Combi cold/static loop pressure.

[Andrew Healey]

Hi, folks.
Please could somebody help with my general query about combi boiler theory/operation?

In short, my question is, "Why do combi boiler heating systems have to have a cold/static pressure (typically around 1bar) in the radiator water circuit?"

I understand that the boiler's internal 'pressure vessel' accommodates the increase in water and pressure when the system is hot. But why must there be a maintained pressure in the loop when the system is cold?

To my mind of thinking, a boiler with zero pressure (when cold) will work satisfactorily when heated, so long as the pressure vessel is sound?

I just don't understand what is achieved with a static/cold pressure?

Is the answer that the gauge pressure provides visual reassurance that the radiator loop is in good health (ie filled with water)?

Thanks for your thoughts.

 

[Agile]

Boilers circulate heat by using hot water. The hot water needs to be pressurised.

Some nuclear reactors use liquid sodium.

Tony

 

[bernardgreen]

Boilers circulate heat by using hot water. The hot water needs to be pressurised.

Why ? Water gets hot without being under pressure.

Could it be to ensure there is water in the system before the boiler fires up. With a sealed system there is no header tank to replace water lost in leaks so a sealed sytem could lose enough water to become unsafe for the boiler to fire up. Requiring the sealed system to be pressurised before the boiler can fire allows for the detection of leaks and thus loss of water by a simple pressure switch.

 

[Roger928]

I thought it was to leave the system easier bled and keep air out.
And of course no header tank required and less problems with pumping over.

 

[picasso]

Boilers circulate heat by using hot water. The hot water needs to be pressurised.

Some nuclear reactors use liquid sodium.

Tony

Meanwhile back on planet earth, boilers have pressure sensors that aren't exactly expensive and cant monitor pressure down to minute levels so a rough "around 1 bar" is recommended.

 

[Andrew Healey]

Thank you all for your thoughts and answers. Much appreciated.