Bathroom exhaust vent fan and kitchen exhaust vent fan duct Q&A

Is it acceptable for a bathroom exhaust fan to share a kitchen exhaust fan duct? Not really.

What about sharing a common exhaust vent fan and duct system that draws air from both kitchen and bath areas.

We explain why such shared ductwork is both prohibited by model building codes such as the IMC and also why it's a bad idea.

This article series describes how to install bathroom ventilation systems, fans, ducts, terminations. We include bathroom venting code citations and the text also 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. We discuss bathroom exhaust vent codes, specifications, advice.

It's Not OK to Share Bathroom Fan Duct with Kitchen Exhaust Fan Duct

Question: bath fan and kitchen fan use the same exhaust pipe?

2018/06/01 John said:

Can the bath fan and kitchen fan use the same exhaust pipe through the roof?

Reply: No.

Simply put, NO.

You cannot (by code) and should not (by practical reasoning) share exhaust venting between a kitchen exhaust and a bathroom exhaust fan.

The design requirements and materials are different between bath exhaust vent systems and kitchen exhaust vent systems, and such sharing is prohibited by the model mechanical codes and shared ducting are likely to be prohibited by local building codes and code inspectors.

Authority: 2015 International Mechanical Code, Section 505, Domestic Kitchen Exhaust Equipment, 505.1, Excerpt:

Such ducts shall have smooth inner walls, shall be air tight, shall be equipped with a backdraft damper, and shall be independent of all other exhaust systems. - retrieved 2018/06/01, original source, UTAH adoption copy of the IMC, www.up.codes/viewer/utah/imc-2015/chapter/5/exhaust-systems#505.1

Details

That’s because a kitchen exhaust fan system that has to handle greasy exhaust. While there is also discussion of a fire risk in residential kitchen exhaust vent ducts, in part because they are virtually never cleaned.

ASHRAE in HVAC Applications 33.36 notes that

Residential hood exhaust ducts are almost never cleaned, and there is no evidence that this causes fires. … However, grease fires on teh residential cooking surface continue to occur, almost always the result of unattended cooking. - ASHRAE Handbook 2005 HVAC Applications, 33.36-33.37.

Kitchen & Bath Vent Fan shared exhaust duct systems are prohibited by the model building codes such as the IMC

International Mechanical Code 2015, Chapter 5 Exhaust Systems, Section 501 General through section 505, Domestic Kitchen Exhaust Equipment

International Mechanical Code 2015, Chapter 5 Exhaust Systems, Section 501 General through section 505, Domestic Kitchen Exhaust Equipment - available for viewing free or top copy for fee fee from iccsafe at https://codes.iccsafe.org/public/document/IMC2015/chapter-5-exhaust-systems

Venting codes are also available asa free, downloadable PDFs at VENTILATION CODES copies of venting codes from various state authorities who publish for public access their adopted copy of the IMC or other ventilation standards..

501.1 Scope

This chapter shall govern the design, construction and installation of mechanical exhaust systems, including exhaust systems serving clothes dryersand cooking appliances; hazardous exhaust systems; dust, stock and refuse conveyor systems; subslab soil exhaust systems; smoke control systems; energy recovery ventilation systems and other systems specified in Section 502. - retrieved 2018/06/01 original source https://www.up.codes/viewer/utah/imc-2015/chapter/5/exhaust-systems#501.1

501.2 Independent System Required

Single or combined mechanical exhaust systems for environmental air shall be independent of all other exhaust systems.

Dryer exhaust shall be independent of all other systems. Type I exhaust systems shall be independent of all other exhaust systems except as provided in Section 506.3.5.

Single or combined Type II exhaust systems for food-processing operations shall be independent of all other exhaust systems.

Kitchen exhaust systems shall be constructed in accordance with Section 505 for domestic equipment and Sections 506through 509 for commercial equipment. - op. cit.

and for residential kitchens, the following ventilation code applies:

International Mechanical Code 2015, Chapter 5 Exhaust Systems, Section 505 Domestic Kitchen Exhaust Equipment

505.1 Domestic Systems

Where domestic range hoods and domestic appliances equipped with downdraft exhaust are provided, such hoods and appliances shall discharge to the outdoors through sheet metal ducts constructed of galvanized steel, stainless steel, aluminum or copper.

Such ducts shall have smooth inner walls, shall be air tight, shall be equipped with a backdraft damper, and shall be independent of all other exhaust systems.  - https://www.up.codes/viewer/utah/imc-2015/chapter/5/exhaust-systems#505.1

FYI from Use and Occupancy Classification, Section 310 Residential Group R

310.1 Residential Group R. Residential Group R includes, amonmg others, the use of a building or structure, or a portion thereof for sleeping purposes when not classified as an Institutional Group I or when not regulated by the Residential Code in accordance with section 101.2.

505.3 Common Exhaust Systems for Domestic Kitchens Located in Multistory Structures

Where a common multistory duct system is designed and installed to convey exhaust from multiple domestic kitchen exhaust systems, the construction of the system shall be in accordance with all of the following: 

1. The shaft in which the duct is installed shall be constructed and fire-resistance rated as required by the International Building Code. 

2. Dampers shall be prohibited in the exhaust duct, except as specified in Section 505.1. Penetrations of the shaft and ductwork shall be protected in accordance with Section 607.5.5, Exception 2. 

3. Rigid metal ductwork shall be installed within the shaft to convey the exhaust. The ductwork shall be constructed of sheet steel having a minimum thickness of 0.0187 inch (0.4712 mm)(No. 26 gage) and in accordance with SMACNA Duct Construction Standards. 

4. The ductwork within the shaft shall be designed and installed without offsets. 

5. The exhaust fan motor design shall be in accordance with Section 503.2. 

6. The exhaust fan motor shall be located outside of the airstream. 

7. The exhaust fan shall run continuously, and shall be connected to a standby power source. 

8. Exhaust fan operation shall be monitored in an approved location and shall initiate an audible or visual signal when the fan is not in operation. 

9. Where the exhaust rate for an individual kitchen exceeds 400 cfm (0.19 m3/s) makeup air shall be provided in accordance with Section 505.2. 

10. A cleanout opening shall be located at the base of the shaft to provide access to the duct to allow for cleanout and inspection. The finished openings shall be not less than 12 inches by 12 inches (305 mm by 305 mm). 

11. Screens shall not be installed at the termination. 

12. The common multistory duct system shall serve only kitchen exhaust and shall be independent of other exhaust systems.
- https://www.up.codes/viewer/utah/imc-2015/chapter/5/exhaust-systems#505.3

505.4 Other Than Group R

In other than Group R occupancies, where domestic cooking appliances are utilized for domestic purposes, such appliances shall be provided with domestic range hoods. Hoods and exhaust systems shall be in accordance with Sections 505.1 and 505.2. - https://www.up.codes/viewer/utah/imc-2015/chapter/5/exhaust-systems#505.4

Practical Issues with Combined Kitchen & Bath Exhaust Ducting

Practical issues with shared exhaust ducting are discussed by quite a few inspectors and websites who may not have gone back to the model codes for authority.

IF the exhaust duct for the shared kitchen and bath exhaust vent fans meets the building code requirements for a kitchen exhaust vent duct to the exterior, including using smooth sheet-metal ducting and inclusion of a backdraft-preventer, the duct system may be functional for both exhaust vent fans.

But there are some potential issues with shared bath/kitchen exhaust including

A common exhaust fan to vent both areas must have adequate CFM capacity

Cross-draft odor transmissions: since usually the two exhaust fans will not be running simultaneously, depending on the location of the backdraft preventer in the duct system bathroom odors could appear in the kitchen - obviously undesirable, and kitchen cooking odors could appear in the bathroom.

IMC Chapter 5 Exhaust Systems, Section 505 Domestic Kitchen Exhaust Equipment

505.1 Domestic systems. Where domestic range hoods and domestic appliances equipped with downdraft exhaust are located within dwelling units, such hoods and appliances shall discharge to the outdoors through sheet metal ducts constructed of galvanized steel, stainless steel, aluminum or copper. Such ducts shall have smooth inner walls, shall be air tight, shall be equipped with a backdraft damper, and shall be independent of all other exhaust systems.

Practical concerns beyond code:

Kitchen & Bath Vent Fan shared exhaust duct systems are prohibited by the model building codes such as the UMC.

Practical issues with shared exhaust ducting are discussed by quite a few inspectors and websites who may not have gone back to the model codes for authority.

IF the exhaust duct for the shared kitchen and bath exhaust vent fans meets the building code requirements for a kitchen exhaust vent duct to the exterior, including using smooth sheet-metal ducting and inclusion of a backdraft-preventer, the duct system may be functional for both exhaust vent fans.

But there are some potential issues with shared bath/kitchen exhaust including the following:

Grease from a kitchen exhaust vent that runs back into the bath vent section of the ducting increrases fire risk.

A common exhaust fan to vent both areas must have adequate CFM capacity

Cross-draft odor transmissions: since usually the two exhaust fans will not be running simultaneously, depending on the location of the backdraft preventer in the duct system bathroom odors could appear in the kitchen - obviously undesirable, and kitchen cooking odors could appear in the bathroom.

Commercial Kitchen Venting Materials,

Using the UMC as example, UMC Chapter 20, Ventilation, Sec. 2002 (a) Kitchen Venting Systems Materials, Grease Ducts. Grease ducts and plenum sserving a Type I hood shall be constructed of at least 0.055 inch-thick (No. 16 manufacturer’s standard gauge) steel or stainless steel at least 0.044 inch in thickness.

Bath & Kitchen Exhaust Vent & Ducting Codes

See BATHROOM VENTILATION CODES SPECS and also see VENTILATION CODES

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-02-24 by (mod) - ICC code guidelines permit sharing kitchen & bath exhaust ducts: go with the more-restrictive code of the two

@Bill Goodspeed, thank you that's a helpful additional detail.

On 2021-02-24 by Bill Goodspeed

I formally inquired with the ICC. The response, seeing both sections, was to refer to section 102.1, go with the more restrictive code when they conflict. Hopefully future codes will modify one of these sections.

On 2021-02-11 - by (mod) -

Thank you, I really appreciate the continued discussion - we are both professionally-neutral and looking for the best information we can find. Our number one concern is safety. Second is function.

It seems reasonable that a qualified expert could design a shared exhaust vent system that

- can be fire-safe

- is protected against backdrafts

- provides adequate exhaust venting for all of the connected vent fans

- remains readily accessible for future cleanout (as is needed with a kitchen exhaust vent system)

- will be accepted by local code officials

My initial reluctance on this score is from an OPINION that too many folks might just slap some vents together to make an unsafe system or one that doesn't work.

I just got off the phone with retired Journal of Light Construction Editor and my friend Steve Bliss who reminded me by quoting someone else who says, paraphrasing,

"For every complicated problem there exists a simple solution ... that doesn't work"

You might want to check with one of the engineers at a kitchen exhaust vent manufacturer to see what suggestions they have and of course stop by your local code official to be sure your plan will be accepted before spending on it.

On 2021-02-11 by Bill Goodspeed

After further review, I now feel that 501.2 and 505.1 contradicts itself even though 505.1's commentary specifically states that it is consistent with section 501.2. From looking at the 2 sections' commentary over and over, I am interpreting that they are "inconsistent" with each other.

Dan, I agree that backdraft dampers should be utilized especially for the kitchen fan and I also agree that metal ducting is best for fire hazards. Many retrofit situations for historic preservation reasons, such as my current project, want to minimize exterior wall penetrations, which is why I google searched this issue and found your article as well as dug into the code since I initially felt that sections 501.2 and 505.1 contradicted itself.

On 2021-02-11 by danjoefriedman (mod) - IMC section 505.1 - downdraft domestic kitchen range hood

Bill

Thank you for those added references; we'll be sure to keep them with this article & series.

Watch out: we need to take great care here when discussing combining exhaust vents, as there are fire and life safety concerns involved.

For example common bath exhaust vent ducts use plastic-and-wire flex-duct for the duct run to the building exterior (not that I think that's the best material) because the fire- and grease- accumulation issues don't pertain.

But no kitchen exhaust vent can use such materials as there would be grave fire hazards.

In a shared exhaust duct system as well, typical bathroom exhaust fans do not include hardware that prevents against backdrafting - which could be a terrible mess if the bath exhaust is shared with kitchen exhaust, even if both use safe materials. We may end up, depending on duct routes and connections, sizes, etc. blowing our bacon grease and cooking odors back into the bathroom, or our bathroom odors into the kitchen cooking area, particularly because the fans in the two locations vary in timing of usage and in fan CFM capacity.

On 2021-02-11 by Bill Goodspeed - IMC 501.2 allows "Environmental Air" exhaust to be combined.

IMC section 505.1 refers to a downdraft domestic kitchen range hood, from a quick search, these include the larger than typical exhaust fans, 300 cfm and higher, not your typical domestic range hood of 100 cfm exhaust.

IMC 501.2 allows "Environmental Air" exhaust to be combined. IMC 2015 501.2 commentary states: This section does not prohibit exhaust systems exhausting environmental air from sharing common ducts..... IMC chapter 2 definition of "Environmental air" is domestic kitchen range exhaust, bathroom exhaust, etc.

On 2020-12-12 - by (mod) -

Martin:

Thank you for the question.

From the article above we have;

You cannot (by code) and should not (by practical reasoning) share exhaust venting between a kitchen exhaust and a bathroom exhaust fan, with possible exception if your bath venting meets fire and other requirements for the kitchen exhaust and if approved by your local code inspector. .

On 2020-12-11 by Martin

I have just noticed a bathroom, kitchen and then another bathroom...in that sequence using the same horizontal exhaust duct going to the stand alone exhaust fan. I don’t think this is to code, am I right...thank you

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