Creating a vacuum on a sealed system?

[tolly]

Hi there, Is it possible to create a vacuum on a sealed system like you can do on a open vented system where you can cap the OV and bung the CF to create a vacuum. I'm guessing it's not possible, but i would like to know the science behind it

sorry for a novice question!

Tolly

 

[dancinplumba]

yes, just drop pressure first

 

[PlumbBobBob]

as dancinplumba said just drop the pressure on the system, then as long as no rads are vented elsewhere on the system you will be fine to work on it

 

[tolly]

thanks for the speedy replys,

So are you saying, On a sealed system...If i connected a hose to a DOC without opening any Rad bleed vents and slowly opened the DOC it would create a vacuum.

I know from doing it on a OV system that a bit of water is discharged before the vacuum takes a hold, Is it about the same amount of water on a sealed system or does it depend on Volume of water and the pressure its under?

 

[tolly]

I only ask as im sure ive drained down sealed systems before without opening any bleed vents, just with a hose on the DOC.

Or am i mistaken?

 

[dave1]

Yeah or prv it although could open up a can of worms with it not seating correctly again after.

 

[PlumbBobBob]

yes just put a hose on doc and let the pressure off

 

[Best]

I only ask as im sure ive drained down sealed systems before without opening any bleed vents, just with a hose on the DOC.

Or am i mistaken?

You couldn't have drained down a sealed system without opening vents, - it would only have taken the pressure off the water in the system.
If you want to create a vacumn in sealed heating, just make sure there are no auto vents anywhere - or close the caps on any & also only slacken one point of system as if you do two connections at once, then one will allow air to enter.

 

[tolly]

thanks for the replys, will have to give it a go!

 

[tamz]

There is usually an auto vent on the boiler.
That is not a problem if the pipes rise off it.

 

[tolly]

thanks tamz, i allready thought of that....i thought i would just turn off the isolation valves underneath the boiler (providing it has them), after turning off boiler and controlls that is.

 

[tamz]

Don't touch the valves. Most will leak when you set them back.
If the pipes rise it will be fine.
If they fall it "might" be fine for a while but don't depend on it.