On Thu, 06 May 2010 13:40:47 +0100, Theo Markettos wrote:

A non-sealed-flue[1] gas boiler means there needs to ventilation in the room,
which is provided by air brick(s) (on the opposite wall to the gas voiler).
This tends to produce a howling gale of cold air in winter, especially when
the wind is blowing against the wall. Is there any kind of heat exchanger
that would attempt to recover some of this heat without impeding air flow
too much?

No.

I'm guessing it wouldn't work without forced airflow (which is possible, but
complicates matters)? Or is there some kind of passive (eg convection)
solution?

Not possible. Where are you going to recover the heat from? The cold air
is coming in to replace air being burned in the appliance and going up the
flue as Carbon Dioxide and Water (as steam). There isn't a corresponding
flow of warm air out of the room, as there is with mechanical heat
recovery ventilation units.

The sensible energy-efficient approach is to replace the inefficient and
potentially-unsafe open-flued boiler with a high-efficiency room-sealed
one.

What sort of airflow is required for safe combustion in the average gas
boiler?

Approx 5cm^2 per kW of heat input to the appliance, less 7kW of the
appliance's rating (a certain amount of ventilation is assumed to be
available "adventitiously", i.e. from natural draughts). So for a 14kW
appliance you need 7 * 5 = 35cm^2.

Heating-only boilers (which heat hot water in a cylinder, rather than
instantly as a combi does) tend to be rated 10-15kW.


Thanks
Theo

[1] I don't know the exact term, but it's the type of gas appliance which
operates like an open fire, in that the oxygen comes from the room, rather
than outside, but the combustion gases go up the flue.

Open flue or conventional flue.



--
John Stumbles -- http://yaph.co.uk

Procrastinate now!