Carbon Dioxide has no smell
The boiler is designed and built to protect people in the house from any escape of dangerous gasses - the fan extracts the products of combustion from the room sealed boiler and the Flue directs them outside. If the case seal is defective so that gas might escape into the house a pressure switch will prevent the boiler from lighting - similarly if the fan is not operating properly.
The big danger from gas appliances is not Carbon Dioxide but Carbon Monoxide resulting from incomplete combustion - colourless, odourless, tasteless and deadly poisonous in time.
http://en.wikipedia.org/wiki/Carbon_monoxide_poisoning#Epidemiology
Whatever you are smelling in the Garden it will not be either CO or CO2.
Your indoor detector would be better and safer for you if it were a Carbon Monoxide device rather than one for Carbon Dioxide.
If you are still concerned about the safety of your boiler have it serviced annually - this will mainly be a safety check. The engineer should be able to reassure you about what you're smelling outside.
Peccavi
May 2009
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