This article defines & describes Drain Fixture Units or DFUs.

DFUs are a unit of measure of the wastewater volume or loading of a drain waste or vent system.

DFUs are used to specify the size of waste piping, vent piping, or related drain devices such as air admittance valves or Studor valves.

Here we provide a helpful table giving the number of DFUs for various plumbing fixture types.

Definition of Plumbing Vent DFU Ratings: Drain Fixture UNITS

Definition of Drain Fixture Unit or Definition of DFU:

The Uniform Plumbing Code defines Drainage Fixture Unit as follows:

Drainage (dfu). A measure of the probable discharge into the drainage system by various types of plumbing fixtures.

The drainage fixture-unit value for a particular fixture depends on its volume rate of drainage discharge, on the time duration of a single drainage operation and on the average time between successive operations. - UPC 2006 

Drain Fixture Unit, or DFU, is a plumbing design factor, or a relative measure of the drain wastewater flow or load for various plumbing fixtures.

Here are two quantitative measures of DFUs:

• 1 DFU = 1 cubic foot of water drained through a 1 1/4" diameter pipe in one minute.
• 1 DFU ≈ (approximately) 7.48 US GPM or ≈ 0.47 liters/second
  
  Note: 1 cubic foot = 7.48 US Gallons.

Notice in the table below that the DFU factor for a plumbing fixture will vary depending on the drain and trap size or diameter.

By adding the DFU load rating of all of the individual fixtures on a single drain to be served by a single air admittance valve (AAV), the plumber or designer can select an AAV with sufficient capacity.

As we discuss separately at AIR ADMITTANCE VALVES AAVs, Oatey, an AAV manufacturer, provides the following helpful DFU Load Table:

Notes to the table above

1. Oatey Corporation, "Oatey Sure-Vent® Air Admittance Valves Technical Specifications", Oatey® Corporation, - retrieved 2016/05/08, original source: http://www.oatey.com/doc/aavtrifoldlcs420c101812lr.pdf The company provides AAVs rated at 6, 20, 160, and 500 DFUs.

2. 1 toilet at 1.6 gpf, 1 bathtub with shower, 1 sink

3. 1 toilet at 1.6 gpf

Watch out: While it is acceptable to oversize a Sure-Vent®; however, an undersized Sure-Vent® (Oatey) or Studor Vent (like the Studor Mini-Vent®) or other AAV product will not allow the plumbing system to breathe properly.

Illustration: Studor's Mini-Vent® DFU (Drain Fixture Unit) sizing chart from the

STUDOR PRODUCTS MANUAL [PDF] (10th ed) (see p. 8) Studor IPS Corporation, retrieved 2019/08/14 original source: https://ipscorp.com/pdf/studor/Studor_Technical_Manual.pdf.

See details and more AAV DFU charts

at AIR ADMITTANCE VALVES AAVs

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

How many fixture units is a trench drain

How many fixture units is a trench drain (interior applications). Specifcally Zurn Z886 8601 (rated at 93 GPM). - by Ryan

Reply by (mod) - the Zurn 886 is a zero DFU system.

Ryan,

Correct me if I've misunderstood, but from my reading of the company's specifications at zurn.com, these are trench drain inserts, not building wastewater disposal system components.

Therefore the Zurn 886 is a zero DFU system.

The Zurn 886 series refers to the 6" Zurn Z886 Trench Drain - a modular drain trench channel - that handles flowing wastewater or runoff at 93 gpm (6 LPS) (0.2 CFS).

Other products in the series accept up to 530 gpm. These might be used, for example, to intercept water running down a sloped drive to conduct it away from a building.

These products have nothing to do with sewage or wastewater processing and thus are not related to drain fixture units discussed in the article above.

 

How many DFUs do we count for a furnace?

How many fixture units is a drain on a furnace by Fred

Reply by (mod) - 2 or less

What an excellent question.

Since a furnace is a forced air heating system it won't have a drain.

Or are you asking about a condensing gas fired furnace that produces a small amount of condensate? If so we'd look at air conditioner condensate drain DFUs.

My OPINION is that the condensate produced in that case is less than the standard 2 DFUs used for "semi continuous condensate drain systemns"

as per

Values for Continuous Flow.

For a continuous or semi-continuous flow into a drainage system, such as from a pump, ejector, air-conditioning equipment or similar devices, two DFUs shall be considered to be equal to each gallon per minute (gpm) of flow. Amended at 38 Ill. Reg. 9940, effective April 24, 2014)

But perhaps I'm being too fussy and you mean a hydronic boiler or hot water heating system.

Nobody counts a heating boiler drain among the building's DFUs since you would not normally be draining it at all, certainly not regularly, and when you do need to drain a boiler for service or repair, you'd typically run a hose outside to a ditch rather than draining into a septic system.

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Continue reading  at DRAIN & SEWER PIPING - topic home,or select a topic from closely-related articles below, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR ADMITTANCE VALVES AAVs
• PLUMBING VENT CODES, DEFINITIONS, TYPES
• WATER CONSERVATION MEASURES
• WATER QUANTITY USAGE TABLES

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PLUMBING DRAIN FIXTURE UNITS DFUs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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