Mike Lyle wrote:
snip
:: At all costs, as has already been pointed out, airflow in and
:: exhaust flow out mustn't be restricted. So we don't want anything
:: growing actually _on_ the protective grille, which rules out
:: climbers.

This is what worries me the most - people think 'ah well, the heat from the
boiler will kill off any growth near the inlet' - It doesn't...in the summer
months, when the boiler's inactive and the plants are going berserk, this is
when it gets blocked...come Autumn, fire up the boiler and a housefull of
carbon monoxide is the result.