PEX Pipe-in-Pipe Relining: Sizing, Clearance and When It Beats Repiping

Pex pipe in pipe relining pulls a smaller PEX tube through an existing host pipe and terminates it at both ends. The old pipe becomes a sleeve. The new tube carries the water. It is the least destructive way to save a failing supply line, and it usually fails for one predictable reason: a wrong liner size.

The trap is dimensional. PEX tubing is outside-diameter controlled under ASTM F876 and sized on copper tube dimensions, so its outside diameter equals copper’s at every nominal size. A same-nominal liner cannot physically enter a same-nominal host. Every sizing decision in this guide follows from that one constraint.

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What PEX Pipe-in-Pipe Relining Actually Involves

Insert relining threads a new pressure tube through a host pipe that still holds its shape. The host becomes a protective sleeve along the whole run. Only the two ends get opened up. That is what separates the method from full replacement: the path survives, and the flow surface is new.

Two lookalike methods cause confusion in the field. Spray lining coats the host wall with epoxy and leaves the original material in service. Cure-in-place pipe inflates a resin-soaked liner inside the host and hardens it there. Insert relining differs from both: the liner is a finished, pressure-rated tube pulled through in one piece.

PEX suits the job because it coils, bends, and carries potable water ratings under ASTM F876, which controls the outside diameter in SDR9 wall. Long coils matter too, since a single continuous length can traverse an entire run with no intermediate joints. The category has matured enough that manufacturers now ship purpose-built relining tube certified to standards such as AWWA C904 and listed under NSF 14 and NSF 61. Insert relining is no longer a field improvisation.

Abandoned branches either stay sleeved or get capped at the last fitting. Either way they carry no water after the cutover, so the plan must say which one happens where. Unmapped dead legs inside a host are the kind of detail that surfaces during a future demolition.

Which Host Pipes Accept a PEX Liner

The host does two jobs at once: it guides the pull and it protects the liner for the life of the system. Pick the wrong host and both jobs fail.

Sound Copper Is the Ideal Host

Copper gives the liner a smooth, dimensionally stable bore. Type L internal diameters run 0.527 in for 1/2 in tube, 0.785 in for 3/4 in, and 1.025 in for 1 in, per the Copper Development Association tube handbook. Those bores accept a liner one size class smaller with room to pull. Copper hosts rarely fight the installation unless the wall has already eaten through.

Galvanized Needs a Wall Check First

Schedule 40 galvanized bores start larger, at 0.824 in for 3/4 in nominal and 1.049 in for 1 in, so fit is usually the easy part. Corrosion is the hard part. Tubercles build inward and steal bore in patches that no nominal table predicts.

Feed a liner into a heavily scaled run and it hangs on a rust ridge. Push harder and the tube gouges before it seats. Cut a sample section at an end and inspect the wall before quoting. Heavy scaling or soft wall means the answer is repipe, not reline.

Some hosts are disqualified on sight. Active leaks, thinned walls, and collapsed sections end the sleeve story before it starts. The liner needs a host that will hold still for decades.

Path obstructions are a separate failure mode. Old repair couplings, crushed sections from past freeze events, and soldered offsets left by previous work can all stop a pull halfway. Run a fish tape or a pilot line through the host first. Whatever stops the tape will stop the liner.

Sizing Logic That Matches Liner to Bore

Sizing a reline is subtraction. The liner must clear the host’s internal diameter with room left for pull tension and small bore imperfections. Start from published dimensions, not from the label.

The OD Match Rule

Work the rule on a 3/4 in Type L host with a 0.785 in bore. The 3/4 in PEX candidate has a 0.875 in outside diameter and does not enter. The 5/8 in tube, at 0.750 in OD, passes with 0.035 in of annular clearance. That clearance sounds small, and it is the entire game.

Galvanized bores run larger than copper at the same nominal size, yet the rule still bites there. A 3/4 in PEX liner measures 0.875 in OD against a 0.824 in bore in 3/4 in Schedule 40 pipe, so it stays outside even the iron host. Fit comes easier in galvanized only once you drop the size class, and only when scale stays out of the way. The table below pairs the common hosts with the largest SDR9 PEX liner that physically enters, using sourced OD and ID values throughout.

Fit is only half the answer. Flow area drops with the square of the liner-to-host ID ratio, and the computed retentions in the table make that trade honest. A 1/2 in liner in a 3/4 in copper host keeps about 36 percent of the original area. Velocity climbs at the same flow rate, so check fixture demand before promising anyone pressure they will not get.

Clearance and Bend Radius Limits Inside Existing Runs

A liner that fits the bore still has to travel the path. Elbows, offsets and tee pockets set the real limits, and they are geometric rather than negotiable.

The Six Times OD Rule

PEX bends to a minimum radius of six times its outside diameter. That computes to 3 in for 3/8 in tubing, 3.75 in for 1/2 in, 4.5 in for 5/8 in, and 5.25 in for 3/4 in. Any tighter and the wall buckles on the inside of the bend.

Now look at a hard 90 degree elbow in a wall cavity. The liner has to complete its bend inside a space roughly the size of the elbow body. If the cavity cannot host the radius, the tube kinks or refuses to feed. Forcing it only work-hardens the mistake into the run.

Planning the Pull Path

Walk the run before cutting access. Count the bends and score each one against the radius figures above. Long sweeps are free; consecutive hard 90s usually mean an access point between them or a rerouted section. Tees and valve pockets are pass-throughs for the liner, yet each one is where a future branch must be cut back in.

Illustrative: a basement-to-second-floor run with an offset at the joist bay. The straight footage is trivial. The offset’s two close bends decide the job, and one access point at the bay converts an impossible pull into a two-stage feed.

Measure the real path before ordering coil length. Pull a tape through the host and read the true travel, which typically runs longer than the building’s straight-line run. Order the liner off that number plus termination allowance at both ends. A short liner discovered after the pull is a full redo.

Terminations and Connections at Each End

The liner carries pressure along the run, but the system reconnects to normal tubing at each end. Everything an inspector can touch happens at those terminations.

From Liner to Branch Tubing

Terminate the liner with any standard PEX connection family. ASTM F1960 covers cold expansion fittings with reinforcing rings. F1807 covers metal insert fittings with copper crimp rings, and F2159 covers the plastic insert equivalents. Choose by the tools your crew already carries. The connection is made on the liner outside the host, so the joint stays serviceable.

Close the annular gap where the liner exits the host with a mechanical seal or a termination fitting rated for the purpose. Support the host within a few inches of each exit so the assembly cannot lever against the old pipe. Unsealed entries also let moisture migrate between the layers.

On the supply side, Hitze stocks the tubing this work runs on: SKZ-tested PE-Xb pipe with no MOQ on general ordering and a sample-first policy for new accounts. The product warranty runs 50 years, and WRAS product approval is asserted for the PEX line. Request the reference and verify it in the public WRAS directory before submittal.

When Relining Beats Full Repiping

Relining is not a discount repipe. It is a different trade-off, and it wins on specific job conditions.

Where the Reline Wins

Access is the first deciding vote. Repiping a slab run or a finished ceiling means demolition, patching and paint in every room the line touches. A reline works through two access openings, and the building finishes stay intact.

Downtime decides commercial and multi-unit work. A zoned reline can keep occupied areas running while one branch is down. A full repipe typically shuts the building’s water over a wider window and a longer calendar.

Continuous length is the quiet advantage. PEX coils ship in 100, 300, 500 and 1,000 ft lengths, so a long run takes one liner with zero intermediate joints. Every joint you remove is one less leak point to watch over the life of the system.

Illustrative: a four-unit row house is losing its 3/4 in copper under a finished hallway. Repipe opens every unit’s ceiling. A reline through end access keeps ceilings closed and takes each unit offline for roughly one working day.

Budget phasing fits the method too. A building can reline the worst riser this quarter and schedule the remaining branches next year, because each run stands alone. Repipe budgets resist that split, since the demolition and patching mobilization gets paid again on every visit.

When access and occupancy drive the price, take the reline. That is the honest read.

When You Must Repipe Instead

The same checklist that approves a reline rejects it on other jobs. Name the disqualifiers early and you save the estimate, not just the embarrassment.

Flow demand disqualifies more runs than people expect. The sizing math keeps only 36 to 53 percent of the original area on common pairs. If the system already runs marginal at peak demand, that loss shows up as complaints rather than numbers. Run the demand check first, and repipe when the math lands badly.

Host condition is the second gate. Galvanized wall loss, repeated pinhole history or soft sections mean the sleeve cannot do its job. Relining over a weeping host traps water between the layers, and it exits at the first low point, usually inside a finished wall.

Jurisdiction and insurance close the list. Some authorities and insurers expect full replacement for potable systems in certain building classes. Confirm before quoting, because a failed inspection converts a finished job into a free repipe. The reline is only as permanent as the pipe around it.

Code Acceptance and Commissioning After Relining

The last mile is paperwork and pressure. Relined potable lines live under the same scrutiny as new pipe, and the acceptance path is procedural rather than codified.

The Commissioning Checklist

Acceptance of insert relining varies by jurisdiction, and no universal clause settles it for you. Put the question to the authority having jurisdiction before the quote, not after the pull. Then commission the run the way you would defend it in a file review.

• AHJ acceptance: confirm insert relining is accepted for potable service in your jurisdiction before quoting.
• Pre-pull test: pressure test the liner before it enters the host, so you have a baseline.
• Post-pull test: test again after the pull, because the path can abrade the tube.
• Flush and disinfect: follow the local potable-water requirements before handover.
• Documentation: record liner markings, test pressures and dates in the job file.
• Labeling: tag both terminations with liner size and host run.

Test pressures sit against the tubing’s ASTM F876 marking: 160 psi at 73.4 °F, 100 psi at 180 °F and 80 psi at 200 °F. Local requirements set the actual test; the marking tells you the headroom.

Conclusion

Pex pipe in pipe relining trades demolition for geometry. The OD match rule decides which liners fit, the squared ID ratio decides what survives of the flow, and the host condition decides whether the job should exist at all. Get those three checks in order and the reline is usually the most economical honest path to a new flow surface.

If you are quoting a reline and want the sizing checked, send over the host material, nominal size and run length. Hitze stocks PE-Xb tubing with no MOQ on general ordering, so you can confirm the fit on a short length before you commit the job.

Frequently Asked Questions