Draining sealed combi system

The filling loop should be 'off' anyway; the system shouldn't be open to the water main.

It really isn't a problem to drain and refill - not sure what potential issues you're worried about?

David

Thats the problem really, all the books & web sites I've looked at just say "its a job for an expert" and don't say why or give any other info.

Chances are you would not need to drain down unless it is necessary to alter pipework or solder joints. Release the pressure and everything usually holds back on a vacuum. It normally only takes seconds to change the valve.

Draining down a sealed system is very little different from a gravity system. Hose on lowest drain c*ck and open up rad vents to get flow going.

If you can do a conventional vented system, then a sealed one is even easier. You only need to drain down as far as the height you are working

- so upstairs rads require less draining than downstairs. You may need to open a bleed valve on a high radiator to allow air into the system and water out. No need to worry about tying up ballcocks etc.

Lob some inhibitor in an empty rad before refilling (via the blanking plug at the other end of the bleeding valve is easy). Fill as per Ed's sealed system FAQ.

If (a very big if) the isolating valves on the boiler work well enough to isolate the pipework, you could get away with leaving the boiler pressurised. There then should only be small amount of water to remove from the pipes to reduce the pressure to atmospheric, and a quick change with lots of rags to soak up any leakage, or as above, drain the minimum to allow a more leisurely change.

Are you sure that you are not mixing a sealed system up with an unvented system? Everyone says to leave the unvented to the pro for some reason.

If it is just a normal combi set up there is no differece between what you are used to doing on a conventional system and what you will have to do on a combi setup.

Adam

There is a FAQ for this. See below.

Are they not one & the same? Whats the difference?

A combi system is by nature unvented surely?

There could be some confusion if one refers to an unvented hot water system like a megaflow that stores mains pressure hot water. This is distinct from a boiler that has an unvented primary circuit (i.e. a sealed system).

Not necessarily. You can get combis that at least have the option to run the radiators with a conventional header tank.