UFH Pipe: 16mm PEX-a and PERT-AL-PERT for Wet Underfloor Heating, Plain English

A leak in a buried UFH pipe after the screed is down is one of the most expensive mistakes on a kitchen extension. Tracing the leak needs a thermal camera and an acoustic listener (£400 – £800 for a specialist call-out). Repairing it means cutting a trench through finished screed, splicing the pipe, then patching the floor and waiting weeks for it to dry again before flooring can go down. All of which is preventable. Pressure-test the pipe at 6 bar before the screeder turns up, hold the pressure during the pour, and the chance of this happening drops to near-zero. Most homeowners don't know to insist on it because nobody tells them.

What it is and what it's for

UFH pipe is the network of plastic tubing buried in the floor that carries warm water from a manifold (the brass distribution block on the wall) out across the room and back. Hot water enters at one end of each loop, gives off heat through the floor as it travels, and returns cooler to the manifold to be reheated. A typical kitchen extension has two to four loops, each between 50m and 100m long, all fed from one manifold.

The pipe is almost always 16mm outside diameter and made from one of three plastic constructions:

• PEX-a, cross-linked polyethylene, the original UFH pipe material. Flexible, kink-tolerant (a heat gun can reverse a kink), 50+ year service life.
• PE-RT, polyethylene of raised temperature resistance. A simpler thermoplastic without the cross-linking step. Cheaper, slightly less forgiving with kinks, performs the same in service at the 35-50°C flow temperatures used for UFH.
• PERT-AL-PERT (or PEX-AL-PEX), a five-layer pipe with an aluminium core sandwiched between PE-RT (or PEX) layers. The aluminium gives the pipe shape memory: it holds bends instead of springing back. Much easier to lay.

All three are sold as coils of 100m, 120m, 200m, or 240m. Two-hundred-metre coils are the sweet spot for most extension-sized kitchens.

The other thing every domestic UFH pipe needs is an oxygen barrier (more on that below). Look for "EVOH" or "barrier" or "DIN 4726" on the label. Without it, you'll destroy the boiler.

The governing standards are BS EN 1264 (the European standard for water-based UFH design and commissioning) and DIN 4726 (the German oxygen permeation standard adopted across Europe, capping permeation at ≤0.1 g/m³/day). Building Regulations Part L (energy efficiency) applies to any UFH installed in an extension, and from June 2025 the Future Homes Standard requires all new and replacement wet heating systems to be designed for a maximum flow temperature of 55°C.

Why the EVOH oxygen barrier matters

This is the single biggest specification mistake a homeowner can make: buying or accepting non-barrier pipe to save a few pounds.

Plastic is permeable to oxygen. Standard polyethylene pipe lets a slow trickle of oxygen diffuse through the wall and into the system water. In an open-vented system this is harmless. In a sealed central heating system it's a slow-acting poison.

Dissolved oxygen reacts with the steel in the boiler heat exchanger, the cast iron in the circulation pump, and any ferrous valves on the system. Over two to five years the corrosion produces black magnetite sludge that clogs the boiler, kills the pump, and eventually destroys the heat exchanger. Replacing a heat exchanger on a modern combi is £600 – £1,200 plus labour. A new boiler is £2,500+. All to save the £30 – £50 difference between barrier and non-barrier pipe on a typical extension.

EVOH (ethylene vinyl alcohol) is a thin internal layer that blocks oxygen diffusion. It's laminated between PE-RT or PEX layers in a five-layer construction (inner pipe, adhesive, EVOH, adhesive, outer pipe). The German DIN 4726 standard caps oxygen permeation at 0.1 g/m³/day at 50°C. EVOH-barrier pipe meets or exceeds this. Non-barrier pipe doesn't.

PERT-AL-PERT goes one better. The aluminium core is a perfect oxygen barrier (metal is impermeable to gas) and it gives the pipe the shape memory that makes installation so much easier. For a domestic UFH system in 2026, PERT-AL-PERT or EVOH-barrier PE-RT/PEX is the only sensible choice.

Types and specifications

The variants you'll see on UK merchant shelves break down like this.

For the typical question asked in forums, "is PEX-a worth the extra money over PE-RT?", the honest answer is no, not for buried domestic UFH. PEX-a's advantages (kink recovery, very high temperature ceiling) don't matter when the pipe is laid carefully on insulation, clipped, and operated at 35-50°C flow temperatures. PE-RT or PERT-AL-PERT is the value choice. Only specify PEX-a if the system designer specifies it (some heat pump suppliers do, for headroom).

Pipe centres: spacing explained

"Pipe centres" is the spacing between adjacent loops as they snake across the floor. Tighter spacing puts more pipe per square metre, more heat into the floor, and a more even surface temperature.

This is where most homeowner guides oversimplify. Spacing isn't a single number for the whole room. It varies by zone based on heat loss.

A typical kitchen extension uses 200mm centres across most of the floor and tightens to 100-150mm in a one-metre band along the patio doors and external walls. The pipe path doesn't change brand or coil; it just curves more tightly through the perimeter zone before opening out to 200mm in the centre.

If you're future-proofing for a heat pump (and from June 2025 you should be), drop the standard spacing to 150mm. Heat pumps run at lower flow temperatures (35-40°C versus 50-55°C for a gas boiler), which means lower heat output per metre of pipe, which means you need more pipe per square metre to maintain the same room temperature. The cost difference between 150mm and 200mm spacing on a 30m² extension is one extra coil (around £75) and an extra port on the manifold (£20 – £30). Trivial against the rest of the build.

Loop length: 100m maximum for 16mm

Each continuous run of pipe from the manifold flow port, around the room, and back to the return port is one "loop" or "circuit". The maximum loop length for 16mm pipe is 100m. Some forum threads quote 80m, but that figure applies to 12mm pipe. For 16mm, 100m is the practical UK industry standard, confirmed across BuildHub, Snug UFH, Uheat, and EngineerFix.

The reason for the limit is pressure drop. Water loses pressure as it flows through pipe due to friction. Beyond 100m of 16mm pipe, the friction loss is high enough that the manifold can't push enough flow through to balance against shorter loops on the same manifold. You end up with one cold loop and three hot ones.

For a 30m² kitchen extension at 200mm centres, you need ~150m of pipe. That splits into two loops of ~75m each, plus the manifold tails (the bit of pipe between manifold and the start of the loop in the floor). A single 200m coil covers it with margin.

For the same 30m² at 150mm centres (heat-pump-ready), you need ~200m of pipe across three loops of ~67m each. One 200m coil plus a short top-up coil, or two 100m coils.

How to work with it

UFH pipe arrives in tightly wound coils. The coil has memory, it wants to stay coiled, and that's the first thing you fight when laying it.

Warm the coil before laying. In cold weather (anything below 10°C), bring coils inside the warm part of the building overnight before installation day. Cold pipe is stiff, springs back hard, and is much harder to clip neatly. PERT-AL-PERT is more forgiving than pure PE-RT but still benefits from being warm.

Use a decoiler if the coil is over 100m. A pipe decoiler is a stand that holds the coil and lets it rotate as you pull pipe off. It removes the twist that builds up if you pull from a static coil. Continal, Uponor, and most brand suppliers either sell or hire decoilers. For a single 100m coil it's optional. For 200m, it saves an hour of fight.

Lay onto insulation, then clip down. UFH pipe sits on top of foil-faced PIR insulation (or specialist UFH insulation panels with castellated tops). The insulation must be specified before the pipe, the screed depth and finished floor level depend on it. Pipes are clipped at 300-500mm intervals using either:

• Plastic UFH clips fired into the PIR with a clip gun (fast, reliable, won't pull out)
• Castellated panels where the pipe pushes between studs (no clipping required; more expensive panels)
• Gripper rail or wire mesh where pipe is fixed with cable ties (older systems; rarely specified now)

Don't use staples. Standard insulation staples can damage the pipe wall. Use proper UFH clips designed for the job.

Keep pipe at least 100mm from walls. Pipes need expansion room and they need to be far enough from the wall edge that the screed can flow underneath them. The conduit (a sleeve of softer pipe) goes over the pipe at any wall penetration where it transitions from horizontal floor run to vertical riser up to the manifold. The conduit absorbs movement and prevents the pipe from being kinked at the bend.

Spiral or serpentine pattern? Serpentine (back-and-forth like a snake) is simpler to lay and works fine for most kitchens. Spiral (a square spiral inward then back out) gives a more even surface temperature because the hot supply pipe sits next to the cooler return pipe in alternating bands. For larger rooms (over 25m²) or open-plan spaces, spiral is worth the extra design effort. For a typical kitchen extension, serpentine is fine.

Photograph everything before screeding. This is the step nobody tells homeowners about and it matters for the rest of the building's life. Before the screeder arrives, photograph each loop with a tape measure visible. Mark the wall-to-pipe distance every metre or so. Save the photos with the loop numbers. When someone drills into the floor in fifteen years to fit a kitchen island, those photos are what stops them from putting a hole through a buried pipe.

Click to enlarge

Pressure testing: the step you cannot skip

This is the single most important practical instruction in this guide.

Before the screeder arrives, the pipe must be filled with water, vented of air, and pressure-tested at 6 bar for a minimum of one hour. The system stays pressurised throughout the screed pour and the initial cure. This is BS EN 1264-4 and it's not optional if you want a working system and a valid pipe warranty.

Why the test:

• A leak found before screed is poured costs nothing to fix (re-clip and re-test)
• A leak found after screed is poured but before flooring goes down costs £400 – £800 to locate and a few thousand to repair
• A leak found after flooring is down costs five figures, easily

The protocol (BS EN 1264-4 and aligned with manufacturer guidance):

1. Fill each loop with water, opening the bleed valves on the manifold to vent air. Close bleeds when water runs clean.
2. Connect a wet pressure test pump (Rothenberger RP50 or similar, hire kit, around £30 – £50/day) to the manifold drain port.
3. Pump pressure up to 2 bar. Hold 10 minutes, watching the gauge for any drop.
4. Pump up to 6 bar. Allow 15 minutes for the system to stabilise (pressure may dip slightly as the pipe expands; that's normal).
5. Top back up to 6 bar and hold for 45 minutes minimum (one hour total at 6 bar from the stabilisation start).
6. Photograph the gauge reading. Record start and end pressure, time, ambient temperature.
7. Reduce pressure to working level (3 bar) and maintain through the screed pour and the initial 24-48 hour cure.

Use water, not compressed air. Compressed air stores enormous energy and a pipe failure at 6 bar of air can fire a fitting across the room hard enough to injure. Water is incompressible, so a pipe failure just dribbles. Water also hydraulically expands the pipe properly, which a gas can't.

How much do you need

The calculation is straightforward once you know the spacing.

Pipe metres = floor area (m²) × multiplier

Where the multiplier comes from the spacing:

• 100mm centres: 10 m/m²
• 150mm centres: 6.67 m/m²
• 200mm centres: 5 m/m²
• 300mm centres: 3.3 m/m²

Add 5-10% for the manifold tails (the unheated section between the manifold and the start of the heated zone in the floor) and for any pattern adjustments.

Worked example: 30m² kitchen extension at 200mm centres (gas boiler standard)

• Base pipe: 30 × 5 = 150m
• Manifold tails (allow 5m per loop, 2 loops): 10m
• Total: 160m
• Coil to buy: one 200m coil

Two loops of ~75m each fits comfortably under the 100m per loop limit.

Worked example: 30m² kitchen extension at 150mm centres (heat pump ready)

• Base pipe: 30 × 6.67 = 200m
• Manifold tails (3 loops × 5m): 15m
• Total: 215m
• Coil to buy: one 200m coil plus a 100m coil, or two 120m coils

Three loops of ~67m each, all comfortably under the 100m limit.

Worked example: 50m² open plan extension at 200mm centres with 100mm perimeter band

• Central area (≈40m² at 200mm): 200m
• Perimeter band (≈10m² at 100mm): 100m
• Manifold tails (4 loops × 5m): 20m
• Total: 320m
• Coils to buy: two 200m coils (640m total, generous margin for any rework)

Four loops of ~80m each.

Cost and where to buy

UFH pipe pricing is consistent across UK suppliers, with three pricing tiers based on construction quality and brand premium:

• Generic PERT-AL-PERT (5-layer) is the value choice. The UFH Group sells 16mm × 100m for around £62 inc VAT and 16mm × 200m for around £140 inc VAT. WRAS-approved with 40-year warranties. This is what most independent UFH installers fit when not tied to a specific system.

• EVOH-barrier PE-RT (3-layer or 5-layer) sits in the middle. The Underfloor Heating Company's 100m × 16mm × 2mm EVOH PE-RT runs around £74 inc VAT. TIO 16mm PE-RT EVOH 3-layer × 100m is around £85 inc VAT.

• Branded retail (Warmup, Polypipe, Uponor) is the premium. Warmup PE-RT at Screwfix is £0.95/m in any length: a 100m coil is around £95, a 50m coil around £48. Convenient for click-and-collect when you need pipe immediately. Uponor Comfort Pipe Plus 16mm × 120m is around £146, premium PEX-a, fits Uponor's proprietary push-fit manifolds.

For a typical 30m² extension, the pipe cost is £140 – £160 for one 200m PERT-AL-PERT coil, roughly 5-8% of total UFH supply cost. The manifold (£200 – £500) and labour (£800 – £1,500) dominate.

Where to buy:

• The UFH Group (theufhgroup.co.uk), best value on PERT-AL-PERT generic
• The Underfloor Heating Company, good for EVOH PE-RT
• The Underfloor Heating Site, competitive on 200m coils
• Screwfix, Warmup branded, click-and-collect
• Toolstation, Wickes, Travis Perkins, Jewson, Polypipe and merchant own-brand at trade pricing if you have an account
• City Plumbing, Solfex PEX-AL-PEX trade range

Compatibility: Eurocone vs proprietary connections

The pipe end connects to the manifold via a brass compression-style fitting. There are two camps.

Eurocone (DIN 5482) is the universal European standard. A Eurocone fitting has a 24mm thread (3/4" Eurocone) and works with any 16mm × 2mm UFH pipe regardless of brand. Most independent and trade-channel manifolds (Continal, Wundatherm, Polypipe, plus most UFH Group own-brand) use Eurocone connections. This is the safest choice for a homeowner because it means you can mix brands or replace components later.

Proprietary push-fit connections are used by some major-brand systems, most notably Uponor's Q&E system and JG Speedfit's UFH manifolds. These are quick to assemble (no spanners, push-fit only) but lock you into that brand for replacement components. Mixing a proprietary manifold with another brand's pipe doesn't work; the connections don't fit.

Alternatives

There aren't many real alternatives to plastic UFH pipe in a wet system. A few worth knowing:

• Copper pipe for UFH was used in the 1990s and earlier. It works, but it's expensive, less flexible (large bend radius), and a copper installation costs three to four times the plastic equivalent for no performance gain. Effectively obsolete for new installations.

• Multi-layer composite (PEX-AL-PEX) is functionally equivalent to PERT-AL-PERT for UFH duty. The choice between the two usually comes down to which manifold system is being used and which pipe the supplier stocks.

• Electric UFH is not a pipe alternative; it's a different system entirely. Resistive cable laid in the floor, fed from the consumer unit. Cheaper to install, much more expensive to run. Suitable for small bathrooms and conservatories, not for whole-room kitchen heating in 2026 with rising electricity costs (unless paired with significant solar PV and battery storage).

• Underfloor heating mats (warm water, pre-spaced pipes on a mesh) speed up installation in conservatories or small additions. Same pipe as standard UFH, just pre-laid on a fabric backing. Expensive per square metre but quick.

For a wet UFH system in an extension, the choice is really between PERT-AL-PERT, PE-RT EVOH, and PEX-a EVOH. All three are correct answers; PERT-AL-PERT is the value-and-ease-of-installation pick.

Where you'll need this

UFH pipe shows up at three points during any extension or renovation that includes wet underfloor heating:

• Underfloor heating, pipe layout, clip-down, and pressure test before screed
• First fix plumbing, manifold installation, copper supply pipework to the manifold, system fill and bleed
• Screeding, pipe stays pressurised throughout the pour; screed depth is 35-40mm above the pipe for liquid screed or 65-75mm for sand-cement

These references point to kitchen extension tasks because that's the first project tree on buildwiz.uk, but UFH pipe is the same product whether you're fitting it in a single-storey rear extension, a loft conversion bathroom, or a garage conversion. The specifications, the pressure test, and the EVOH barrier rule don't change.

Common mistakes

Forgetting to pressure-test before the screed pour. This is the most expensive mistake on the list. By the time the screed has cured, finding and fixing a leak is a major job. The fix costs two to three times the price of the entire UFH system. Insist on a pressure test, witnessed, with photos and a signed certificate, before the screeder arrives. If your installer says "we don't usually bother", get a different installer.

Not photographing the layout. When the kitchen island gets fitted three months later, when the dishwasher needs replacing in five years, when the next homeowner wants to tile the kitchen in fifteen years, somebody will drill into the floor. Without photos showing where the pipes run, that drill bit will hit a pipe sooner or later. Take 20 photos before the screeder arrives, with a tape measure visible for scale, and save them with the build documentation.

Running pipe too close to walls. Pipes need at least 100mm clearance from walls so the screed can flow underneath. Closer than that and you get voids that crack later, plus the pipe has no expansion room. Use clip rails to maintain the setback automatically.

Using non-barrier pipe. As covered above. The £30 – £50 saving is the worst false economy in plumbing. Look for "EVOH" or "DIN 4726 compliant" or "PERT-AL-PERT" on the label and reject anything else.

Stapling instead of clipping. Standard insulation staples can pierce the pipe wall during firing or pinch it tight enough to weaken it. Use proper UFH clips fired with the right gun, or use castellated insulation panels that need no clipping at all.

Not warming the coil before laying. Cold PE-RT and PEX is stiff and springs back. Bring coils into the warm building overnight in winter. Twenty minutes of preparation saves an hour of cursing.

Mixing pipe brand with proprietary manifold. If the manifold is Uponor or JG Speedfit, the pipe must be the same brand. Eurocone manifolds accept any 16mm × 2mm pipe. Check before buying.

Designing the system yourself. Most UFH suppliers (UFH Group, Continal, Wundatherm, Snug) offer a free CAD design service when you buy the system from them. They take the floor plan, the building's heat loss calculation, and the heat source type (boiler or heat pump) and produce a layout drawing with circuit lengths, pipe centres, and zone boundaries. Use it. DIY pipe layouts on the back of an envelope produce systems that under-perform or over-supply specific zones, both of which are hard to fix once the screed is down.

Skipping the conduit at wall penetrations. Where pipe rises out of the floor screed up the wall to the manifold, it needs a softer conduit sleeve over it. The sleeve absorbs movement (the pipe expands and contracts as it heats and cools) and protects the pipe from being scored by screed or mortar at the penetration point. A small detail with a long-term consequence if missed.