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Frequently Asked Questions about Mainline Backwater Valves

Q: Can I convert an older (Model 4963 ABS or PVC) Mainline Fullport Backwater Valve into a "Top Check" model valve?
A: Yes! Find the ML TCKit about 4 items down the page at

Q: How much slope is required in a drain line in order to use a backwater valve?
A: In all installations, make sure the slope is adequate.  It must be at least 2% (or 1/4" per foot) to insure proper operation (NOTE: this requirement does not apply to SF [Straight -Fit] and ML-FR-4 valves).

PLEASE NOTE: 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.  The Fullport 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 the "loss" of fall built in to each valve.

Q: Are Mainline Backwater Valves available in different sizes and materials?
A: At this point in time, they are only produced in 4" ("Schedule 40" ABS and PVC.)  We've asked the Mainline factory for a 6" backwater valve and believe they will be available in the future.

Q: What are the Inside and Outside Dimensions of Schedule 40 DWV (Drain/Waste/Vent) Pipe?
A: 3" ABS or PVC DWV is approximately 3.03" inside diameter and 3.500" outside diameter.
     4" ABS or PVC DWV is approximately 3.99" inside diameter and 4.500" outside diameter.
     6" ABS or.PVC DWV is approximately 6.02" inside diameter and 6.625" outside diameter.

Q: What is Difference between PVC and ABS?
A: These are two different chemical formulations of plastic commonly used in Drain, Waste and Vent (DWV) systems and are joined by solvent cementing.  PVC stands for Poly Vinyl Chloride and ABS stands for Acrylonitrile-Butadiene-Styrene.  Different solvent cement is required depending upon the type of pipe being used.

Q: Should I use ABS of PVC?
A: First, your local plumbing code governs which material must be used in your area, but it is also important to know that you should not ever mix the 2 materials as the solvent may not bond them together properly.

Q: How do I adapt to Clay or Cast Iron pipe.
A: A company called Fernco makes a wide variety of Neoprene Rubber Adapters that clamp to the outside diameter of the pipe and the backwater valve with stainless steel clamps and sleeves.  We do not supply these adapters as various plumbing codes require different Fernco models which can generally be purchases at your local home-center or plumbing supply.  Again, you must check your local municipal code to be sure this type of adapter is permitted.

Q: Can I increase from 3" pipe to a 4" Backwater Valve or Adapt-A-Valve?
A: From a mechanical perspective, you may increase from 3" to 4" on the INLET (uphill) side of the valve with a concentric or eccentric reducer, but must only use an eccentric reducer on the OUTLET (downhill) side.  Reducing on the outlet side with a concentric adapter would cause the bottom of the smaller diameter 3" pipe to be higher than the bottom of the valve, causing the gate to be submerged with reduced flow over it's surface.  With a reduction in the self-cleaning flow, sediment would build up and thwart the gates' ability to close.  You must also check your local municipal plumbing code to be sure that the use of a reducing or increasing adapter is permissible in your area.

Q: Why do both ABS (Black in color) and PVC (White in color) Mainline Backwater Valves bear an "ABS" stamped right into the material on the bottom of each valve?
A: Mainline currently has only one mold, and it was stamped ABS because that was the only material they produced when the company began production.  Molds are incredibly expensive and the same mold is used for both ABS and PVC, now that PVC valves are being produced.

Q: I live in a city that routinely flushes out its sewers with highly pressurized water.  Do you have any experience with the normally open Backwater Valve in a flushing environment.  Will it close in time so as to prevent damage?
A: Most municipal sewers are flushed with high pressure on a fairly regular basis.  The Fullport Backwater Valve and Adapt-A-Valve should both work if installed properly and maintained (check that the gate moves freely) on a regular basis.  The combination of air and water will close the gate.

Q: What is the "Laminar Flow Rule?"
A: Laminar flow is a term described the smooth movement of a fluid through a pipe.  Openings in the side of a pipe's uniform walls interrupt this smooth flow.  The "Laminar Flow Rule" states that no branch lines should enter the pipe nearer than 2' upstream of the Backwater Valve.  This 2' or greater margin between branch line attachment and the valve installation point will allow laminar flow to resume by the time wastewater is flowing across the gate of the valve.  Laminar flow aids the self-cleaning process as the flow moves across the gate, and insures the gate is not constantly bobbing up and down in the midst of turbulent flow.

Q: What is the leakage rate of the normally open Fullport Backwater Valve or Adapt-A-Valve?
A: Mainline provides that the allowable leakage rate up to 2 ft of head is 1 Liter per 10 minutes, as head pressure increases to over 2 ft there is no leakage.

Q: How much pressure can a Fullport Backwater Valve or Adapt-A-Valve withstand??
A: Mainline provides that they have independently tested their valves to withstand at least 50 psi, the weight of a 100 foot column of standing water which is ample in most residential applications.

Our Specs/Measurements page at links to the official IAPMO page at which can be printed in 8-1/2x11" format with Adobe Acrobat Reader.

This IAPMO "Certification of Listing" states:

"Products are in compliance with the following code(s):
-- Uniform Plumbing Code (UPC®)
-- National Plumbing Code of Canada
-- International Plumbing Code (IPC®)

Products are in compliance with the following standard(s):
-- ASME A112.14.1-2003(R2008)
-- CSA B181.1-2011 and CSA B181.2-2011

ASME A112.14.1-2003(R2008) and CSA B181.1-2011 and CSA B181.2-2011 codes state backwater valves in general must tested to 5 psi, and backwater valves with an integrated cleanout (like the 4963 Fullport line) must withstand 15 psi.

Mainline valves far exceed US and Canadian code requirements. 

Q: What is the Mainline Fullport Backwater Valve and Adapta-A-Valve warranty?
A: Mainline Backwater Valves and Adapta-A-Valves are warranted by the manufacturer for 1 year against manufacturer's defects. Here is the link to the Mainline Valve Warranty as a printable Adobe Acrobat .pdf file.  The actual installation of the valves can not be warranted by Mainline.

Q: Can the Adapt-A-Valve Test Gate be used in an emergency to shut off back-flow?
A: Yes.  The Test-Eze Gate can be used as both a Test Gate and as an Isolation or Knife Valve.

Q: With a Normally Open Gate Fullport Backwater Valve or Normally Open Gate Adapt-A-Valve, what is the chance of the gate not closing?
A: As long as the valve has been properly installed (proper slope, laminar flow rule followed) and is clean and maintained, it will close.  You should inspect your valve every 3 months, removing any buildup noted, and hosing the gate and valve surfaces clean with water pressure as needed.

Q: My installer lost the sticker shipped with my Mainline 4963 ABS or PVC Backwater Valve.  What did it look like and what did it say?
A: Here is a link to Installation Tips photographs we have taken of the front and back of the Backwater Valve sticker we ship with each valve we sell..

Q: My 4963 Backwater Valve arrived, but it seems the yellow hinged gate hinge pins are binding and the gate is not opening and closing freely.
A: We inspect and the gate on each valve we ship to be sure it is hinging freely, and even place a cardboard band over the gate to protect it from flopping around during shipment, but sometimes a carrier drops a box during transport in such a way that the hinge pins bind up.  This is not a defect, and the hinge pins of the yellow gate can be gently pressed back down into their sockets (molded into the bottom of the valve body) as seen in the series of detailed photos at

Q: I've seen Access Boxes with Steel bolts and Access Boxes with Plastic Push Fasteners. Is there a difference?
A: The PE-2013 Access Box was produced with steel bolts holding the lid to the box until early 2009 when it was found that plastic push fasteners worked equally well, and also removed the possibility of the nut falling out loose into the box where recovery in order to properly refasten the top was difficult.

Q: Are there any known problems reported from using chemical drain cleaners with Mainline valves?
A: Chemical drain cleaners are not recommended by many plumbers because they create a tremendous amount of heat buildup. However, they are a fact of life and are used every day.  That means that they have been used in Mainline brand valves, and every other valve on the market. Mainline has never received a report that drain cleaners have adversely affected their valves, and they do not anticipate that the cleaner would hurt any components of their valves.

Q: What are the dimensions of the 4963 Backwater Valve?
A: 4963 Backwater Valve: Width: 10" (254 mm), Length: 15.5" (394 mm), Height: 11" (280 mm ).

Q: What are the dimensions of the PE-2013 Access Box?
A: PE-2013 Access Box:: Overall dimension of the open base is 14-7/8" X 20" and steps down to 11-1/8" X 14-7/8" at the top of its 16" height.  Removable gray high density polyethylene (plastic) top access panel is 10-7/8" X 14-1/2".

Q: Can chemical drain openers be used after a Backwater Valve has been installed?
A: The ABS or PVC valve bodies and neoprene seals should be unaffected, however, the yellow gate material could be affected if drain cleaner stays in contact with it for a prolonged period of time.  If you have the proper 2% or greater slope through your valve, the chance of a problem would be unlikely, but you should monitor the condition of your gate for visual deformation after using a chemical drain opener.  The gate easily snaps in place and we always have replacement gates in stock.

Q: What happens if my installation is not perfectly flat side-to-side?
A: The gates within our backwater valves are designed to work with proper slope (2% or greater) from inlet to outlet and flat from side to side. Visualizing a clock, a cross section of the valve would be aligned from 9 o'clock to 3 o'clock with the top of the valve pointed straight up to 12 o'clock.  The gate may not seal perfectly if the valve is not level from side-to-side.

Q: I've removed the clear plastic lid from my Backwater Valve and don't want to crack it when re-tightening the bolts.  Is there a tightening procedure / sequence, and what torque should I use?
A: You should use a hex-driver and hand tighten (don't use a ratchet as that could allow you to apply too much torque).  On their assembly line, Mainline sets their air tool at 80 psi. Snug down the bolts in a criss-cross pattern.

Q: Does wastewater freeze near the cap of the riser to the surface of an Adapt-A-Valve?
A: Assuming your cap is air tight, and it should be or pressure would cause leakage, No.
Two things are at work: As water attempts to push up the riser, the air in the riser compresses and pushes back down on the water column never allowing it to reach the top. Wastewater is also  biologically active and freezes at a lower temperature than 32 degrees Fahrenheit. In Mainline's experience, freezing has never been a problem even in the coldest climates.

Q: What is a Fio Valve?
A: Mainline has acquired the rights to produce and distribute the Fio Valve / Fio Drain that was popular in the 1980's and 90's. Now designated as the Mainline ML-FR4 Backwater Valve, this valve, which is less particular about minimum slope requirements is easier to use in retrofit applications without much fall in the line.

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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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