# Technical Pressure Question

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#### cantaloup63

Following on from a couple of threads I've read lately about boiler pressure. Most gravity fed systems would have operated quite successfully at less that 1 bar pressure based on FE tank height above the lowest rad being well under 10m.

So why do so many recommend that a closed system be pressurised up to 1 to 1.5 bar cold?

#### John506

More water, more velocity, more heat?

My opinion, don't know exact reason

C

#### cantaloup63

Interesting opening theory which may have some mileage in it.

#### Agile

Note that there are no replies by industry professionals!

Tony

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#### cantaloup63

I estimate my own current house as originally having the FE at a bit under 5m, which would provide 0.5bar. So why do I have to have my combi set at approx 1 bar cold?

#### Dan Robinson

Note that there are no replies by industry professionals!

Tony

Go on then .

C

#### cantaloup63

I could understand an explanation which suggested that water at pressure can hold a greater latent heat due to it's increased relative density, or indeed that 1 to 2 bar is an optimum level for diminishing gas content of water, which this would mean that the pump would be working efficiently and/or less need for removing gases via bleeding is required.

But I'd prefer a pro to confirm any of the above or shoot it all down as nonsense (which it probably is anyway since I made it up as I typed it).

But there's gotta be a reason for the pressure required in a system

G

#### gaswizzard

Because it does thats all.

#### doitall

I fink da answer lies in the soil.

#### simond

Many sealed system boilers rely on a pressure transducer to determine that they have water in them. A boiler starting without water in it is in danger of terminal damage.

The typical cut off for that pressure sensor is 0.5bar. Any less than this, as discussed, would not prove that the system was sealed.

If you were to set the system pressure to 0.6bar, it would work, but the tiniest leak or air vent bleed would stop the boiler. So 1.0bar is a good figure to recommend. Higher than this puts the pipe work under stresses which are unnecessary.

Also, many domestic installations would not be found with a water supply capable of pressuring the system in excess of 1bar.

Using my basic knowledge of physics, I would expect kettling to be reduced on the surfaces of the heat exchanger due to the sealed system pressure raising the boiling point of the water. This would also, I'd imagine, lower pump cavitation.

#### dumbrill

I could understand an explanation which suggested that water at pressure can hold a greater latent heat due to it's increased relative density

No, water is incompressible so there is no change in 'relative density' and it would be Specific Heat Capacity, not Latent Heat.

#### dumbrill

So why do so many recommend that a closed system be pressurised up to 1 to 1.5 bar cold?

A sealed system eliminates the feed and header tank and so the risk of pumping over or air being drawn down an incorrectly plumbed vent. The arrangement of feed pipe, vent pipe and pump causes a lot of problems and so eliminating the F&V is a good thing.

and

As water is drawn into a pump it enters an area inside the pump where the pressure is lower, just infront of the impellor. As water gets hotter its vapour pressure increases. It's possible in the right conditions for the vapour pressure to exceed the pressure inside the pump and then you get cavitation. If the whole system pressure is lifted by 0.5 to 1 bar then the risk of cavitation is eliminated.

The first point is the main reason and then once the header tank goes you need to pressurized the system to keep the radiators full at the top of the system. With lower system pressures cavitation becomes a risk and so the second point becomes relevant.

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#### cantaloup63

Thank you all for your replies

#### Agile

I normally recommend a pressure of 1.5 Bar because this gives added cover to the benefits of a sealed system and will extend the period before repressurisation is needed. Particularly important for tenented property.

1 Bar is adequate for a two story property with the boiler downstairs. This should be increased by 0.3 bar for each additional floor.

One advantage of pressurised boilers is that as kettling is reduced that enables greater heat exchanger surface temperatures to be used which means smaller surface areas and this reduces the manufacturing cost.

Another advantage not mentioned above is that as the system is under pressure, air is unable to enter and oxidation of metal components does not occur. Thats particularly important with steel radiators.

Tony

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Absolute drivel

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