Updated 2026-10-18
The letter tells you how the polyethylene was cross-linked, not how good the tube is. Both are tested to the same standard, at the same dimensions and the same pressure ratings. What genuinely differs is how the tube handles in a bay, how it takes a kink, and which fitting system and tool it is approved for.
Best overall
Either, matched to its fittings
Same ASTM F876 tube, same SDR 9 bore, same ratings. The fitting standards on the printline decide the job, not the letter.
Best for tight retrofits
PEX-A on cold expansion
The softer tube in a cramped bay, a joint built on the tube's own memory, and the maker's claim that a kink can be recovered.
Best for a crimp-tool van
PEX-B on insert fittings
Crimp, clamp or press with no expander on the van, on a tube held to the same F876 ratings as PEX-A.
| Factor | PEX-A | PEX-B | PEX-C |
|---|---|---|---|
| Cross-linking process | Peroxide (Engel) | Silane, moisture cure | Electron beam |
| When it cross-links | During extrusion | After extrusion, with heat and moisture | After extrusion |
| Tubing standard | ASTM F876 | ASTM F876 | ASTM F876 |
| Inside diameter, 3/4" | 0.671" | 0.671" | 0.671" |
| Minimum pressure ratings | 160 psi 73°F · 100 psi 180°F | Same | Same |
| Usual fitting family | Cold expansion, F1960 | Crimp, clamp or press insert | Per printline |
| Flexibility (Viega's rating) | Commonly high | Moderate to high | Varies by product |
| Kink fix | Heat on some products, or cut and couple | Cut and couple | Cut and couple |
| Chlorine and UV rating | Read the code | Read the code | Read the code |
The letter is not a grade
All three start as high-density polyethylene. Cross-linking ties the polymer chains together, which is what gives PEX its strength at temperature, its toughness and its flexibility. The three common processes differ in when and how the links form.
Organic peroxide in the compound releases free radicals as it is heated in the extruder, so the tube cross-links as it is formed. Viega reads ASTM F876 as requiring at least 70% cross-linking for this method. Uponor's AquaPEX is a PEX-A line.
A silane is grafted onto the polyethylene, the tube is extruded, and the links form afterwards when it is cured with moisture and heat, steam curing for instance. Viega reads the minimum as 65%. Viega's PureFlow is a PEX-B line.
Finished tube is passed under a beam of high-energy electrons, which forms the links by a physical process rather than an additive. Same 65% minimum by Viega's reading. Treat it exactly like the other two and read its printline.
PEX cannot be solvent cemented or heat fused, so every joint is mechanical: something squeezes the tube against a fitting. There are two families. Insert fittings go into the tube as it is and a ring, clamp or sleeve is squeezed around the outside. Cold-expansion fittings go into tube that has been stretched first, and the tube shrinks back down onto them. Which family a given coil may use is printed on the coil itself.
| Standard | Fitting | How it seals | Tool |
|---|---|---|---|
| F1960 | Cold expansion, PEX reinforcing ring | Tube and ring expanded, fitting inserted, tube recovers onto it | Expander, manual or powered |
| F2080 | Cold expansion, metal sleeve | Tube expanded, then a brass sleeve pulled over axially | Expander plus sleeve tool |
| F1807 | Metal insert, copper crimp ring | Ring crushed around the tube over a barbed insert | Crimper sized to the ring |
| F2159 | Plastic insert, copper crimp ring | Same as F1807 on a sulfone-plastic insert | Crimper sized to the ring |
| F2098 | Stainless clamp for F1807 / F2159 | Ratcheting clamp in place of the copper ring | Clamp (cinch) tool |
| F3347 / F3348 | Metal or plastic press insert | Factory-fitted stainless sleeve pressed onto the tube | Press tool and jaws |
| ASSE 1061 | Push-fit | O-ring on the outside of the tube, locked by a collar | None beyond a square cut |
In the field the split usually lines up with the letters. Cold expansion relies on the tube's memory, and PEX-A makers build on it: Uponor says its ProPEX rings are extruded to “capitalize on the shape memory of PEX-a”. PEX-B is most often joined with insert fittings, crimp, clamp or press, and Viega, a PEX-B maker, says the tubing, fitting, sleeve, tool and press profile must be approved as one configuration. Expansion rings are also sold for some PEX-B, which Uponor's own literature argues against. Neither maker's marketing settles it. The printline does.
Geometrically, yes, a little. An insert fitting sits inside unstretched tube, so its wall takes a slice out of the tube's bore at every joint. An expansion fitting sits inside tube that was stretched first, so its bore can run closer to the tube's own. We could not find a primary source that publishes a like-for-like pressure loss for the two, so we are not putting a number on it. What is certain is that the tube bore is the same for both letters: SDR 9 fixes 1/2" PEX at 0.475" inside and 3/4" at 0.671", PEX-A or PEX-B.
Joints are where the loss lives
ASTM F876 requires the tube to carry a printline at least every five feet. Two fields on it answer most of this comparison: the four-digit material designation code and the fitting standards the tube maker approves. On a recirculating hot-water loop the chlorine digit is the one that matters: a pump fitted to cure a long hot water wait time changes how many hours a day the tube sits at 140°F.
1/2" SDR 9 · PEX 5306 · ASTM F876 F877 · F1960 · 160 psi 73°F · 100 psi 180°F · NSF-pw
This is where the letters genuinely feel different on the job. PEX-A is generally the softer tube; PEX-B is generally the stiffer one, which is why it tends to want more elbows and bend supports in a tight bay. Viega, which makes PEX-B, rates PEX-A flexibility “commonly high” and PEX-B “moderate to high”, and warns against assuming every PEX-A beats every PEX-B: compare the bend radius each maker actually publishes.
| Nominal size | Actual OD (in.) | Min. free bend radius (in.) |
|---|---|---|
| 3/8" | 0.500 | 3.0 |
| 1/2" | 0.625 | 3.8 |
| 3/4" | 0.875 | 5.3 |
| 1" | 1.125 | 6.8 |
Six times the outside diameter is the industry floor for an unsupported bend, measured at the middle of the bend. It applies to every letter unless the manufacturer states a different figure, so the one to work to is the one in your coil's installation instructions. Where a bend has to sit close to a fitting, use a bend support so the joint is not carrying the load.
Uponor's own repipe comparison ticks “kink reparability” for its PEX-A and crosses it for insert-fitted PEX-B: the claim is that warming a kink in PEX-A lets the tube recover its shape. Viega's view is that heat repair works on some products but is slow and leaves a spot more likely to kink again, and that the reliable fix is to cut the kink out and join it with an approved fitting. The Plastics Pipe Institute (PPI) guide sides with the fitting: approved couplings may repair kinked or damaged lines, and tube that has been kinked should not be installed. If you heat-repair anything, do it only where the tube maker's own instructions allow it.
PPI treats freeze tolerance as a property of PEX, not of a letter: the tube stays flexible below -40°F, expands with the ice and in many cases does not burst, though it can. PPI also credits fewer fittings with fewer ice blockages at transitions, one more argument for home runs. If a line freezes, thaw it with warm water, a warm towel, a space heater or a hot-air gun, keeping the tube under 180°F, and never with a flame. The frozen pipes guide covers the order to work in.
No torch near any PEX
Neither model code names a letter. IPC 2021 Table 605.4 lists cross-linked polyethylene tubing to ASTM F876 or CSA B137.5 for water distribution, and Table 605.5 lists the fittings, including F877, F1807, F1960, F2080, F2159 and ASSE 1061. Section 605.16.2 then requires the tube to be factory marked with the fitting standards its maker specifies, which is what makes the printline enforceable. The 2024 IRC (P2906.10.2) and the 2024 UPC (605.9.1) say the same thing in their own words; the UPC also keeps any PEX out of the first 18 inches of piping at a water heater (604.13).
Which edition applies, and any local amendment, is set by your authority having jurisdiction (AHJ). The IPC vs UPC comparison covers how adoption works. If the decision is still PEX against a metal, start with PEX vs copper, and for a whole-house job the three-way repipe comparison.
Before the coil goes on the van