On Tue, 09 Mar 2004 22:53:33 +0000, Jim Hatfield local@localhost
wrote:

On Tue, 9 Mar 2004 21:23:02 -0000, "IMM" wrote:

Firstly, a combi is a "combination" of the heating and water system in one
case, eliminating external tanks and cylinders, and generally supply hot
water at high main pressure.

Does this mean that all the radiators are at mains pressure? If so,
does changing from a conventional system to a combi mean replacing all
the radiators?

Jim

No it doesn't. The radiators form a separate circuit.

People often make the confusion when the word "conventional" is used.

There are actually at least four parameters defining a boiler where
the word "conventional" is used to mean that it doesn't have/do the
particular thing.

Open vented vs. sealed
=====================

This means that the primary heating circuit is topped up and expansion
allowed using a small tank in the loft (open vented) vs. circuit
topped up via a non-return valve from the mains - connected when
required - and expansion done with a closed vessel with diaphragm.

non-combi vs. combi
==================

whether or not the boiler (within its case) heats the domestic hot
water directly or not - description by IMM.

non condensing vs condensing
===========================

Whether or not the boiler has an internal arrangement to recover the
latent heat of condensation from the exhaust gases and thence achieve
a higher efficiency.

non system vs system
===================

whether or not the boiler has the pump and possibly controls and
expansion vessel inside the case.


There are typical correlations. Combi boilers more often than not
require to run on sealed radiator circuits. They quite often have the
expansion vessel and pump inside as well.

So the real question is more one of "does a sealed system imply a
radiator change?" and the answer to that is no, as long as they are in
good condition. Radiator valves may need to be changed if they are
old and tending to weep.

The pressure used is typically under 2 bar so rather less than mains
pressure and not hugely greater than an open vented system in the
scheme of things. It is possible and may be beneficial to convert an
open system to sealed anyway if the boiler will allow it (read boiler
spec). It eliminates air being drawn in to the system, makes
filling easier and saves a little bit of space.

More details at Ed Sirett's sealed system FAQ







..andy

To email, substitute .nospam with .gl