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Sources of makeup air for a bath vent fan (C) Daniel FriedmanBath Vent Fan Makeup Air Supply
Make-up air is needed for fan effectiveness or for building heating equipment safety in tight buildings

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Bathroom exhaust fan make-up air requirements for tight buildings:

This article explains why we need to provide make-up air in to bathrooms to permit the exhaust fan to operate effectively and safely. We include safety warnings about possible back-drafting heating equipment in tight buildings where exhaust fans are used.

Bath vent fan installation, troubleshooting, repair: this article series explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan and vent-duct installation details.



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Provide Bath Vent Fan Makeup Air - venting tight bathrooms

Sources of makeup air for a bath vent fan (C) Daniel FriedmanFor the bath fan to be effective, and considering that usually the fan is running while the bathroom door is closed, provide an opening to allow makeup air to enter the bathroom from an adjacent building area. Makeup air might be supplied by any of the following:

Watch out: Combustion air interference: The possible safety hazards caused by any building vent fan and combustion air for heating appliances are discussed in detail at COMBUSTION AIR REQUIREMENTS and at COMBUSTION AIR for TIGHT BUILDINGS. Fan manufacturers such as Fantech and Nutone all recommend similarly (quoting Fantech):

The combustion airflow needed for safe operation of fuel burning equipment may be affected by this unit’s operation. Follow the heating equipment manufacturer’s guidelines and safety standards such as those published by the National Fire Protection association (NFPA), the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) and the local code authorities.

Question: effect of draught-proofing the whole perimeter of a bathroom door on the bath fan operation?

What will be the effect of draught-proofing the whole perimeter of a bathroom door on the fans operation? - Rob Findlay

Reply:

If the bathroom is air-tight the vent fan will have trouble exhausting moisture during hot showers etc. unless a source of make-up air is provided.

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