If you’ve looked online for boiler replacements, you’ve probably noticed that homeowners are often encouraged to choose a combi boiler. But whilst combi boilers are great, they’re not right for everyone. Often a system boiler would be a much better alternative.
System boilers are a perfect solution for larger homes with greater demands for heating and hot water. And homes that are interested in adding renewable technology to their heating system.
In this blog, we look at what a system boiler is, and discuss their major pros and cons. Hopefully this will help you to decide whether a system boiler might be a good option for you.
You might also find our related blogs: ‘Your options with a boiler replacement’ and 6 top tips for buying a new boiler helpful.
What are system boilers?
When it comes to choosing a new boiler, you have three types of boiler to choose from: heat-only boilers; system boilers; and combi boilers.
A heat-only boiler is a system that produces your heating and hot water using two tanks; a hot water cylinder and the boiler itself.
Combi boilers produce both your heating and hot water from one compact unit; without the need for external tanks or cylinders.
A system boiler shares some similarities with both combi and heat-only boilers.
A system boiler is similar to a heat-only boiler in that it stores hot water in a cylinder. But it doesn’t need any additional water tanks; because like a combi, it takes its cold water supply directly from the mains.
Since a system boiler holds some similarities to the other two types of boiler; it’s useful to compare them so that you can see what makes system boilers different:
How are system boilers different from heat-only boilers?
System boilers are sometimes confused with heat-only boilers because they too have a separate hot water cylinder.
The storage of hot water means that both heat-only and system boilers can meet high demands for hot water. And are good choices for larger homes with multiple bathrooms.
But there are some very important differences between a system boiler and a heat-only boiler.
A system boiler contains an expansion vessel and pump allowing the boiler to be installed in a sealed system. They use a pressurised heating circuit, topped up by mains water.
Whereas a heat-only boiler can be used on either an open vented system; which has a tank in the loft, or in a sealed system with the addition of an external pump and expansion vessel.
System boilers take up less space in the home than a heat-only boiler. Because they take their cold water supply directly from the mains. They also have more control options than heat-only boilers.
Read the pros and cons of heat-only boilers here.
How are system boilers different from combi boilers?
A combi or ‘combination’ boiler provides heating and hot water from the boiler itself. And doesn’t need any external tanks or cylinders.
An obvious advantage of a combi boiler over a system boiler is that it requires less space. So combi boilers are a good choice for smaller homes or flats that can’t accommodate a separate hot water cylinder.
A system boiler needs a hot water cylinder. Because although it takes it’s cold water directly from the mains (like a combi); it heats the water and then stores it until it’s needed.
This is different from a combi, which heats the water instantly when a tap is turned on. And therefore doesn’t need to store it.
Since combi boilers heat water on demand; they aren’t well suited to homes that have a high demand for hot water. In other words, where multiple taps or showers are being used at the same time.
System boilers, on the other hand, can store large quantities of hot water for extended periods of time. And can supply it to multiple outlets simultaneously. This makes them the ideal choice for a home with multiple bathrooms.
Read the pros and cons of combi boilers here.
How do system boilers work?
System boilers take their cold water supply directly from the incoming mains supply. The cold water is passed over the boiler’s heat exchanger, which heats up the water.
Once the water is hot, the boiler’s pump sends some of it to the hot water cylinder. It’s kept her until it’s needed to supply hot water to taps, showers, and baths.
The rest of the hot water is pumped to the radiators to warm up the home.
Since there’s a large store of hot water in the cylinder; a system boiler can provide a steady supply; even where families are using multiple taps and showers at the same time.
Pros of system boilers
Here are the main advantages of choosing a system boiler:
Pro: System boilers can meet a high demand for hot water
The main advantage of system boilers is their ability to supply multiple bathrooms and taps with hot water at the same time.
Unlike combi boilers, system boilers store large quantities of hot water. This means that they can meet a high demand for hot water.
This is perfect for larger homes; or for households where family members all want to be using hot water at the same time.
Pro: System boilers require less space than a heat-only boiler
Although a system boiler needs a hot water cylinder, it doesn’t need a cold water tank in the loft. This means that it takes up less space in the home than a heat-only boiler.
So, a system boiler can be a good option where there is high demand for hot water; but there is little or no loft space available.
Pro: System boilers are compatible with renewable energies such as solar power
Another advantage of system boilers over combis; is that they are easier to integrate with renewable heating systems, such as solar power.
If you’ve read any of our previous blogs on the future of gas boilers; climate change; heat pumps; and COP26; you’ll know that increased use of renewable energies is key to the UK meeting its obligations under the Paris Agreement.
And the bonus is that by reducing your carbon footprint, you can save you money on your energy bills too.
Solar power is a renewable energy source which uses free (and environmentally friendly) heat from the sun to heat your water; rather than your boiler burning gas, which you have to pay for.
Solar panels can be used in conjunction with system boilers if the correct cylinder is connected to the system. This can be a cost-effective and environmentally friendly way to heat a home.
A solar water heating system won’t provide 100% of the hot water required throughout the year. But a boiler can make up the difference.
Pro: System boilers can be installed to low temperature, high efficiency heating
Another great thing about system boilers is that they can be installed to low temperature, high efficiency heating.
Low temperature heating (LTH) systems are a more cost effective and environmentally friendly way of running gas boilers. But it doesn’t mean your house is cold!
Low temperature heating is a heating system where the temperature of the water that’s flowing around your heating system is reduced; without compromising your comfort.
In a traditional heating system; the temperature of the water flowing around your radiators is between 70 °C and 85 °C.
With a low temperature heating system; the temperature of the water could be 25 °C to 50 °C; whilst still keeping you nice and warm.
Running your heating system like this is much more efficient. Your boiler has to burn less gas to do its job; which will save money on fuel bills, as well as being better for the environment.
Pro: System boilers can be fitted with an immersion heater
Another bonus of having a system boiler over a combi; is that it can have an electric immersion heater fitted to its hot water cylinder.
An immersion heater can act as a backup and heat your water instead of your boiler. This is handy because it means you won’t be left without hot water if your boiler ever suffers a breakdown.
Cons of system boilers
Let’s look at the main drawbacks of a system boiler:
Con: System boilers take up more space than a combi boiler
Whilst system boilers take up less space in your home than a heat-only boiler; you still need space for a hot water cylinder.
If you don’t have space such as an airing cupboard for a cylinder; but you have a high demand for hot water, you could consider a storage combi boiler.
Con: System boilers can temporarily run out of hot water if your heating controls are not set up properly
Combi boilers never run out of hot water because they heat water instantly on demand. But as we’ve seen, system boilers don’t work like that. They heat water and then store it in a cylinder until it’s needed.
So the amount of hot water that you can use is limited by the capacity of your hot water cylinder.
If lots of hot water is used and the cylinder is completely emptied; you’ll run out of hot water and have to wait for the cylinder to refill. This generally takes between 20 and 30 minutes depending on the model.
But if you choose an expert engineer to install your size and install your boiler, this shouldn’t be an issue.
Your heating engineer will be able to design your heating system and cylinder to satisfy your hot water needs and set appropriate controls. If this is done correctly you shouldn’t run out of hot water.
Con: System boilers will lose heat from their cylinders if not insulated properly
One of the reasons that combi boilers are highly efficient; is because they heat water instantly on demand, and so you only pay for what you use.
A potential pitfall with a system boiler; is that you can waste hot water by having an uninsulated cylinder which allows the heat to escape.
If you’ve read our blog: What is heat loss and why is it important? You’ll know that reducing heat loss is vital in optimising your energy efficiency and saving money.
Fitting a hot water cylinder jacket (which cost as little as £20) will insulate your cylinder and the reduced rate of heat loss will help maintain the temperature of the water for longer.
The Centre for Sustainable Energy say that heat loss from a cylinder can be reduced by over 75%; saving around £40 per year on your energy bills.
Final thoughts…
System boilers are a great choice for larger homes with a high demand for hot water. They come with the added advantage of being compatible with low temperature heating and renewable energy sources.
We hope that you’ve found this quick guide useful.
For tailored advice on which is the best type of boiler for your home; call one of our friendly team today to book your free survey.
