Why are boilers pressurised?

magicmushroom666 · 2 Feb 2021

I'm curious about why boilers are now designed to be pressurised at around 1 bar, and even feature protection to stop them being operated without pressure in the system.

Is it just to raise the boiling point of the water, or is there another reason?

Mottie · 2 Feb 2021

Only on combi and system boilers though. Possibly a safety reason? Having no feed and expansion tank, the pressure let’s the system know there’s water in it maybe? I’ve had my heat only system converted to sealed and there’s no such safety on mine though.

Can a pressure sensor be easily added on a system that’s been converted to a sealed one?

Harry Bloomfield · 2 Feb 2021

The principle is so as to avoid the need for a header tank to provide the open vented pressurisation. No need for the header tank means the system can be simply made more compact, thus they are well suited to flats, where there is no space or height available for header tanks.

They disable below a set water pressure, so as to ensure the system does have adequate water in it.

oldbuffer · 2 Feb 2021

If you think about it, all boilers are pressurised. The traditional "F&E in the loft" will pressurise the ground floor at around 0.5 bar (2 x 2.4 meter storeys).

oilhead · 2 Feb 2021

oldbuffer said:
If you think about it, all boilers are pressurised. The traditional "F&E in the loft" will pressurise the ground floor at around 0.5 bar (2 x 2.4 meter storeys).

Hence the reason why "open" systems were often fitted with Altitude gauges. A circulator (in common parlance, pump) cannot circulate if there is no head of water in the system.

sxturbo · 2 Feb 2021

pressurised systems are supposed to be cleaner aswell as they do not allow air into the system (or at least shouldnt)

though in practice i think the difference is negligable

vulcancontinental · 2 Feb 2021

The boiler is then in a sealed system with no open vent at a higher pressure. A low pressure switch is not compulsory if converting an OV boiler but a second high limit thermostat non-self setting, a pressure gauge, filling loop with double check valve and a pressure relief valve is.

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Why are boilers pressurised?

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