How to Join Dissimilar Pipes

Family Handyman

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Joining dissimilar pipes

When you go to add or replace plumbing lines in a house that’s more than 15 years old, chances are you won’t find new pipes that are the same kind as the old ones. That’s no big deal—hardware stores and home centers carry hundreds of different kinds of transitional fittings to help you make the connections. What is a big deal is that those hundreds of different kinds of fittings don’t all install the same way. Some fittings need to be soldered; others need just a wrench or pliers. Several require specific crimping tools, and there are newer styles that simply push together. We asked Les Zell, a master plumber, how he deals with the ones he encounters the most.

Icemaker
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Icemaker line

Installing a brass compression tee body is a good way to provide water to your refrigerator’s icemaker. Although the brass ferrule seals the copper pipes to the tee, Les still uses a little pipe dope on the ferrule to assist in even compression. Many municipalities don’t allow 1/4-in. icemaker lines to be covered by finished walls, floors or ceilings, so you may have to run a 1/2-in. line to the fridge instead.

Water Filter Line
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Water filter line

This angle stop adapter valve is a great solution if you want to install an under-the-sink, in-line water filter. It fits between the shutoff valve and the braided line that runs to your faucet. No pipe dope or pipe thread tape is necessary because rubber washers seal the connections.

PEX to Copper
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PEX to copper

If you want to connect PEX to copper by soldering on a transitional copper fitting, make sure you wait for the fitting to cool before attaching the PEX. The downside of PEX is that many of the connectors are proprietary, and it’s extremely important that you use manufacturer-approved connectors. Les likes to work with the brands of PEX that can be connected with cinch clamps. Just slide the cinch clamp over the PEX tubing, and then tighten the clamp with a cinch clamp tool. Cinch clamp tools cost about $40 at home centers.

PEX to CPVC
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PEX to CPVC

It doesn’t get much easier than this. Glue the CPVC end (there is a band of CPVC in the female end of this copper fitting), and secure the PEX with a cinch clamp. Let the glue fully cure before turning the water back on (some CPVC cements require several hours to cure).

Plastic to galvanized
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Plastic to galvanized steel

The inside of galvanized pipes gets thinner over the years. And the male ends, where the threads were cut into, can become so thin that they leak when you try to screw on a new female fitting over the end of them. So try to make the transition at an existing female fitting. When working with galvanized pipes, Les wraps pipe thread tape on the threads and dabs on a little pipe dope as added insurance against leaks.

ABS to PVC
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ABS to PVC

One inexpensive method of connecting ABS to PVC drainpipes is to use male and female fittings. Apply pipe thread tape before screwing them together, and then glue the pipes into the fittings using the proper cement. Arrange the fittings so the water flows past the threads on the male fitting, not into them. This helps solid materials flow by the connection without getting hung up.

Copper to PVC
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Copper to CPVC

Plastic and metal expand and contract at different rates. This can be a problem when joining CPVC to copper, especially when using threaded connections. One option is to use union adapters. The rubber washer should flex enough to keep the connection sealed. If you choose to solder the copper side, make sure you do that first or you’ll melt the plastic side.

Shielded Rubber Coupling
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Shielded rubber couplings

These couplings are often referred to as “mission couplings,” and they work great to connect dissimilar drain lines: galvanized steel to plastic, cast iron to plastic, ABS to PVC. Make sure you use a fully banded coupling, because the couplings with just the two individual hose clamps may not be allowed in some situations.

Copper to Galvanized Steel
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Copper to galvanized steel

Connecting two dissimilar metals can cause galvanic corrosion, which deteriorates metal over time. A dielectric union is a fitting designed to isolate the two metals from each other. There is some controversy as to the effectiveness of dielectric unions, but the bottom line is that if you connect a copper pipe to a galvanized one, some plumbing inspectors are going to require one. Certain municipalities consider a brass fitting a suitable dialectic union, but others do not. Your best bet is to ask your local inspector.

Push Fitting
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Push fittings are immediate

Push fittings haven’t been around all that long, but they are without a doubt the easiest way to join two pipes. Even though push fittings are easy, you still have to do your homework. Read the instructions on the type of fitting you’re using to see how far the pipe needs to be inserted, and make a mark on the pipe at that length to ensure that it’s pressed in all the way. And be certain that the outside portion of the pipe that will slide into the fitting is free of burrs and scratches.

Control valves for joinery
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Control valves for joinery

When Les joins dissimilar pipes, he likes to install a control valve whenever possible so the water in the rest of the house can be turned back on while he’s making his repairs. Some control valves are designed to be transitional fittings, but most can be converted by adding various fittings at either end.

Don't guess
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Don’t guess

Just because a fitting is the right size or configuration doesn’t mean it can be used in any situation. Some can be used above ground but not below. Others work perfectly fine in the open but can’t be buried behind drywall. When in doubt, ask your inspector. Here are a few examples of connectors that may seem like the perfect solution but could be rejected by your inspector, or worse, fail to work and cause thousands of dollars in water damage:

Dresser couplings: These will pull apart on pipes that aren’t completely immobile.

Quick elbows: Drain snakes can poke right through them.

Saddle valves: These valves are not always allowed. They clog easily and don’t always shut off reliably.

No-hub mission couplings: These couplings are for cast iron to cast iron connections only.

Buy approved products
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Buy approved products

Exceptionally fastidious inspectors will want you to show them that you’re using approved building materials. If you have to choose between two similar products, buy the one that is clearly labeled as being approved by nationally recognized organizations. ANSI and ASTM are examples of widely accepted product certification organizations. And save your labels until after the inspection is completed. For more information on what these markings mean, check out safeplumbing.org/product-markings.

Meet the Expert
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Meet the expert

Les Zell has been a plumber for 30 years. Many of his customers have several different types of pipes in their homes, and the number of different kinds of fittings he carries in his van has tripled. Knowing how to join dissimilar pipes has been essential.