Pressure, conventional v system

[rhinosaw]

Going from vented to a pressurised system.
One installer told me that you can keep the pressure lower on a conventional boiler (with pressure vessel and pump in the airing cupboard) compared with a system boiler.
Is that correct and if so why?
Thanks

 

[Mottie]

I would imagine it's because there’s no pressure valve that prevents the system from running if the pressure is too low (or too high).

 

[bernardgreen]

Some system boilers will shut down if the pressure is too low for them.

The reason they have to have pressure is to prove there is water in the system before they allow themselves to fire up.

A heat only boiler with feed and expansion tank ( what used to be called a conventional system ) will be happy with a low pressure ( 0.5 bar ).

 

[rhinosaw]

Thanks Bernard. So a heat only boiler (conventional) with separate E . Vessel will not shut down with low pressure? I'm talking probably about a W Bosch.

 

[bernardgreen]

I cannot answer that specific question re that boiler. You would have to ask the manufacturer.

I am led to believe that most boilers working into a sealed system ( expansion tank ) will require a minimum pressure to "prove" there is water and that the system has not drained due to a leak.

Personally if you are concerned about loss of heating then I would stay with your vented system as that will keep water in the system when there is a small leak. ( obviously the leak could cause some damage and needs to be fixed ). A small leak in a pressurized system will almost certainly cause the boiler to eventually shut down ( or not fire up ) until someone re-pressurises the system using the filling loop.

 

[rhinosaw]

Thanks Bernard, I would like to keep to a vented system but the f& e tank is level with the boiler (property is a flat), looks like it has to be pressurised

 

[bernardgreen]

f& e tank is level with the boiler (property is a flat), looks like it has to be pressurised

How does the existing vented system work ? I accept that in a flat a pressurised system can be the better option but the decision vented or unvented is frequently biased towards un-vented because the installers cannot cope with the design of an ultra low head system.

 

[rhinosaw]

It does work... Just, Bernard, installed like that when flats were built 40 years ago.
Hot water ok but struggles a bit when heating is on, then can get some pump over

 

[bernardgreen]

40 years ago there were good installers. One novel solution was to put the F&E tanks in the communal stair well one floor up.

Make sure your installer checks the incoming mains pressure at your flat is higher than the minimum pressure that the boiler requires in the heating system.

One "disaster" was a low rise block of new build apartments circa 1980 where the mains water pressure at the top floors was just above the minimum legal supply pressure but too low to re-pressurise the heating systems. Fortunately the need to re-pressurise did not arise often.