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

Sources of Make-up Air For Bathroom Vent Fans

• 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.
• For safety, also see the relation between building exhaust fans and heating appliance combustion air hazards discussed
  
  at COMBUSTION AIR REQUIREMENTS and
  
  at COMBUSTION AIR for TIGHT BUILDINGS

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.

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.? D.K. 10/19/2013

Reply:

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 is your home? What country & city?

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.

Reader Question - flex duct vs semi rigid duct as far as air resistance

(Oct 22, 2017) AK said:
I've been unable to find any information on flex duct vs semi rigid duct as far as air resistance. It seems like the semi rigid would be easier to work with as far as angles and it supporting itself for the most part, but wouldn't all the ridges would cause a lot of air resistance?

It also seems flex duct would have to be held down somehow, to get it properly tight after running a short distance along the floor of the attic before heading upward, as we've read should be done.

We do not have the tools to attempt using rigid duct, though we know that's best. Suggestions?

Reply: Research on optimuym air duct geometry & length

AK

Round, flex-duct exhaust vents made of flexible stainless steel (or in some cases more-fragile aluminum) can combine economy with forcing the installer to make gentle bends, and are generally self-supporting with a minimum of attachment points and supports needed.

In my OPINION

In general ductwork with a

• smooth, round interior (Şahin 1998) with
• a minimum of sharp bends or elbows and
• kept to as short distance as possible
• vented to the building exterior

will perform best (all other factors being equial) and will have measurably less air-flow resistance than vent ducts that involve right-angle bends or corrugated or spiraled uneven surfaces.

• As the article above on this page notes, in a tight bathroom with inadquate makeup air, when the bathroom door (and windws) are shut the vent system may be ineffective.

Below in the references section I include scholarly research on optimum air duct properties that will pertain for bathroom exhaust vent ducts as well as other HVAC ductwork.

Shown above: NovaFlex flexible stainless steel ducting T/L-SS Triple-Lock non-insulated flexible ductwork cited in the references below.

Bathroom Vent Fan Makep Air Supply & Ventilation Design References

• 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 Contact Fantech:

  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 www.fantech.net; info@fantech.net

  Fantech in Canada 50 Kanalflakt Way, Bouctouche, NB E4S 3M5 Phone: 800.565.3548; 506.743.9500 Fax: 877.747.8116; 506.743.9600 www.fantech.ca; info@fantech.ca

• 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: http://www.nutone.com/PDF/InstallGuides/8663RPins61784.pdf
• ENERGY SAVERS: WHOLE-HOUSE SUPPLY VENTILATION SYSTEMS, [PDF] U.S. Department of Energ, original source: energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
• ENERGY SAVERS: WHOLE-HOUSE EXHAUST VENTILATION SYSTEMS [PDF] U.S. Department of Energy, original source: energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
• ENERGY SAVERS: VENTILATION [PDF] U.S. Department of Energy
• ENERGY SAVERS: NATURAL VENTILATION [PDF] U.S. Department of Energy
• ENERGY SAVERS: ENERGY RECOVERY VENTILATION SYSTEMS [PDF] U.S. Department of Energy, original source:energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
• ENERGY SAVERS: DETECTING AIR LEAKS [PDF] U.S. Department of Energy
• ENERGY SAVERS: AIR SEALING [PDF] U.S. Department of Energy
• HUMIDITY LEVEL TARGET What indoor humidity should we maintain in order to avoid a mold problem?
• Bomberg, Mark, Tomasz Kisilewicz, and Katarzyna Nowak. "Is there an optimum range of airtightness for a building?." Journal of Building Physics 39, no. 5 (2016): 395-421.
• Brinkworth, B. J. "Optimum depth for PV cooling ducts." Solar Energy 80, no. 9 (2006): 1131-1134.
• Karmare, S. V., and A. N. Tikekar. "Experimental investigation of optimum thermohydraulic performance of solar air heaters with metal rib grits roughness." Solar energy 83, no. 1 (2009): 6-13.
  Abstract:
  
  Artificial roughness has been found to enhance the heat transfer from the collector plate to the air in a solar air heater. However, it would result in increase in frictional losses and hence, power required by fan or blower. This paper presents the results of an experimental investigation of thermohydraulic performance of roughened solar air heaters with metal rib grits. T
  
  he range of variation of system and operating parameters is investigated within the limits of, e/Dh: 0.035–0.044, p/e: 15–17.5 and l/s as 1.72, against variation of Reynolds number, Re: 3600–17000. The study shows substantial enhancement in thermal efficiency (10–35%), over solar air heater with smooth collector plate. The thermal efficiency enhancement is also accompanied by a considerable increase in the pumping power requirement due to the increase in the friction factor (80–250%).
  
  The optimum design and operating conditions have been determined on the basis of thermohydraulic considerations. It has been found that, the systems operating in a specified range of Reynolds number show better thermohydraulic performance depending upon the insolation.
  
  A relationship between the system and operating parameters that combine to yield optimum performance has been developed.
• NovaFlex, T/L-SS Triple Lock Stainless Steel Flexible Ducting,Novaflex Industries 1024 Industrial Dr West Berlin, NJ 08091 tel 856-768-2275 fax 856-768-2385 800-225-0215 sales@novaflex.com retrieved 2019/04/22, original source: http://www.novaflex.com/productcart/pc/T-L-SS-Triple-Lock-Stainless-Steel-61p564.htm
  
  The company has multiple offices in the U.S, Canada, and in the U.K. at Novaflex Limited 18 Candy Park, 2 Power Rd Bromborough, Wirral CH62 3QT tel 44-(0)151-334-0873 fax 44-(0)151-334-7145 sales@novaflex.co.uk
• Şahin, A. Z. "A second law comparison for optimum shape of duct subjected to constant wall temperature and laminar flow." Heat and Mass Transfer 33, no. 5-6 (1998): 425-430.
• Şahin, Ahmet Z. "Irreversibilities in various duct geometries with constant wall heat flux and laminar flow." Energy 23, no. 6 (1998): 465-473.
  Abstract:
  
  A second-law comparison of irreversibilities is used to determine the optimum duct geometry which minimizes losses for a range of laminar flows and constant heat flux.
  
  The duct geometries used are circular, square, equilaterally triangular, rectangular with an aspect ratio of 1/2, and sinusoidal with an aspect ratio of .
  
  Hydraulic diameters are used for the different geometries.
  
  The circular geometry is best, especially when the frictional contributions of entropy generation become important. Triangular and rectangular duct geometries are inferior choices for both entropy generation and pumping power.
• Telejko, Marek, and Dorota Koruba. "Microclimate in Bathrooms of Multi-Family Buildings." In IOP Conference Series: Materials Science and Engineering, vol. 471, no. 9, p. 092035. IOP Publishing, 2019.
  Abstract
  
  A large number of single-family and multi-family buildings, managed accommodation facilities and institutional buildings both in Poland and throughout Europe are equipped with a natural ventilation system. The operation of this system depends, among other things, on weather conditions, and the system itself is prone to design and operational errors.
  
  The analyses of the functional programme of typical flats in multi-family residential buildings as well as of single-family houses indicate that in certain rooms, such as kitchens and bathrooms, the values of the basic microclimate parameters can differ significantly from the parameters' values of the remaining parts of the flat or building. Kitchens and bathrooms particularly require appropriate ventilation. In their day-to-day operation, these rooms are exposed to a more elevated level of relative humidity than other rooms in the flat or building.
  
  The paper presents the results of the study conducted in autumn and winter in five flats located on different floors in three multi-family buildings in Poland. All the flats under study were fitted with a natural ventilation system with exhaust ducts in their kitchens and bathrooms.
  
  The outdoor air was supplied through window vents installed in the window frames in the selected rooms. Special attention was given to the variability in the levels of relative humidity in these rooms. However, the variability in the indoor air temperature as well as the concentration of carbon dioxide were also considered in the analysis.
  
  Only buildings with a natural ventilation system and windowless bathrooms were selected for the study. The results of the analysis showed an elevated level of relative humidity persisting in the rooms in focus throughout the entire period of the study.
  
  This salient fact should be noted, since the elevated relative humidity levels combined with the relatively high temperature of the indoor air kept in such rooms in winter are conducive to the development of moisture of the partitions, and in extreme cases, to the development of mould fungi on their surface.
• Walker, Iain S. Best practices guide for residential HVAC Retrofits. No. LBNL-53592. Lawrence Berkeley National Lab.(LBNL), Berkeley, CA (United States), 2003.

...

Continue reading at BATHROOM VENT DUCT LENGTHS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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