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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.
Provide Bath Vent Fan Makeup Air - venting tight bathrooms
For 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:
A warm heating air supply register in the bathroom (if your building is heated by warm air)
A supply grille covered opening cut near the bottom of an interior wall between bathroom and adjoining room, or in the bottom of a bathroom door.
A supply grille covered opening cut near the top of an interior wall between bathroom and adjoining room, or in the top of a bathroom door.
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
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.
Reader Question: is it OK to vent a bath vent fan straight-up, vertically out through the roof? Is it ok to vent the bath vent fan through a larger duct size than the fan's outlet diameter?
I am going to install a new bath fan, I am having a new roof put on the house and decided now would be a good time to put the vent on the roof.
My question is I got a vent for 6" ducting, I will need a reducer at the fan end to 4" Would this be a good size duct for the fan.?
Also I an using metal ducting and it's about six feet from the fan to the roof, Should I angle the duct a little or would it be ok to go straight up.?
You've raised several key topics, and your question helps us realize where we need to work on making our text more clear or more complete. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. For example on site I might notice something about your attic and roof structure, ease of routing venting, placement of insulation, and even very basic stuff like - where the heck is your home? Bath ventilation worries may be a bit different in a cold climate than in a warm dry one and different again in a wet humid climate.
That said I offer these comments:
Local Climate Affects Good Bath Vent Fan Designs: freezing vs hot and humid
For freezing climates we don't want to risk ice accumulation in the vent system - ice can collect from freezing condensate that arrives inside the bath vent duct during hot steamy showers;
For hot humid climates we don't want to have condensate accumulation in air conditioning systems and A/C ductwork, but a bath vent run through a hot attic is less likely to raise that same issue.
Bath vent routing vertically up through roof - not my first choice
I prefer to run a bath vent to outdoors via a horizontal line that goes across an attic and out through a gable-end wall or one that vents down and outside through a roof overhang or soffit. The vent run needs to be designed to drain any condensate outside not back into the bathroom ceiling; in a freezing climate I'd insulate such a vent line as well; If we run a bath vent vertically up through a roof we have two risks I'd prefer to avoid:
The vertical run guarantees that any condensation runs back down into the fan (risking damaging the wiring or fan motor) and back into the bath or bath ceiling.
The vertical run also means another roof penetration. I prefer to minimize the number of roof penetrations on any building since every penetration is a potential leak point, more so if the penetration flashing is not installed correctly.
Bath vent diameters & vent duct materials
The vent fan manufacturers installation instructions typically give maximum run lengths and recommended vent diameters for their products; long vent runs and vents that use plastic dryer-type flex-duct (not your case) cut the effectiveness of the fan by adding airflow resistance and thus increase the risk of accumulated moisture too. Metal duct work (your case) is in my opinion always a better installation: smooth interior means better airflow. Metal fan vent ducting also reduces the risk of duct crush or collapse.
I am guessing that for a very short bath vent duct run, going to a larger duct size is fine - it'd make no difference but you're probably not gaining a thing on a short run by using a 6-inch duct to vent a fan that expects to vent through a 4-inch duct.
Bath vent fan capacity
In my experience inspecting and troubleshooting buildings, I've seen many bath vent fans that seemed ineffective. A fan that nobody uses because it's too noisy means a bathroom that is rarely vented adequately (risking mold, smells, even wet insulation). A fan that is under-powered means even if the fan is used it doesn't do anything.
The fan capacity you need depends on the size of the bathroom being vented - usually calculated in cubic feet. That figure is matched against the fan manufacturer's recommendations for fan capacity measured in cubic feet per minute (CFM). The CFM rating of the fan in turn presumes that the vent routing, diameter, length, and number of obstructing turns and bends is within the company's specifications. In the article above we explain how to calculate the required bathroom vent fan capacity. Also, for bathrooms over 100 sq ft, the HVI
recommends a ventilation rate based on the number
and type of fixtures as shown in Table 6-12 - data discussed in more detail at BATHROOM VENTILATION DESIGN
Sorry that these notes are a bit long on arm-waving and short on more specific details, but as we've got no information about your particular installation except what's in your original note, I have to stop here.
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Questions & Answers on bathroom vent fan and fan ducting installation procedures, codes, standards.
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"About the House - Bathroom Vents", Henri deMarne, New England Builder, November 1985
"Bathroom Vent Fan Beats Open Window", James Dulley, Poughkeepsie Journal, 11/4/1987 p. 12D.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: firstname.lastname@example.org
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References for Bathroom Vent Fan Installation
Fantech Installation, Operation, and Maintenance Manual, PB Series Premium Bath Fans. These fan models use a remote fan motor and
are available in 4" duct and 6" duct models. Web search 7/26/11 - original source http://fantech.net/docs-resi/412889-pb-install.pdf
Fantech in the United States
10048 Industrial Blvd.,
Lenexa, KS 66215
Phone: 800.747.1762; 913.752.6000
Fax: 800.487.9915; 913.752.6466
Fantech in Canada
50 Kanalflakt Way,
Bouctouche, NB E4S 3M5
Phone: 800.565.3548; 506.743.9500
Fax: 877.747.8116; 506.743.9600
Nutone Bathroom Exhaust Fan/Light Combination Installation Instructions, Model 8663RP, 8673RP, 8664RP suitable for use
in shower or tub enclosure when used with GFCI protected branch circuit. Suitable for use in insulated ceilings.
Nutone, 4820 Red Bank Road, Cincinnati, Ohio 45227, web search 07/27/2011, original source:
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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"Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
"Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
"Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
"Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
"Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
Fiberglass: Indoor Air Quality Investigations: Health Concerns About Airborne Fiberglass: Fiberglass in Indoor Air from HVAC ducts, and Building Insulation
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
Re-Bath, tub lining products is a bath tub relining manufacturer and distributor located in Tempe, Arizona - see rebath.com
Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or
ISBN 10: 1-933264-01-2,
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