Mainline ML-FR4 Backwater Valve
(formerly Fio Valve)

The newly reintroduced ML-FR4 ABS Automatic Backwater Valve is a great choice for retrofit situations where you have a limited amount of slope to work with.

This is a normally-open design, automatic backwater valve that permits the free circulation of air throughout the plumbing system, allowing gases to move just as if the valve wasn't even there.

The valve will close automatically when sewage backup occurs, reducing the risk of sewage backflow throughout the dwelling.

The ML-FR4 is designed to be installed in building main lines to prevent backup through floor drains, shower drains, toilets and sink drains.

It closes automatically when the municipal sewer line or septic system starts to back-up or a reverse flow occurs.

The clear plastic lid allows for easy visual inspection and accessibility for cleaning.

The design doesn't restrict the withdrawal of a sewer snake since the gate remains in the open position, except when a backflow is in progress.  The ML-FR4 drops 1/2" from inlet to outlet over its 12" installed length and is the outside diameter (male) of 4" Schedule 40 ABS on both ends making it easy to fit either end into a female coupling or neoprene rubber with stainless steel clamp (Fernco brand type) adapter.

Patented USA 4503881 and Canada 1196835

Normal flow through a Mainline ML-FR4 ABS Backwater Valve Back Flow situation (reversal) in the ML-FR4 Backwater Valve

Mainline ML-FR4-A ABS Backwater Valve installation tips in basement floor.

Normal flow through a Mainline brand automatic Fio ML-FR4 backwater valve        Back-flow through a Mainline brand automatic Fio ML-FR4 back-flow valve

Fio Valve ML-FR4 Top View 1Fio Valve ML-FR4 Top View 2

ML-FR4 Fio Valve end view closedML-FR4 Fio Valve end view open

ML-FR4 Fio Valve cutaway view openML-FR4 Fio Valve end view open 2

ML-FR4 Fio Valve als offered with PE-2013 Access Box


Fio Valve or Fio Drain Valve by Mainline (ML-FR4) brochure in Adobe Acrobat pdf file format - Mainline ML-FR4 ABS Valve Brochure (Adobe Acrobat pdf) - mainline ML-FR4 backwater valve pricing


top check backwater valves backwater check valves backwater valves adapt-a-valve information ml fr4 fio valve information termex valve information straight fit backwater valves backwater valve diagram backwater valve features and specifications backwater valve history testimonials backflow prevention information helpful links backwater valve pricing frequently asked questions backwater valve specifications backwater valve installation tips backwater valve animation backwater valve videos contact us backwater valves main page backwater valves backflow prevention links


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adobe acrobat pdf filesAdapt-A-Valve Brochure (Adobe Acrobat pdf)

Adapt-A-Valve Inspection Chamber

Backwater Valve Diagram
Backwater Valve Features
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Backflow Links
Backflow Preventers
Backwater Valve Animation
Backwater Valve Videos

Basement Drain Backing Up

 contact us about your backwater valve needsContact Us

Flood Alarm

Frequently Asked QuestionsBack water valve questions

adobe acrobat pdf filesFullport Backwater Valve Brochure (Adobe Acrobat pdf)

how to install your backwater valveINSTALLATION TIPS;  how to install backwater valves 

Termex Backwater Valves
Termex Backwater Valve Brochure (Adobe Acrobat pdf)

NEW: TOP-CHECK BACKWATER VALVESnew Top-check backwater valves

Mainline ML-FR4 Backwater Valve (Fio Valve)
adobe acrobat pdf filesML-FR4 Mainline Backwater Valve Fio Valve Brochure (Acrobat pdf)

6" Backwater Valves - Mainline STRAIGHT FIT Backwater Valves

Mainline History

backwater valve measurements and specificationsMeasurements, Specifications and IAPMO Certifications


Mainline Fullport Backwater Valves offer the right solution for your sewer backup problems.


Protect Your Home with a Backwater Valve

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copyright 2004 to 5/30/2024 at 8:43:13 ET,
Akron, OH, USA.
All rights reserved.

backflow prevention links is independently owned and resells products produced and manufactured by Mainline of Canada and assumes no liability for the installation of backwater valves and related products.

Be sure to check with the plumbing authority in the municipality in which your valve is to be installed to insure that your installation is in compliance with local code, and be sure have your valve inspected by your local plumbing inspector after installation. Inspect your valve frequently; the Backwater Valve through its clear plastic housing top, and the Adapt-A-Valve, by removing the gate cassette.


Pipe end-to-end, the Adapt-A-Valve takes up 6" of run in your line and the outlet is 1-1/2" LOWER than the inlet, so in just 6" the level of the pipe at the outlet side needs to be 1-1/2" lower.

Pipe end-to-end, the Backwater Valve takes up 12" of run in your line and the outlet is 3/4" LOWER than the inlet, so in just 12" the level of the pipe at the outlet side needs to be 3/4" lower.

Depending upon the amount of slope in your line (2% minimum is required for proper operation), you may need to replace several feet of pipe downstream from the valve to average out this "loss" of fall.

NOTE: New Straight-Fit and ML-FR4 require less slope. (refer to detail pages for these products)


Mainline Valve Warranty
(in printable Adobe Acrobat .pdf file)

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