Compression fittings
Entry in Joining methods · Updated · Written by the Know Your Pipes editors from the sources listed under References
Compression fittings are threaded fittings that join tube by squeezing a soft ring, called a compression sleeve or ferrule, between a nut and the fitting body so it grips the outside of the tube.[1] They are made in brass for copper, PEX and other plastic tubing, and need no heat, solder or special crimping tool.[1]
One maker lists their uses as drinking-water and non-drinking-water systems, instrumentation, and hydraulic and pneumatic lines, and states that they are not for gas connections of any kind.[1] Small fixture shutoff valves, such as the stops under a kitchen sink, are made with compression inlets for copper tube.[3]
1 Parts and how the seal forms
The joint has three pieces: the nut, the sleeve and the fitting body.[1] The nut and sleeve slide onto the tube with the nut's threads facing the fitting, the tube goes into the body until it bottoms out, and the nut is threaded on and tightened with two wrenches, one holding the body steady.[1]
The tube end has to be cut square and be free of dents, burrs, paint, putty and gouges, any of which can keep the sleeve from sealing.[1] One maker forbids putty or gasket-type sealants on its compression joints because they can cause leaks, while allowing a drop of oil or thread sealant on the nut threads to ease assembly.[1] It also warns that overtightening can cause failure and leaks long after installation.[1]
2 Tubing and inserts
Compression fittings suit thin-wall tubing. One maker excludes thick-wall tube such as type K copper, and its supply stops with compression inlets are for type L or M copper only.[1][3] Wall thickness by type is compared under copper pipe types K, L and M.
Plastic tube needs support from the inside. An insert stiffener is a liner that sits inside polyethylene, polybutylene or PEX tube and supports it all the way around under the sleeve.[2] Without one, the plastic can slowly creep away from the joint, relax the grip and cause a leak or a blow-off.[2] The stiffener should be long enough to support the tube wherever the fitting contacts it, and the insert maker confirms the right size.[2]
| Tubing | Sleeve | Insert stiffener |
|---|---|---|
| Copper, types L and M | Brass | None |
| Copper, type K | Not recommended by one maker | — |
| PEX, 1/2 in OD and larger | Brass | Stainless steel insert |
| PEX, 3/8 in OD and smaller | Plastic | None |
The copper and PEX rows follow one maker's compression fitting sheet and supply-stop instructions.[1][3] Polyethylene water service pipe joined with compression couplings relies on insert stiffeners in the same way.[2]
3 Access and reuse
One maker's compression-inlet stops are marked for use with water in accessible locations only.[3] The same instructions say not to solder within 12 in of the valve, not to reuse it, and to inspect it every year, replacing it if damage, deterioration or corrosion appears.[3] Whether a compression joint may be concealed in a wall is set by local code and the maker's listing, and permanent alternatives include soldered sweat joints and press fittings.
Because one fitting style accepts both copper and PEX tube, compression adapters are one kind of transition fittings.[1] Brass fittings in aggressive water are covered under brass fittings and dezincification.
See also
- Polyethylene water service pipeFlexible black or colored PE tubing buried between the water main or well and the house; cold water only.
- Shutoff valvesThe valves that stop water in a house: ball, gate and fixture stop valves, how they differ, and the rules that apply to them.
- Brass fittings and dezincificationBrass is a copper-zinc alloy used for valves and fittings; dezincification is corrosion that removes its zinc.
- Soldered (sweat) jointsHow a sweat fitting is soldered to copper tube by capillary action, which solders are lawful for drinking water, and how brazing differs.
- Push-fit fittingsFittings that seal by pushing tube into an O-ring and locking device; covered by ASSE 1061 for PEX, copper, CPVC and PE-RT.
- PEX crimp ringsCopper rings squeezed around PEX over a ribbed insert fitting; made to ASTM F1807 or F2159 and checked with a go/no-go gauge.
References
- BrassCraft, "Compression Fittings Installation Instructions, 225.01" (2020) — d2cl5to9q9oami.cloudfront.net
- A.Y. McDonald Mfg. Co., "The Importance of an Insert Stiffener" (2019) — aymcdonald.com
- BrassCraft, "Dual Outlet and Dual Outlet/Dual Shut-Off Stops, Compression Inlet: Installation Instructions, 425.11" — d2cl5to9q9oami.cloudfront.net