Connecting metal pipes like copper or galvanized steel to PVC pipes is a common plumbing task. There are a few methods for joining these dissimilar materials depending on the application. The most important factors to consider are finding the right fittings to transition between the pipes, using the proper cements and solvents, and preparing the pipes correctly for the connection. With the right techniques, metal and PVC pipes can be joined securely.
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What Are the Challenges of Connecting Metal to PVC?
Metal and PVC have very different properties which make joining them together difficult:
– Metals like copper and galvanized steel are rigid while PVC is flexible plastic. The joints need to accommodate this difference.
– PVC pipe uses glues and cements to weld pieces together while metal joints are threaded, soldered, or brazed. Some transition fittings are required.
– PVC expands and contracts with temperature changes more than metal. The joints need to be flexible.
– PVC is not compatible with the fluxes used for soldering copper pipes. The metals need protection at joints.
Common Ways to Connect Metal to PVC
There are three main methods for joining metal pipes to PVC:
Threaded Fittings
One way is to use threaded adapter fittings. A male or female threaded fitting is attached to the metal pipe and a slip or threaded PVC fitting connects to it. For example:
– A galvanized pipe male adapter threads into a PVC female adapter
– A copper male threaded adapter goes into a PVC slip x thread connector
Pros:
– Simple with readily available fittings
– Allows for easy disassembly
Cons:
– Doesn’t accommodate temperature changes well
– Can come loose over time
Union Connections
Another option is to use union fittings. These consist of a threaded end, a slip end, and a union nut to join them. For example:
– A galvanized steel coupling threaded end connects to a PVC union with slip end
– A copper union tailpiece threads to a PVC union
Pros:
– Compensates for temperature changes
– Quick to take apart
Cons:
– More expensive fittings
– Bulkier connections
Direct Solvent Welds
For some applications, metal pipes can be directly solvent welded into PVC fittings. This involves:
– Deburring and sanding the metal pipe end
– Applying primer and PVC cement just like joining regular PVC
– Inserting the pipe into the fitting and holding until bonded
Pros:
– Permanent and reliable connection
– No special fittings required
Cons:
– Not reusable
– Careful preparation needed
How to Choose the Right Connection Method
Consider these factors when deciding the best way to join the pipes:
Pipe Sizes
Threaded and union connections typically come in fewer sizes. Check availability for the pipe diameters. Solvent welds can work for any size combination.
Application
Permanent and rigid bonds like solvent welds are best for buried piping. Threaded or union connections allow easier repairs.
Differences in Pipe Materials
Take the metal’s hardness and PVC’s softness into account. Threaded ends on hard steel can damage PVC threads over time.
Pressure Ratings
Solvent welds and unions provide full pressure ratings. Threaded adapters have reduced pressure capacities.
Code Requirements
Some codes restrict threaded joints and certain fitting materials for underground installations. Check regulations.
How to Connect Copper Pipe to PVC
Copper is commonly used for water lines. Here are some tips for connecting it to PVC:
Cleaning and Preparation
– Cut the copper pipe squarely and deburr the inside and outside edges.
– Clean the end of grease, flux, and grime so the cement bonds.
– Sand the end lightly with emery cloth to improve adhesion.
Fitting Options
– Use a copper male adapter to connect to a PVC female threaded or slip adapter
– A copper coupling tailpiece can thread into a PVC union
– For direct solvent welds, use a copper x PVC transition fitting.
Making Threaded Connections
– Wrap the copper threads with Teflon tape to prevent leaks.
– Tighten joint to hand tight plus 1/2 turn. Over tightening can crack the PVC.
Solvent Welding
– Apply primer to the inside of the fitting and outside of the pipe.
– Use heavy bodied PVC cement and apply a thick layer on both surfaces.
– Insert copper immediately all the way into the fitting and hold 30 seconds.
Supporting the Joint
– Support direct solvent welds with a no-hub connector band for extra strength.
– For buried lines, encase unions and threaded joints in a watertight sleeve.
How to Connect Galvanized Pipe to PVC
Galvanized steel pipe is often used for drains. Here are tips for joining it to PVC:
Cleaning and Prep
– Wire brush galvanized pipe to remove zinc coating from the ends.
– Sand the end lightly to rough up surface for gluing if solvent welding.
Fitting Options
– Use galvanized threaded adapters into PVC threaded or slip fittings
– Galvanized couplings work with PVC unions
– Directly solvent weld galvanized pipe into PVC fittings
Threaded Joints
– Use fresh galvanized fittings and wrap threads with Teflon tape
– Tighten joint by hand plus 1/2 turn max
Solvent Welds
– Apply primer and medium bodied cement to pipe and fitting
– Push galvanized pipe fully into fitting and hold 1 minute
– Allow 24 hours curing time before pressure testing
Dealing with Corrosion
– Use dielectric unions when connecting galvanized pipe to copper to prevent galvanic corrosion
– Consider painting buried galvanized steel pipe connections for added corrosion protection
How to Connect Stainless Steel to PVC
For potable water systems and corrosive applications, stainless steel is a good choice. Connect it to PVC like this:
Fitting Options
– Use stainless steel threaded nipples with PVC threaded fittings
– Stainless steel flanges mate with PVC flange adapters
-Weld stainless flanges to pipe and bolt to PVC flanges
Making Threaded Connections
– Wrap male stainless threads with Teflon tape
– Use strap wrenches and tighten carefully to avoid cracks
Flanged Joints
– Install gasket between flanges
– Use stainless steel bolts with nylon washers
– Tighten bolts gradually in a crisscross pattern
Dealing with Thermal Expansion
– Allow room for pipe movement with expansion joints
– Use adequate hangers and supports on stainless steel pipe
transition Methods Comparison
Here is a comparison of the most common metal to PVC connection methods:
| Method | Pros | Cons |
|---|---|---|
| Threaded fittings | Easy to source parts Can be disassembled Works for many applications |
Prone to leaks if not tightened properly Less reliable long-term Not good for temperature changes |
| Unions | Allows for thermal expansion Reusable connection Good alignment |
More expensive Bulky fittings Lower pressure rating |
| Solvent welds | Permanent and reliable bond No special fittings Maintains full pipe strength |
Not reusable Careful prep work required Useful for limited materials |
Conclusion
Connecting metal pipes like copper, galvanized steel, and stainless steel to PVC lines is made possible with the use of adapters like threaded fittings and unions. For more rigid connections, direct solvent welding lets you bond the pipes. Choosing the right method depends on the application, pipe sizes, and expected service conditions. With the proper fittings and preparation, reliable mixed pipe connections can be made.