Laying underfloor heating pipes

Laying the pipe itself is only part of the work. A good result depends on a well prepared base, the right order of layers and a thorough test before anything is covered.

The layers under the pipes

Thermal insulation goes down first on a flat, clean base. This is usually EPS polystyrene with a compressive strength rated for floors. Over the ground or above an unheated cellar the insulation is usually much thicker than between two heated storeys. Otherwise a share of the heat would simply leak downwards. An edge insulation strip is then fixed around the perimeter of every room, around columns and at door frames. It separates the screed from the walls so the screed can expand as it warms up without cracking.

Depending on the chosen system, the insulation is then covered with a film, a system panel with raised nubs, or clip rails that hold the pipe in place.

Laying the circuits

The pipe is unrolled straight from the coil, ideally with a pipe uncoiler, so it does not twist or kink. Each circuit starts at the manifold, runs through the room according to the plan and returns to the manifold. The installer keeps to the pipe spacing set in the design, usually somewhere around 10 to 20 cm. Along external walls with windows a denser edge zone is often laid, because that is where cold from the glazing is felt most.

There are two basic layouts:

• Spiral (snail), where the flow and return runs sit side by side and alternate, so the floor temperature is more even. This is the most common layout in living spaces today
• Serpentine (meander), where the pipe runs back and forth in parallel strips. It is simpler for narrow and small areas, but the floor is warmer at the start of the circuit than at the end

Manifold, sleeves and labelling

Many pipes run close together near the manifold. Here they are usually laid in protective sleeves or with insulating sleeves so the floor around the manifold does not overheat. Sleeves are also used wherever a pipe crosses a movement joint in the screed or passes under a threshold between rooms. Each circuit is labelled at the manifold, so it is clear which pair of pipes belongs to which room.

Pressure test and pouring the screed

Before anything is covered, the whole system is filled with water, bled of air and pressurised. The test pressure and duration are set by the system manufacturer and the design. If the gauge shows no drop, the result is recorded in a test report. The screed is then poured while the pipes stay under pressure. If a pipe gets damaged during the pour, you see it straight away, not after the tiles are down.

Underfloor heating is not all the same. The system affects how quickly the work goes, how high the floor build-up ends up and how well the pipe stays put during the screed pour.

Wet systems in screed

• Tacker system: insulation boards with a film printed with a grid, and the pipe is fixed with plastic staples using a tacker tool. It is quick and copes well with irregular room shapes. If the stapling is poor, though, the pipe can float up in liquid anhydrite screed.
• Nub panel (castellated system panel): the pipe is pressed between the raised nubs and holds by itself. Spacing is kept almost automatically and the pipe cannot float. The panel is stiffer, and on complicated floor plans there is more cutting.
• Clip rails: plastic rails with grooves are fixed to the insulation and the pipe clips into them. These are mostly used on large, simple areas.

Dry and low-profile systems

In renovations there is often not enough height for a classic build-up with screed. That is where dry systems come in. The pipe sits in grooves in insulation boards or gypsum fibre boards fitted with aluminium spreader plates that distribute the heat, and dry flooring boards go on top. Other options are thin-layer systems with smaller diameter pipes, or milling grooves into an existing sound screed. These solutions are light and low, but they need precise work and they react faster to temperature changes.

Cement or anhydrite screed

Cement screed tolerates moisture well and is suitable for bathrooms too. It is usually laid at least around 45 mm above the pipe. Liquid anhydrite (calcium sulphate) screed wraps around the pipe completely, conducts heat better and, according to the manufacturer, can often be thinner. It is more sensitive to moisture, though, and its residual moisture must be checked carefully before tiling. With anhydrite it is also essential that the pipe is firmly fixed down.

Only pipes made for underfloor heating with an oxygen barrier belong in the floor. Without the barrier, oxygen gets into the water and gradually causes corrosion in metal parts of the boiler, pump or manifold, along with sludge build-up.

• PE-X (cross-linked polyethylene): very common and resistant to heat and pressure. The variants PE-Xa, PE-Xb and PE-Xc differ in how they are made. According to manufacturers, a kinked PE-Xa pipe can often be restored with a hot air gun.
• PE-RT: more flexible and easier to bend, popular in low-temperature systems.
• Multilayer pipe PE-RT/Al/PE-RT: the aluminium layer holds its shape, so the pipe does not spring back after bending. It suits layouts with many bends but is stiffer to work with.

The most common sizes are 16 × 2 mm and 17 × 2 mm, with smaller sizes in thin-layer systems. A single circuit has a length limit. With 16 mm pipe it usually does not go beyond roughly 100 m, but the exact figure comes from the manufacturer and the design. An overly long circuit has high resistance, too little water flows through it and the room does not warm up properly.

Most problems start when the heating installers turn up on a site that is not ready for pipe laying. Before work begins, the following should be in place:

• A design or at least a calculation: heat losses for each room, pipe spacing, number and length of circuits. Without it the pipes are laid by eye, and balancing the system later becomes almost impossible
• Windows and plastering finished: internal plastering should be done so nobody walks over insulation and pipes with scaffolding and a mixer
• Services in the floor in place: electrical cables, water and drainage in the floor must be finished and brought level, ideally within a levelling layer or the insulation layer
• A flat, clean base: significant unevenness in the slab or subfloor must be levelled beforehand, otherwise the insulation will not sit properly and the screed will vary in thickness
• Manifold cabinet and manifold fitted: at a clear height and position, so pipes can be brought to it without crossing
• Floor heights agreed: the finished floor level, the thickness of the floor covering and how it meets thresholds and doors
• A furniture plan: where the kitchen units, bath and built-in wardrobes will go. Pipes are usually not laid under fixed furniture

In a flat, it is worth speaking to the building management or the owners' association before changing the heating. Connecting underfloor heating to a shared central heating system can affect the balancing of the whole riser system and the heat metering, and a deeper floor build-up adds load to the slab. If the pipe laying is part of wider building work, check with your local building authority what the local building regulations require in your case. Do not assume that no approval or notification is needed.

The effort involved varies a lot, even for the same floor area. This explains why two quotes for the "same" house can differ noticeably.

Easier

• Large open spaces with simple shapes, where circuits run without interruption
• Wider pipe spacing in rooms with low heat losses
• A nub panel system, which makes keeping the spacing much faster
• A prepared, flat and clean base and good access to the floor
• A clear design with a drawing of the circuits, so nothing has to be worked out on site

Harder

• Many small rooms, corridors, bathrooms with recesses and sloping walls
• Dense spacing and edge zones, which mean more pipe and more bends
• Renovations with a low build-up, dry systems or milling into old screed
• Other trades' services crossing the floor
• Working in the cold, when the pipe stiffens and is harder to bend
• Rooms far from the manifold and long feed runs

Underfloor heating is teamwork. The pipes sit between the work of the builders and the floor layers, and any lack of coordination ends up affecting them.

• Electrician: runs cables to the room thermostats and to the controls at the manifold. The valve actuators and controls are wired in, so this is work for a qualified electrician. A forgotten conduit for a thermostat cable usually means chasing out the wall later.
• Screed contractor: needs to know where the movement joints will be. These are usually planned for larger areas, in doorways and on irregular shapes, and they must match the way the circuits are divided.
• Floor layer and tiler: the floor covering must be suitable for underfloor heating. Tiles conduct heat well, timber and vinyl floors need to be chosen with their thermal resistance and the manufacturer's advice in mind, and thick carpets slow the heating down considerably.
• Heat source installer: a low-temperature circuit needs a mixing unit or a heat source with a low flow temperature, such as a heat pump or a condensing boiler. If work is done on a gas boiler, it must be carried out by a qualified, registered gas engineer.

In practice it pays to photograph every room before the screed is poured, with a tape measure laid on the floor. Later, when someone drills for a door frame, a door stop or to fix a toilet, it is possible to see where the pipes run.

• Kinked or crushed pipe: the circuit has weak flow and the room stays cold. Under screed this is very hard to fix.
• Missing edge strip: the screed pushes against the wall as it heats and cracks, and footstep noise can carry through.
• Circuits of very different lengths without a design: short circuits overheat and long ones never get warm enough. Balancing at the manifold only partly makes up for it.
• Pipes too close to the wall or under the kitchen units: heat goes into areas where nobody spends time, and food or furniture can get too warm.
• Floating pipe in anhydrite screed: a poorly fixed pipe rises towards the surface, the cover above it is too thin and the floor can crack over it.
• No pressure test, or pouring without pressure: damage from a wheelbarrow, shovel or boots is only found after the floor is finished.
• Missing sleeves at movement joints: movement in the screed stresses the pipe and can damage it over time.
• Unlabelled circuits: during balancing and servicing it becomes guesswork which circuit belongs to which room.
• Heating up too fast: running the heating at full power into fresh screed causes cracks and loose tiles.

Laying the pipes is usually the quickest part of the whole underfloor heating job. An experienced team will generally finish a typical family house in one to a few days, depending on the area, the layout and the system. Dry systems and milling take longer.

What follows takes much more time. The screed has to cure, and only then is the heating started gradually according to a heat-up schedule. With cement screed the heat-up usually begins after roughly three weeks, with anhydrite earlier, often after about a week, but always according to the screed manufacturer's instructions. The water temperature is raised in small steps, and the residual moisture of the screed is measured before the floor covering goes down. Anyone who wants tiles two weeks after the pour is risking cracks and loose tiles.

A handy and patient homeowner can lay pipes in a nub panel system, as long as there is a design with a drawing of the circuits. Leave the connection to the heat source, the electrical wiring of the controls and the balancing to professionals. If the heat source is a gas boiler, only a qualified, registered gas engineer may work on it. Have the pressure test confirmed by a heating installer.

1. Check that the base is flat, dry and clean and that all services in the floor are finished.
2. Fix the edge insulation strip around the perimeter of the rooms, around columns and at door frames.
3. Lay the insulation and the nub panel according to the manufacturer's instructions, and clip the joints together firmly.
4. Mark the route of the circuits on the panel according to the design, including edge zones and areas without pipes.
5. Connect the start of the pipe to the flow side of the manifold and unroll the pipe from an uncoiler without twisting it.
6. Press the pipe between the nubs, bend it smoothly and keep to the manufacturer's minimum bending radius.
7. Run the pipes in sleeves near the manifold and across movement joints.
8. Connect the end of the circuit to the return side, label the circuit and note its length from the markings on the pipe.
9. Fill the system, bleed it and carry out the pressure test with the heating installer, recording the result in a test report.
10. Photograph every room with a tape measure and keep the pipes under pressure while the screed is poured.

Never join pipes within the floor unless there is no other option. If a pipe is damaged, ask the heating installer whether a press fitting approved for embedding in screed is available for that type of pipe.

Before the screed is poured is the last chance to check everything and fix it without breaking anything out. At handover, go through these points:

• The edge insulation strip runs around the whole perimeter and at all door frames and columns
• The insulation sits firmly on the base, does not move and has no gaps
• Pipe spacing and edge zones match the design
• The pipes are not kinked, crushed or damaged and there are no unnecessary joints in the floor
• No pipes run under the kitchen units or fixed furniture where the design excludes them
• Pipes run in sleeves near the manifold and across movement joints
• The circuits are labelled at the manifold and you have a list of rooms with circuit lengths
• You have a pressure test report showing the pressure, duration and a signature
• The system stays under pressure while the screed is poured
• Conduits for thermostat and control cables are in place
• You have photos of every room with measurements visible
• You have received the recommended screed heat-up schedule from the heating installer or screed manufacturer