Viessmann Vitoladens 300-W Installation And Service Instructions Manual page 3

Safety instructions
Extractors
Operating appliances that extract air to the outside
(cooker hoods, extractors, air conditioning units, etc.)
can create negative pressure. If the boiler is operated
at the same time, this can lead to reverse flow of the
flue gas.
Danger
The simultaneous operation of the boiler and
appliances that extract air to the outside can
result in life threatening poisoning due to
reverse flow of the flue gas.
Fit an interlock circuit or take suitable steps to
ensure an adequate supply of combustion air.
(cont.)
Safety instructions
3