System Boilers

What are system boilers and how do they work?

what are system boilers?

This design is generally fitted for central heating systems which are “sealed circuits” where the all the plumbing work connecting to cylinders is “closed”.

The fact that the water flow circuit is sealed, and not open to the surrounding atmosphere, maintains water pressure and so, combined with the fact that most of the important elements already built into the boiler, takes away the requirement for an expansion tank & feed system.

However, in some cases where the hot water cylinder is not pressurised they may require a tank for cold water.

This type of hot water & central heating is compatible with using Solar Thermal & Solar Thermodynamic Panels

The operating method is opposite to that of a Combi Boiler in that it makes use of storing water then heating it, rather than using the mains flow and heating the water as it comes into the home.

All new & replacement boilers must be fitted by a “Gas Safe” registered company or engineers, do not consider any offer that comes from an unregistered installer – find out more about certification here: Gas Boiler Installer Certification

If you are not sure about which product suits you best you can take look at different Boiler costs

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  • Great for larger properties (2 or more bathrooms)
  • No drop in water pressure if lots of taps are on at the same time
  • Can be very cost effective to run
  • Easy installation as many components built into the boiler itself
  • They can be used with Solar Thermal Heating Panels
  • Energy Efficient – lower monthly heating bills


  • Installation costs higher than Combi Boilers
  • Need a cupboard for the main hot water cylinder
  • Hot water not “instant”
  • Hot water cylinder needs to be well insulated
  • If your cylinder is small & you use too much water you may need to wait for more to be heated

Applications for a System Boiler

Really advantageous for larger families, this design is comparatively less complex to install due to many of the “workings” such as the pump & safety valves etc. being built-in, meaning less materials, which can also help keep down the cost of maintenance.

However, compared to a combi, they do use a bit more room as you will need to have the hot water tank stored in a cupboard (usually the airing cupboard), but that is offset somewhat by not needing the extra tanks in the loft.

Get the largest storage cylinder that you can fit, so you have a good reserve of hot water, but make sure it is really well insulated and then you can enjoy constant hot water to as many taps or showers as you like without suffering any pressure drop.

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