Installing Radiant Floor Heating

The Emitter Has a Temperature Ceiling

A radiant floor is not a heat source buried beneath the finish. The finish is the heat source, and its output is capped by what bare feet tolerate rather than by boiler capacity, tube count or how much pipe fits in the room. Comfort criteria converge on a surface around 29 °C (84 °F) in continuously occupied areas, a few degrees higher in bathrooms and wet rooms where feet are wet and dwell time is short, and higher again in a narrow perimeter band along glazing. Which figures actually bind a given job depends on the standard the jurisdiction adopts — EN 1264 and ISO 11855 across much of Europe, ANSI/ASHRAE Standard 55 for comfort criteria in North America — layered with local building regulations and whatever the floor covering manufacturer will warrant.

Translate that ceiling into watts and the rest of the design falls out of it. A surface held roughly nine degrees above a 20 °C room gives up something near 100 W/m², a little over thirty Btu per hour per square foot, radiation and free convection combined. That figure is the budget for the whole room. It does not grow because the water is hotter or the tubes are closer together; hotter water and tighter spacing change only how fast and how evenly the surface arrives at its limit.

Before a single roll comes off the trailer, compare design heat loss against available floor area — net of kitchen runs, baths, built-in wardrobes, the fridge footprint, anything that sits on the deck and traps emission. A room losing 130 W/m² across a floor of which a quarter is unusable has already failed as a floor-only scheme, and the honest moment to say so is at the drawing, not the commissioning. Adding a panel radiator or a fan coil after the screed has cured is a very different conversation, held with a client who already paid for a floor sold as sufficient.

Working Backwards From Surface to Water

With the surface temperature fixed, three dials reach it: flow temperature, spacing and depth of cover. Flow temperature does the coarse work. A well-insulated slab at 150 mm spacing under 50 mm of screed may reach its ceiling on water in the mid-thirties Celsius, while a suspended timber build-up with plates between joists wants considerably more to drive the same watts through the extra resistance in the path. Every degree of flow temperature shed is condensing range on a boiler and coefficient of performance on a heat pump, which is why the two technologies pair so well and why a needlessly hot design quietly throws the pairing away.

Spacing does the fine work, and depth mediates both. Tighter centres raise average surface temperature at a given water temperature and even out the peaks; wider centres save pipe and pressure drop but demand hotter water for the same output. Loop length caps come from pressure drop rather than heat — circulator head and the balancing you can achieve at the manifold set the practical limit, commonly in the region of 90 m for 16 mm tube and shorter for smaller bore. Confirm the figure against the tubing specification you are actually laying, referenced to ASTM F876 or its regional equivalent, and against the pump curve.

Keep the pipe run continuous from manifold to manifold. Buried joints are a category of risk that no test regime fully retires, and a leak under 60 mm of screed and a fitted kitchen is the most expensive mistake available on this trade.

Everything Under the Tube

A floor radiates in both directions and is entirely indifferent to which one you intended. Beneath a slab on grade, downward and edge losses become a serious fraction of total output when insulation is thin, discontinuous, or crushed by traffic before the pour. Three details prevent most of it: continuous insulation under the full slab area with staggered joints, an unbroken perimeter edge strip carried up to finished floor level, and no penetrations that were not on the drawing.

Edge strip earns its own attention. A heated screed grows, and a bay locked against a hard wall will relieve that movement somewhere less convenient — usually a crack running diagonally from a door reveal. Fit the strip before the pour, tape it to the insulation so it cannot slump, and trim it only after the finish goes down.

Between storeys the calculation shifts. You are not losing heat to the ground, you are heating the ceiling of the room below and slowing the response of the floor above. Some of that downward flow is recovered as useful heat if the space below is heated and occupied; none of it is if the space below is a garage, a plant room or a neighbour's dwelling. Insulation value under the tube is a design input rather than an afterthought, and it is worth confirming the assembly resistance rather than accepting a board thickness someone quoted from memory.

The Ripple Between Runs

Surface temperature over a hydronic floor is never flat. It peaks above each pipe and dips midway between, and that variation — striping — is felt underfoot long before it shows on an infrared camera. Wide spacing under a thin, conductive finish such as large-format tile produces the worst of it: warm bands and cool bands about a hand's width apart.

Depth of cover damps the ripple. Screed spreads heat laterally, so a deeper cover blurs the stripes at the cost of slower response and more thermal mass to charge. A thin overlay panel system does the opposite: it responds in minutes and stripes readily, which is why panel manufacturers specify their own spacing rather than letting the installer choose. Follow whichever set of tables the system you are laying was tested against.

Tighten centres where the loss concentrates. A perimeter band along full-height glazing, typically the first metre or so, carries a disproportionate share of the room's loss, and running the flow leg of each loop through that band first puts the hottest water where the loss is. The higher surface temperature permitted in a perimeter zone by many standards exists precisely to allow this — but it is a narrow band along the glass, not a licence to run the whole room hotter.

When the Emitter Is a Wire

Electric floors obey the identical ceiling and reach it by a different route. Output is set at the moment the cable is fixed down, because watts per metre of cable is a property of the cable and watts per square metre of floor is therefore purely a function of spacing. There is no flow temperature to trim afterwards and no balancing valve to open. Get the spacing wrong and the only remedies are lifting the finish or running the floor on a duty cycle that never lets it settle.

Cable cannot be cut or shortened to fit, so the layout has to consume the exact length supplied across the exact net area — which makes the net area survey unusually load-bearing. Measure the heated field, not the room: subtract vanity units, shower trays, the swing of a door where a mat would be crushed, and any fixed cabinetry. A remainder of half a square metre with three metres of cable left over becomes a hot band the client will feel.

Set the floor sensor in a conduit, midway between two adjacent runs, so it reads the dip rather than the peak, and leave the conduit accessible so a failed sensor can be drawn out without breaking tile. Ground-fault protection, circuit sizing and the marking of the heated area are governed by the electrical code adopted locally — NFPA 70 Article 424 covers fixed electric space-heating equipment in jurisdictions that adopt it. Record insulation resistance three times: on delivery, after fixing, and after the screed or adhesive has gone down. Two of those readings are worthless without the third.

The Screed Carries the Heat, Then the Load

Cover over the crown of the tube is a structural and a thermal number at once. Too little and the screed cracks along the pipe lines and stripes badly; too much and response time stretches into hours and the mass overshoots every mild afternoon. Sand-cement screeds generally want more cover than flow-applied calcium sulfate screeds, which are commonly laid thinner and encapsulate the pipe more completely. The screed specification, and in the UK the BS 8204 series on screeds, bases and in-situ floorings, governs the minimum — not the pipe manufacturer and not habit.

Movement joints break the floor into bays sized by the screed system, and each loop should ideally live inside a single bay. Where a pipe must cross a joint, sleeve it, and keep the crossing perpendicular. Manifold tails leaving in a dense fan are the classic hot spot: sleeve or insulate them for the first stretch so the area in front of the cabinet does not run hotter than everything the standard permits.

Curing and commissioning are two separate processes and conflating them cracks floors. The screed cures on its own schedule — the manufacturer's, which for cementitious mixes is typically counted in weeks and for anhydrite in a shorter but still specific period. Only after that does the commissioning heat cycle begin, and it exists to drive residual moisture out and prove the assembly, not to warm the building for the plasterers.

What You Lay On Top Spends the Budget

Every layer above the tube is thermal resistance between the emitter and the room, and it comes straight out of the 100 W/m² budget. Design tables for embedded systems commonly assume a floor covering resistance no greater than about 0.15 m²K/W; exceed it and either the water runs hotter or the room runs cooler, and one of those choices is not available once the flow temperature is capped by the heat source.

Tile and stone are the cooperative finishes — low resistance, high conductivity, and no moisture argument. Engineered timber is workable within the manufacturer's stated surface temperature limit, which is frequently lower than the comfort ceiling and therefore becomes the governing number for that room. Narrower boards and quarter-sawn material move less. Solid timber over heat is a specialist conversation, and the moisture content at installation matters more than the species.

Carpet is the one that surprises people, because the underlay usually carries more resistance than the carpet itself and gets specified by whoever fits it, months after the heating design was signed off. Write the maximum permitted total resistance into the specification and into the handover pack, in the units the flooring trade actually uses. A floor designed at 29 °C and finished with a thick felt underlay is not a system fault; it is a specification that escaped.

Manifold, Loops and the Balance

Loops that differ wildly in length cannot be balanced into equal surface temperatures, only into equal unhappiness. Aim to keep circuits within roughly ten percent of one another, and where a room genuinely needs a long loop and a short one, expect to throttle the short circuit hard and accept the pressure drop. Flow meters on the manifold are not a luxury — without them balancing is guesswork, and guesswork shows up as a cold corner in a bathroom the following winter.

Differential across a circuit typically sits in the five to eight kelvin band. Wider and the far end of the loop runs visibly cooler than the near end, which the client reads as a cold patch rather than as physics. Narrower demands more flow and more pump energy for no comfort gain.

Control sits on top of all of it, and the mixing arrangement — thermostatic valve, injection loop, or a heat pump running weather-compensated flow directly — must include a hard high-limit stat on the flow to the manifold. That stat is the last physical guard on the surface temperature ceiling. Resist over-zoning: a house with a thermostat and an actuator in every space spends most of the year closing circuits, short-cycling the source, and starving the loops still open. Zone by how the building is genuinely occupied and by orientation, not by door count.

Pressure, First Heat, and the Record

Pressurise the circuits before the pour and hold the pressure through it, with the gauge in plain sight of whoever is barrowing. A screeder who can see a needle drop stops immediately; a screeder who cannot, finishes the bay. Test to the pressure and duration the tubing standard and local hydronic installation code require — CSA B214 in Canada, the relevant national code elsewhere — and photograph the gauge with a timestamp at the start and end.

Photograph the layout too, comprehensively, with a tape or a marked staff in frame and dimensions taken off fixed building lines rather than off the insulation edge. Somebody will one day drill that floor for a stair balustrade or a kitchen island leg, and those photographs are the only thing standing between them and a repair through the finish.

The commissioning heat-up follows the screed manufacturer's schedule: start well below design flow temperature, raise it in daily increments to the maximum permitted, hold, then bring it back down in the same measured way. Log it. Where timber is going down afterwards, take relative humidity or moisture readings from the slab after the cycle and before the flooring contractor is anywhere near site, and be prepared to say no if the numbers are not there. Hand over a pack with the loop schedule, the flow settings, the commissioning log, the test certificates, and the covering resistance limit stated plainly.

Reading Failures Off the Surface

Almost every complaint about a radiant floor can be diagnosed by walking it barefoot, because the surface reports the fault directly. Distinct warm and cool bands mean spacing is too wide for the cover depth and finish — a design or layout error, not a control one, and no thermostat setting will fix it.

One loop cool along its return half points at a circuit that is too long, starved of flow, or holding air. Check the flow meter reading against the balance schedule first, purge the loop second, and only then start suspecting the pipe. Air collects at high points and behind manifold isolation valves that were never fully opened after the last service.

A floor sitting at its permitted maximum with the room still short of setpoint is not a commissioning problem and no amount of returning to site will resolve it. The emitter is at capacity — the loss calculation, the available area, or the covering resistance was wrong, and the fix is supplementary emission or fabric improvement.

Cupped or gapped boards over a heated floor usually mean two faults compounding: timber laid before the slab moisture came down, and a surface temperature above what the flooring manufacturer allows. Both are recorded in documents that exist before the boards go down, which is the argument for taking those readings and keeping them.