Copper M-press fittings installation guide: tube, insertion depth, pressing and testing

Copper press fittings have an O-ring in a raised bead at each end. The press jaw closes around that bead and squeezes fitting and tube together, so the O-ring seals and the metal grips. There is no flame, no flux and no waiting for joints to cool. A good joint takes seconds.

The steps are simple, but each one matters. This guide covers the tube you can use, the tools, the insertion depth and the spacing between fittings, how to press and how to test. For choosing between press, solder and compression, see copper press fittings vs solder vs compression.

What copper tube can I use with M-press fittings?

Use copper tube to BS EN 1057 in a wall thickness the fitting maker approves. One fitting maker says its copper press fittings suit hard, half-hard and soft EN 1057 tube, and lists the approved wall thicknesses for each temper and size.

• Tempers: R290 (hard), R250 (half-hard) and R220 (annealed or soft), as listed in another maker's application guide for its copper press system.
• Condition: the tube end must be round, undamaged and free of deep scores or scratches. A scratch across the sealing area can leak.
• Check the list: wall thickness limits vary by maker and size. Do not assume a thin-wall or unusual tube is approved.

Copper is a conductor. One press system maker says its copper press installations must be connected into the main equipotential bonding. Leave the bonding checks to an electrician.

Which press tool and jaws do I need?

You need a press tool listed by the fitting maker and M-profile jaws in each size you are pressing. M-press fittings will not press correctly with a U-profile multilayer jaw.

Tool class	Copper sizes	Example use
Compact machine	12 to 35mm	Domestic heating, tight spaces
Standard 32kN machine	12 to 54mm	Larger mains, plant rooms

Those size ranges come from one fitting maker's tool tables. The MaxHeatUFH Electric Hydraulic Press Tool Set (MH-EHCS) takes M-profile jaws for 15, 22 and 28mm copper, and U-profile jaws for multilayer. MaxHeatUFH 35, 42 and 54mm M-profile jaws are made for the High Power tool (MH-HPEHCS) only.

For a fuller look at tools, hire against buying and servicing, read choosing a copper press tool.

How do I prepare the tube?

Cut square with a rotary tube cutter, then deburr the inside and outside. Fitting makers' steps start the same way every time.

1. Cut square. Use a rotary tube cutter, not a hacksaw. Check the tube has kept its round shape.
2. Deburr inside and out. Angle the tube downwards where you can so filings fall out, not in. Both edges should be smooth with no sharp lip.
3. Clean the end. Remove dirt and swarf from the sealing area.
4. Inspect. No deep scores, dents or flats where the O-ring will sit.
5. Check the fitting. Correct size, O-ring present, undamaged and seated in the bead. One maker recommends a small amount of its press fitting lubricant on the O-ring.

Burrs are the main enemy. An outside burr can roll the O-ring out of its groove as the tube goes in. An inside burr catches debris and disturbs flow.

What is the insertion depth for copper press fittings?

Insert the tube until it meets the tube stop, then mark the depth on the tube so you can see it has not moved before pressing. The depth depends on the size and the fitting maker. These are one fitting maker's published figures for its copper press range.

Tube size	Insertion depth	Min gap between fittings	Min tube length
15mm	22mm	10mm	54mm
22mm	23mm	20mm	66mm
28mm	24mm	20mm	68mm
35mm	26mm	25mm	77mm
42mm	36mm	30mm	102mm
54mm	40mm	35mm	115mm

Use the depth your own fitting maker gives if it differs. The minimum gap matters because pressing reforms the tube near each joint. Makers advise leaving a minimum distance between fittings for that reason. The minimum tube length is the shortest piece that can join two fittings and still be pressed properly at both ends.

Turn the tube slightly as you push it in. That helps stop the O-ring being dislodged. Check alignment too: the tube should go in straight, with no side load.

How do I press an M-press fitting?

Fit the M-profile jaw that matches the tube size, place it square with the fitting's bead centred in the jaw groove, and run one full press cycle. Do not press the same joint twice.

1. Fit the right jaw. M-profile, same size as the fitting.
2. Seat it on the bead. The bead must sit in the middle of the jaw groove, with the jaw at right angles to the tube.
3. Check the mark. Your insertion mark should still be at the fitting mouth.
4. Press. Hold the trigger until the jaws close fully and the tool completes its cycle.
5. Mark the joint. One maker suggests marking each finished joint so it is easy to inspect before the test.

The maker is clear that the press cycle should be completed once only, and the fitting not re-pressed. If a joint is wrong, cut it out and replace the fitting.

Plan the access before you start. The jaw needs room around the fitting, so press joints near walls, joists and other pipes before they become hard to reach. Angled jaws help where space is tight.

How do I pressure test copper press joints?

Fill with clean water, test at low pressure first to find any unpressed joints, then raise to the full test pressure and hold it. Fitting makers set out this order.

• Fill and vent. Fill against an open valve at the high point to let trapped air out.
• Low-pressure check. Test at 1 to 2 bar and inspect. One maker's fittings are made to leak if left unpressed, across a range of 0.1 to 6.0 bar.
• Press any missed joints. The same maker says an unpressed joint can be pressed without draining, as long as the tube is fully inserted.
• Full test. Raise slowly to the system test pressure, as set by BS EN 806-4 for water systems. Hold it for at least 30 minutes with no pressure drop.

Another maker makes the same point about its own system: unpressed joints leak during the pressure test, so they are found before the system is in use. For underfloor heating manifolds and loops, see our pressure testing underfloor heating guide.

What pressures and temperatures are copper press fittings rated for?

It depends on the fitting and the use. One fitting maker gives a general maximum of 16 bar and 110°C for its copper press fittings, with lower limits for some uses.

Use	Max pressure	Max temperature
Drinking water (EN 806)	10 bar	95°C
Heating water (EN 12828)	6 bar	110°C
Local and district heating	10 bar	110°C
Chilled water	10 bar	-10°C minimum

A domestic sealed heating system or underfloor heating primary is well inside those limits. Check the data for your own fittings, especially for solar, gas or anything unusual. Gas work must be done by a Gas Safe registered engineer, with fittings approved for gas.

Where do copper press fittings go in an underfloor heating job?

Mostly on the primary pipework: from the boiler or heat pump to the manifold, around the pump and mixing set, and on radiator circuits. The underfloor loops themselves are plastic or multilayer pipe.

Where copper meets multilayer, an adaptor joins the two. The MLCP press fit to copper brass tail has a U-profile press end for multilayer and a plain brass tail that goes into a 15, 22 or 28mm copper fitting, including a copper press fitting. Our UFH manifold primary pipe size guide covers what size to run.

MaxHeatUFH copper fittings come in bags and boxes, for example the equal straight coupling, sold by the bag. See the full copper M-press range.

Frequently asked questions

What is the insertion depth for 15mm and 22mm press fittings?

One fitting maker gives 22mm for 15mm tube and 23mm for 22mm tube. Other makers may differ slightly, so use the figure for your fittings.

Can I press a copper fitting twice?

No. Makers say to complete one press cycle only. If a joint is wrong, cut it out and fit a new one.

Do I need to mark the tube before pressing?

Yes. Mark the insertion depth once the tube is fully home. The mark shows whether the tube has slipped back before you press.

Can I use M-press fittings on soft copper?

One fitting maker says its copper press fittings suit hard, half-hard and soft EN 1057 tube, within its listed wall thicknesses. Check your maker's table for the size you are using.

Will a U-profile jaw press copper fittings?

No. Copper M-press fittings need M-profile jaws. U-profile jaws are for multilayer fittings.

How close can two press fittings be?

One fitting maker gives a minimum gap of 10mm between 15mm fittings and 20mm between 22mm or 28mm fittings, because pressing reforms the tube near each joint.

How do I find a joint I forgot to press?

Test at 1 to 2 bar first. Leak-before-press fittings weep when unpressed, so you can find and press them before going to full test pressure.

Sources: copper press fitting manufacturers' technical brochures and installation guides, and BS EN 1057