"Ed Sirett" wrote in message
...
On Wed, 04 Jun 2008 19:48:59 +0100, Heliotrope Smith wrote:

"Rod" wrote in message
...
We currently have an older gas boiler which is open-flue. It is in a
cupboard (all right, a compartment) which has a door.

We wish to replace this door (because it is tatty). But we need to fit
ventilation grilles. I *know* I found calculations for the requirements
somewhere when last I looked. Now I can't. (I just find references to
sites which have the information for a fee such as BSI.)

The boiler is something like 12 to 15 KW. We need to provide air inflow
for combustion plus air in/out for moderating the temperature in the
cupboard. (I *know* we should get a condensing boiler... :-) )

What effective area ventilation grilles are required? Or where can I
find the calaculations?

--
Rod

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A compartment ventilated to a room or internal space requires 10 cm2 per
kilowatt (net) of the boiler maximum rated input at high level. Low
level requires 20 cm2 per kilowatt (net)
You should find the rating information on the plate fixed to the boiler.
Grills should be non closable and not covered in any mesh/flyscreen
etc.

That will be a slight over estimate. Pretty much all appliances are
quoted in gross. So 9 and 18 are the figures. Also the combustion air
supply is required to the room containing the compartment. This is 4.5cm2/
kW (from outside air), but the first 7kW are discounted.

Not an estimate, but the figures stated in the regulations.
I did add in my post that the figures are net so of course divide 1.11 if
input is stated gross to give the required size of vents.
You are a little out of date with combustion air supply sizing. This has
been 5cm2 per kW (-7kW) for some time now.