"Ed Sirett" wrote in message
...
On Tue, 01 Apr 2008 14:10:31 +0100, David Hansen wrote:

On Tue, 1 Apr 2008 04:52:39 -0700 (PDT) someone who may be tvmo
wrote this:-

Are sealed systems better than open systems?

It depends.

To a certain extent. There are only a few reasons I can think of a

1) The existing system is correctly working and setup correctly and the
new boiler would require additional expenditure to add the sealed
components.

2) The existing system has radiators which are so old the designs can't
take the pressure.

3) The existing system has massive 'period' radiators which have such a
huge volume of primary water that a seal system would be impractical.
In this case I'd recommend that the technology levels are so different
that the new boiler is coupled to the old radiators via a heat exchanger
and athe primary is pressurized whilst the secondary circuit is open
vented.

The problem with sealed system is that the filling loops are left
permanently connected by installers - not supposed to be. Some are a part
of the boiler too. When the expansion vessel deflates, or a leak, the user
just tops it up by turning the tap. This leads to water gushing out of the
discharge pipe, sometime onto public walkways. And the users will do this
for months and years, especially those in rented property. It also
introduces fresh water into the system and accelerates corrosion. Sealed
system expansion vessels fail eventually - usually sooner than later. Sealed
systems are intolerable of minute leaks too. I know of some systems where a
small leak was never found after extensive searching. The pressure
eventually drops and a service call.

A one pipe system with a close fitting lid F&E tank is generally the better
option in reality. Sealed systems are great when installed properly and
don't go wrong. The expansion vessel prevents pump cavitation too and
quietens pump operation too.