What is a System Boiler? System Boilers Explained

What is a System Boiler? System Boilers Explained

What is a system boiler? Depending on the type of household (and after considering many other factors of course) I often recommend a system boiler to customers and I’m always asked what a system boiler is and how it differs to other boiler types. So, to help I’ve decided to write a series of blog posts explaining the main different types of boiler systems that we have here in the UK.

Different boilers systems are better suited for different situations and it can become confusing choosing the right boiler for your home, especially if you don’t really know what these boilers do or how they work.

It’s all well and good having a boiler system recommended to you by a fully qualified gas safe registered plumber but I always advise that you learn the basics of how the system works too. This helps when discussing your wants and needs with the plumber so that they understand exactly what you need.

Worcester Gas System Diagram

System Boiler Diagram

Questions to Ask Yourself When Choosing Your Boiler System

Some of the questions I usually ask my customers to help them figure out which boiler system is best for them are:

How many people live in the house?

What times of the day is the heating going to be on?

How many showers/bathrooms do you have?

Are they likely to be used at the same time? Or at different times?

Do you have space for an unvented stainless steel cylinder or a copper cylinder?

This blog will be about system boilers, but I will be writing dedicated articles for combi boilers and open vent (also known as conventional or regular boiler systems) in the coming weeks.

I like to think of the boiler system as the ‘brains’ of the house so it’s important to choose the right boiler type for your needs and household.

How Does a System Boiler Actually Work?

System boilers are very similar to conventional boilers in that they both work on the principle of stored hot water but they take up much less space. They are heat only and require a cylinder for storing hot water. The main difference between the two is that the major heating and hot water components have been built into the system boiler itself. So, in terms of actual physical size, a system boiler is bigger than a conventional boiler because it has an internal expansion vessel and a pump inside it. Whereas with a conventional boiler these elements are installed outside of the boiler.

If you choose to install a system boiler you don’t need to install an external pump or cold water storage tanks in the loft. There is also no need for an expansion tank to help maintain the water level of the central heating system. This can help to free up valuable storage space in the loft.

System boilers use an internal pump to direct to flow of hot water to your radiators and hot water cylinder. The advantage here is that you are drawing hot water from a reservoir so you can open multiple taps and the water pressure doesn’t drop. In a combi boiler system, if multiple hot water taps or showers are opened at the same time then there is usually a significant pressure drop. However, once the hot water tank is empty, you will have to wait for the water to be heated in the boiler and then pumped back into the cylinder. This means you can temporarily ‘run out’ of hot water. This is why I ask how likely is it that the showers or bathrooms are going to used at the same time in order to calculate the right size of the unvented stainless steel cylinder or copper cylinder.

Boiler Installation Dartford Kent

Getting ready to install a system boiler in Dartford Kent.

To summarise, I have made a list of the pros and cons of system boilers:

Pros of system boilers:

  • Because the main heating and hot water components are contained within the boiler itself, it is relatively quick and easy to install the boiler.
  • Thanks to the separate hot water cylinder you can use hot water at multiple taps in your home at the same time, without losing pressure. This makes it ideal for larger homes and families where hot water is in high demand so that occupants can use multiple showers and/or bathrooms at the same time (this really helps when the entire family is getting ready at the same time in the mornings!).
  • There is no need for a separate cold water storage tank or an expansion tank in the so this frees up storage space in the loft.
  • System boilers are compatible with solar heating systems giving you the option to be economical and ecological with your central heating system.

Cons of system boilers:

  • They require a separate hot water cylinder to operate, making them less space efficient than combi boilers.
  • Unless the hot water copper cylinder is properly insulated it will lose heat which will add to your running costs. It’s important therefore to have your boiler system installed by a qualified gas safe registered plumber to avoid this issue.
  • They usually cost more than combi boilers so initial installation costs are higher.
  • The volume of hot water you can use at one time is limited to the volume of hot water the cylinder can store. Once the hot water in the cylinder has been used, you will have to wait for the boiler to pump newly heated water into the cylinder before hot water can be used again. This is especially problematic for busy households which have a high demand for instant hot water so it’s important to think about this beforehand.
7, 8, 9 or 10 Year Extended Guarantee on Worcester Bosch Boiler Installations

7, 8, 9 or 10 Year Extended Guarantee on Worcester Bosch Boiler Installations

So, there you have it, what a system boiler is, how it works and some pros and cons of having this type of system installed.

Your decision-making process should be easier now that you know a bit more about how system boilers work. If you need a new boiler installed I would recommend getting quotes from three different independent plumbers (make sure that they’re gas safe registered) and asking them their advice in terms of what you should go for. Answer the questions that I wrote earlier in the blog post and be realistic about what you want and need. Biggest isn’t always best. Maybe a system boiler isn’t for you? Perhaps you need a combi boiler? There are many different factors to consider so it’s not always a straightforward choice.

If you’re still confused and need me to clarify anything feel to free to comment down below and I’ll answer the questions myself personally.

I would like to stress once again that it’s always important to get any gas related advice or installation work done from a fully qualified gas safe registered plumber.

Thanks to our reputation as the top plumbing company in Dartford, Bexley, Bexleyheath, South East London and Kent we have been accredited as Gold Installers by the Worcester Bosch Group. This means that all of our boiler installations are covered by at least an 8 year guarantee with the option to extend to a 10 year guarantee.

If you live in South East London or Kent and need some advice in terms of what boiler system to choose for your home then feel free to get in touch. We’d love to hear from you.

Richard Costello

Richard Costello, from Dartford in Kent, is the owner of MultiPlumb and has been involved in the plumbing industry installing bathrooms and boiler systems for over 15 years. If you have any questions or suggestions for future blog posts feel to get in touch at richard@multiplumb.co.uk.


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