Risk of leakage when changing open-vented to sealed-system boiler

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Apologies in advance for the long post.

I currently have a gravity-fed, fully-vented central-heating (CH) and hot-water (HW) system (large cold-water tank and expansion tank in the loft, hot-water tank and pump in airing cupboard, gas boiler in the kitchen), and I need to replace the boiler.

I'm trying to decide between a boiler that would keep the system exactly as it is (open-vented) or switch to a sealed-system (e.g., see http://www.gasapplianceguide.co.uk/central_heating_systems.htm), which means the expansion tank and pump will be cut out as the boiler itself provides the necessary functionality.

The sealed-system boiler was recommended by the plumber, but I'm trying to understand a bit more in order to choose correctly. A specific advantage of the sealed system that the plumber mentioned was that the pump, which can be a bit noisy, would be removed from the airing cupboard, which is closer to bedrooms. However, he also mentioned that a sealed system...

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What is a sealed system?

A heating system that is closed to the atmosphere and does not have a feed and expansion tank.

How does a sealed system work?

Sealed systems will use a

pressurisation unit

to provide automatic replacement of water losses and ensure minimum head requirements are maintained. A dedicated

expansion vessel

will also need to be installed to deal with the effects of heating fluid expansion and contraction that will occur as the heating fluid temperature changes across the entire operating temperature range of the sealed heating system.

Both units need to be accurately sized for the system it is being used on (please see

Hamworthy brochure

for information on Pressurisation unit sizing and expansion vessel sizing).

What are the benefits of a sealed system?

At Hamworthy we recommend the use of a sealed system. Sealed systems take up less space, assist with system cleanliness, improve water quality...

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