On 13/01/2017 12:45, mechanic wrote:
On Thu, 12 Jan 2017 23:23:12 +0000, John Rumm wrote:

On 12/01/2017 12:04, mechanic wrote:
On Wed, 11 Jan 2017 22:54:38 +0000, John Rumm wrote:

Which is why many modern designs run the case at negative
pressure - so if there is a leak it draws air in, rather than
allows combustion products out.

Can't see any mention of this on the Vaillant or Bosch sites?

On my Vaillant, the fan's induction side is connected to a duct
that stops open ended inside the case. The case is open to the
the outer part of the concentric flue - but the duct is not
attached directly to the inlet. So the fan is actually drawing
its combustion air from inside the case, and that in turn will
pull air in through the balanced flue. I believe this type of
design is fairly common these days.

The balanced flue connects to the outside world? How does that
ensure under-pressure?

The fan sucks air out of the case. The case then draws its from the
flue. The fact that air is flowing via the flue into the case means the
case must be at lower than atmospheric pressure.

Combustion air is being pulled into the pre-mix burner (which is
adjacent to the fan) rather than pushed from a fan at the entry point of
the flue like on older designs.


--
Cheers,

John.

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