because there’s less pipework and fewer tanks to install.

When Would a Different Type of Boiler be Better?

Generally speaking, if you have a small household or live on your own, the chances of you needing hot water from two separate taps at the same time are quite low. In such cases, you will probably be fine with a combi boiler.

Another reason would be if you don’t use much hot water — say you have one shower a day and have a dishwasher. It’s probably not going to be worth heating up a whole cylinder of hot water, so again a combi boiler should be enough.

System boilers are enclosed, so work at mains pressure. Heat only boilers deliver hot water using gravity as they are not under pressure. If you don’t have high water pressure where you live, a system set-up might not work very well.

If your home already has a heat only boiler installed (as is the case in most homes built before the 1970s), you might save money by replacing the heat only boiler itself and keeping the rest of the equipment in place.

If you need help deciding which is the best boiler for your household, why not try our quick and easy boiler selector tool? Or for further advice, you’re always welcome to talk to our experts when it comes to picking a boiler that’ll fulfil all your needs.

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