In a typical open-vented system, with the boiler downstairs and the F&E tank in the loft, the boiler pressure will naturally be about 0.5bar. Why, in a sealed system, is it deemed necessary to set the cold pressure higher, typically at about 1.5bar?
Why is a sealed HW/CH system pressurised?
- Thread starter Alec_t
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In an open vented system the pressure at the bottom will depend on how far above the boiler the F & E tank is (10m = 1 bar).
AFAIK running at higher pressure reduces the release of dissolved gas from the primary water (and elevates the boiling point)
AFAIK running at higher pressure reduces the release of dissolved gas from the primary water (and elevates the boiling point)
1.5 bar is not typical.
1 bar is perfectly adequate for the vast majority of homes including those with loft conversions. Some boilers have cheap pressure switches that drift upwards and the inclination is to over presurise to get them working rather than fix the problem.
If the boiler (and expansion vessel) was in a basement and you had several floors above then you would increase the vessel pre-charge and cold fill charge accordingly.
You need sufficient pressure to bleed the air from the highest pipework/rads etc.
And there is a benefit to raising the boiling point..a reduction in noise from kettling etc and less chance of air ingress assuming it's plumbed correctly.
1 bar is perfectly adequate for the vast majority of homes including those with loft conversions. Some boilers have cheap pressure switches that drift upwards and the inclination is to over presurise to get them working rather than fix the problem.
If the boiler (and expansion vessel) was in a basement and you had several floors above then you would increase the vessel pre-charge and cold fill charge accordingly.
You need sufficient pressure to bleed the air from the highest pipework/rads etc.
And there is a benefit to raising the boiling point..a reduction in noise from kettling etc and less chance of air ingress assuming it's plumbed correctly.
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Does it add safety ? If due to failed temperature controls the water reaches 111°C ( boiling point at 1.5 bar pressure ) then when the water is released to atmosphere by a safety device the water will immediate turn into steam. One litre of super heated water produces 1.6 cubic metres of water vapour
At 3.5 bar ( pressure relief valve setting ) the temperature could be as high as 140°C though the over temperature safety device should have operated long before this temperature is reached.
Hence the need for regular servicing of pressurised systems by a properly trained person who is also competent and conscientious about the work (s)he does.
A common misconception I've often read about unvented systems is that if you were to release the pressure from a system operating at 3.5 bar and with a temperature of 140C, that all of the water would instantly turn into steam. This is not true. It takes a lot of energy to change 100C water into 100C steam. A small amount would change into steam and the energy required to do that would drop the temperature of the rest to 100C. When you see one these devices popping off, for example because the expansion vessel has failed, you mostly get hot water and a little bit of steam.
Having said that, the higher the pressure in the system, the more dangerous it becomes. Sealed boilers are typically 3 bar, but unvented cylinders can be at 7 or even 10. At 10 bar water temperature can get as high as 180C.
Having said that, the higher the pressure in the system, the more dangerous it becomes. Sealed boilers are typically 3 bar, but unvented cylinders can be at 7 or even 10. At 10 bar water temperature can get as high as 180C.
Google BLEVE ( 0r Boiling Liquid Expansion Vapor Explosion )
. If a closed container of superheated water ruptures, the superheated water changes state: it flashes suddenly from water to steam. This state change of water from liquid to vapor (steam) releases a tremendous amount of energy.
Definition of superheated water & latent energy: Water is considered superheated when its temperature is above 212°F (100°C).
Superheated water contains latent energy that will be released if the pressure is suddenly released. It is this latent energy from superheated water that causes a Boiling Liquid Expansion Vapor Explosion or BLEVE).
The change of state from water to steam requires a lot of energy. In fact it takes as much energy to convert water at steam both at 100C at it does to heat water to from 100C to just over 600C. This energy has to come from somewhere. If you release the pressure on 101C water, around .2% percent of it will convert to steam.
The super heated water is rapidly cooled by evaporation when it changes to water vapour. That cooling releases latent energy from the water. The amount of energy released is roughly equal to the amount of energy required to raise the temperature of the liquid water to a super heated temperature.
I realize this statement is not correct, as it is not an overheat situation. In an overheat situation, which I don't think I've ever seen, you should see mostly steam, even though it should be mostly water by mass. It would be more steam by volume, but more water by mass.
You get enough pressure for that with only 0.5bar or so in a vented system. I'm still unclear why sealed sytems require pressurising above that level, or the boiler refuses to operate unless above some minimum pressure. So far the anti-kettling advantage seems the best explanation, yet kettling is not usually a problem in a well set-up vented system.
But your F&E cistern is above the system so you can always bleed the rads, if your expansion vessel is in the basement how much pressure do you think you'd need if you had a towel rail in a loft conversion several floors up?
Due to the density difference an air bubble at the bottom of the system would naturally rise to the top (unless there were some restriction), regardless of the prevailing water head. Think of bubbles in your glass of beer, or in a kettle, or from a deep-sea diver. I don't see why increasing pressure beyond that naturally prevailing head is the norm (involving the filling loop). Prevailing pressure should suffice providing system pressure exceeds the pump pressure head to prevent cavitation or air being drawn in.
Edit: I've just realised that the pump input pressure needs to be kept up, to near atmospheric, otherwise the water boiling point could be reduced excessively, which would be undesirable.
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