Radiant floor hydronic heating design field guide

What radiant floor heating is and how it works

Hydronic radiant floor heating is a system that runs warm water through tubing buried in or under the floor, turning the whole floor into the heat emitter. The water gives its heat to the slab or subfloor, the floor surface warms a few degrees above room air, and that warm surface radiates heat upward into the space and into everything in it. There is no hot blast from a register. The heat comes from the floor, low and even, and the room sits warm from the feet up.

The physics is simple and it is why people who live with a good radiant floor rarely go back. A large surface running barely warmer than the room moves a lot of heat at a low temperature, because output is spread across the entire floor instead of concentrated at a few grilles. That is also why the supply water runs cool by hydronic standards, often 90 to 120°F against the 160 to 180°F a fin-tube baseboard wants.

The water has to be moved and it has to be metered. A circulator pushes the flow through the loops, sized and started per the companion hydronic pump install guide, and the flow to each loop gets set so every room gets its share, which is the balancing work covered in the companion hydronic balancing guide. This guide is the design and the install: the tubing, the method, the layout, the manifold, the water temperature, and the things that go wrong when one of those is skipped.

Radiant floor vs forced air: which is more comfortable?

Radiant wins on comfort and quiet, and forced air wins on response and dual use. That is the honest split, and the right answer depends on the building, not on which one the contractor likes selling.

A radiant floor heats evenly with no drafts, no blowing dust, and no fan noise, because nothing is moving air to carry the heat. The floor is warm underfoot, the temperature is the same at the ankle and the head instead of stratified with the hot air at the ceiling, and most people are comfortable a degree or two lower on the thermostat than they would be with forced air. There is no ductwork eating ceiling space and no return-air problem.

The tradeoff is response. A slab full of warm concrete is slow. It takes hours to come up to temperature and hours to coast down, so radiant does not chase a fast setpoint change and it does not cool a building. A house with a real cooling load still needs a second system for air conditioning, and that often means radiant heat plus a separate forced-air or ductless cooling system rather than one set of ducts doing both. Radiant heats well. It does not do everything.

The tubing: PEX and the oxygen barrier

The loop tubing is almost always PEX, cross-linked polyethylene, and for a closed heating system it has to be oxygen-barrier PEX. That barrier is not optional and it is the detail a homeowner-grade install gets wrong. Plain PEX lets oxygen diffuse straight through the pipe wall into the loop water, and that oxygen rusts every ferrous part it can reach: the cast-iron circulator body, the boiler heat exchanger, steel fittings. You will not see it for a year or two. Then the system fills with black sludge and the pump seizes.

Oxygen-barrier PEX carries an EVOH layer that stops the diffusion, and it is the standard product for radiant. The common manufacturing types are PEX-a, PEX-b, and PEX-c, which differ in how the cross-linking is done. PEX-a is the most flexible and has the most forgiving expansion-fitting connection, PEX-b is stiffer and cheaper, and all three work as radiant tubing when they carry the barrier and meet the tubing standards, commonly ASTM F876 and F877.

Loop tubing runs 3/8 in, 1/2 in, and 5/8 in. Half-inch is the workhorse for residential floors. Three-eighths shows up in thin-slab and plate work and on tight bends where the shorter loops suit the smaller pipe. Five-eighths carries more flow and goes longer per loop, which suits larger slabs. Size the tubing to the design flow and the loop length the manifold is built for, not to whatever is on the shelf.

Which radiant install method should I use?

There are four ways to put the tubing in a floor, and the choice is driven by whether you are building new or retrofitting and how much floor height you have to give up. The four are in-slab, thin-slab over a subfloor, plates above the subfloor, and staple-up below the subfloor between the joists.

New construction on grade picks itself: tie the tubing into the concrete slab and pour over it. A remodel with no slab and a finished ceiling below picks itself too: staple up from underneath. The hard calls are the middle cases, a wood-framed floor where you want better output than staple-up gives but cannot lose three inches of head height to a full thin-slab. That is where plates and thin pours compete, and the deciding factors are floor height, the finish floor going down, and the design load the room actually needs.

Each method changes the thermal mass, the response time, and the water temperature the floor needs to hit its output. More mass between the tube and the room means a slower, steadier floor and a lower water temperature. Less mass means faster response but a higher water temperature to push the same heat through. The sections below take the four in turn.

In-slab: tubing in the poured concrete

In-slab is the original radiant floor and still the best performer. The tubing is laid out across the rebar or welded wire mesh, zip-tied or clipped to it, and the concrete is poured over the top so the tube is encased in the slab. The whole slab becomes the heat emitter and the thermal store, which is why an in-slab floor runs the lowest water temperature of any method and holds heat the longest after the boiler shuts off.

Tie the tube to the steel at a spacing that keeps it down in the slab, not floating up to the surface where the trowel finds it. The pour has to come up over the tube with cover, so the tube cannot sit too high. Pressurize the loops with air or water before the pour and leave them pressurized through it, so a screed boot or a rebar chair that nicks a line shows up as a pressure drop while the concrete is still wet and fixable, not after it has cured around the leak. Concrete placement and curing is its own trade; coordinate the pour by topic with whoever owns the flatwork.

The mass is the feature and the limit. A heated slab is slow to respond and unforgiving of a deep thermostat setback, because hauling all that concrete back up to temperature takes hours. You design around steady operation and outdoor reset, not around scheduled swings.

Thin-slab: lightweight overpour on a subfloor

Thin-slab puts the tubing in a thin pour over a wood subfloor, usually gypsum-based underlayment or a lightweight concrete, commonly around 1.5 in thick. It is how you get most of the comfort and the low water temperature of a real slab on a framed floor instead of on grade. The tube is fastened to the subfloor, the pour goes over it, and the finish floor goes on top.

The catch is height and weight. You are adding an inch and a half of floor and the dead load that comes with it, so the framing has to carry it and the door jambs, stair risers, and transitions all have to account for the build-up. Gypsum underlayment is the common choice because it self-levels and pours thin, but it is softer than concrete and needs the right finish floor and sometimes a sealer, so follow the underlayment manufacturer's detail for what goes over it.

Thin-slab sits between in-slab and the dry methods on every axis. More mass and lower water temperature than plates, less mass and faster response than a full slab. For a new wood-framed floor where the height can be found, it is usually the best output for the money.

Plate systems: heat-transfer plates above the subfloor

Plate systems use aluminum heat-transfer plates with the tubing snapped into grooves, installed on top of the subfloor under the finish floor, or in panel products that combine the groove and the plate in one board. The aluminum spreads the heat from the tube across the floor instead of leaving it in a stripe over the pipe, which is the whole point of a plate: even surface temperature without the weight and height of a pour.

This is a dry system, so it responds faster than a slab and it adds little height, which makes it the common pick for a remodel that cannot lose floor height or carry a pour. The price of low mass is that the plate has to actually contact the tube and the floor above to move heat. A sloppy install with the plates loose, the tube not seated, or air gaps between the plate and the finish floor produces hot stripes and weak output, and it shows up as a floor that never quite keeps up on the cold days.

Plates run a higher water temperature than a slab to push the same output, because there is less material conducting heat to the surface. Account for that in the design, especially under a finish floor with any insulating value.

Staple-up: tubing below the subfloor between the joists

Staple-up runs the tubing under the subfloor in the joist bays, almost always with aluminum plates clipped up against the underside of the floor, and it is the retrofit method when the finished floor has to stay and there is access from below. You work from the basement or crawl space, snap plates to the subfloor, run the tube through them, and insulate under the bays to drive the heat up.

It is the lowest-mass and lowest-output of the four, and it asks the most of the water temperature. The heat has to cross the plate, the subfloor, and the finish floor before it reaches the room, so staple-up runs the highest supply temperature of any method and it struggles under a high-R finish floor like thick carpet. Run it bare under the bays with no plates and the output drops further and the response gets worse, because then you are heating the joist cavity air and hoping it conducts up.

Used right, with good plates and real insulation under the bays, staple-up heats a retrofit nicely. Used as a cheap underslung loop with no plates and no insulation, it disappoints, and that is the install that gives staple-up its mixed reputation.

Loop layout: serpentine vs spiral

The tube in a room is laid out one of two ways: serpentine, where it snakes back and forth across the room, or spiral, also called counterflow, where it coils in toward the center and back out. Both work. The difference is how the surface temperature is spread.

Serpentine is the simplest to lay and the easiest to plan, but it lays the hottest water down one side of the room and the coolest down the other, because the supply enters at one edge and gives up heat across every pass. You get a warm wall and a cooler wall. Spiral solves that by running the supply and return tubes next to each other on every pass, so a hot supply leg sits beside a cooler return leg and the surface temperature averages out even across the room. Spiral is the better pattern for a finished living space; serpentine is fine for a garage or a utility area where even-to-the-degree does not matter.

Tighten the spacing along the perimeter and under exterior walls and big windows, where the heat loss is highest. A common move is to run the supply, the hottest water, around the outside of the room first so the perimeter gets the most heat, then spiral inward. The cold comes from the edges, so that is where the tube goes closest together.

How long can a radiant floor loop be?

A 1/2 in radiant loop is commonly held to about 300 ft maximum, and that figure includes the leader tubing running to and from the manifold, so the heated field gets less. The Radiant Professionals Alliance and most tubing manufacturers publish this kind of limit, and it scales with tube size: 3/8 in loops are shorter and 5/8 in loops can run longer. The manufacturer's table for the specific tubing controls the number, not the round figure.

The limit exists because a loop is a pressure-drop problem. Push water through hundreds of feet of small tube and friction eats the head, the flow slows, and the water gives up so much heat along the way that the tail end of a too-long loop runs cool and underheats that part of the floor. Go far enough past the limit and the circulator simply cannot move useful flow through it at all.

Keep the loops close to equal length. This is the rule that makes the whole system balanceable. Loops of roughly equal length present roughly equal resistance, so the flow splits evenly across the manifold without a fight. Mix a 120 ft loop with a 290 ft loop on the same manifold and the water takes the short easy path, the long loop starves, and that room never warms. Each loop home-runs back to the manifold as its own circuit, so a large room takes several equal loops rather than one long one.

Plan the tube count from the area and the spacing, then split it into equal loops under the length limit. If a room needs 900 ft of tube at the chosen spacing, that is three or four loops near 250 ft each, not two loops over the limit.

The manifold and the loops

The manifold is the brass or composite assembly where every loop in a zone home-runs to a common supply and return. The supply side feeds warm water out to each loop and the return side collects it back, and the loop ends connect to the manifold ports with compression or push fittings. One manifold can serve a handful of loops or a dozen, depending on the assembly and the zone.

What earns the manifold its keep is the trim on it. Each loop gets a flow meter or a balancing valve so you can set and read the flow per loop, which is how unequal loops or a heavier room get dialed in and how the whole floor gets balanced, the work the companion balancing guide covers in depth. Each loop can also carry an actuator, a small electric valve head driven by a thermostat, so individual loops or rooms open and close on call. That is how you zone at the manifold without a separate pump per zone.

Mount the manifold where you can reach it, label the loops to the rooms they serve, and leave the flow meters visible. The person who balances the system, and the tech who troubleshoots a cold room three winters from now, both work from that manifold, and an unlabeled manifold buried behind drywall turns a ten-minute adjustment into an afternoon.

What water temperature for a radiant floor?

Radiant floors run low supply water temperature, commonly in the 90 to 120°F range, against the 160 to 180°F a baseboard or fin-tube system wants. The exact number is set by the floor covering, the tube spacing, the install method, and the room's heat loss, with the design calculation and manufacturer data controlling it, not a single rule-of-thumb temperature.

Lower is the goal, within reason. The cooler you can run the water and still meet the load, the more efficient the heat source and the gentler the floor. A bare or tiled slab with close tube spacing and a low heat loss might hold the room on 95 to 105°F water. The same room with a high-R finish floor, wider spacing, or a leaky envelope needs hotter water to push the same heat through, sometimes up near 120°F or more, and at some point the floor surface temperature limit caps what you can deliver no matter how hot the water gets.

The water temperature and the floor surface temperature are not the same thing, and people conflate them. The water can be 110°F while the floor surface sits at 82°F, because the slab and the covering drop the temperature between the tube and the room. You design to a target floor surface temperature for comfort and let the water temperature be whatever it takes to get there.

Mixing and temperature control

A heat source usually makes water hotter than a radiant floor wants, so something has to drop the boiler-temperature water down to the radiant supply temperature. That something is a mixing device: a thermostatic or motorized mixing valve, a three-way or four-way valve, or an injection-mixing arrangement that meters a little hot water into the cooler radiant loop. It blends hot supply with cooler loop return to hold the radiant supply at its low setpoint.

Mixing protects two things at once. It protects the floor, because feeding 170°F water straight into a slab can crack the finish, scorch a wood floor, or run the surface past the comfort limit. And on a conventional non-condensing boiler it protects the boiler, because a mixing valve or a four-way valve keeps the boiler return temperature up while still sending cool water to the floor. A non-condensing boiler return should generally stay no lower than about 140°F to avoid sustained flue-gas condensation that corrodes the heat exchanger.

Pair the mixing control with outdoor reset where you can. Reset slides the radiant supply temperature down as it warms up outside, so the floor runs cooler and steadier in mild weather and only sees its full design temperature on the coldest days. That is what makes a heavy floor feel responsive without ever asking it to swing.

The heat source: condensing boiler and heat pumps

Radiant's low return water temperature is the ideal feed for a condensing boiler, and the two belong together. A condensing boiler only reaches its high efficiency when the return water is cool enough to pull the latent heat out of the flue gas by condensing it, generally with return water below roughly 130 to 140°F. A radiant floor returning water in the 80 to 100°F range keeps the boiler condensing nearly all the time, which is the most efficient way a gas boiler runs. Sizing and piping the boiler itself is its own job; size it to the heat loss by topic, not to the floor area.

The same low temperature suits a heat pump. An air-to-water or ground-source heat pump makes heat most efficiently when it does not have to lift the water temperature high, and radiant's low supply target is a good match, which is why geothermal and air-to-water systems pair well with radiant floors. A heat pump feeding a high-temperature baseboard struggles; feeding a low-temperature radiant floor, it works in its comfortable range.

The pattern across all of them is the same. The lower the water temperature the floor needs, the better the heat source performs, which is one more reason to design the floor for low water: good covering choices, close spacing, and real under-insulation all let you run cooler and let the boiler or heat pump do its best work.

Floor coverings and the design

The floor covering is part of the heating system, and it drives the water temperature more than almost any other choice. Heat from the tube has to pass through the covering to reach the room, so a covering with insulating value, a high R-value, throttles the output and forces the water hotter to compensate.

Tile, stone, and bare or polished concrete are the best radiant coverings, with low R-value and high conductivity, so they let the floor put out the most heat at the lowest water temperature. Thin engineered wood and many resilient floors are a reasonable middle. The problem child is carpet, especially thick carpet over a thick pad, which can insulate the floor so well that the system cannot deliver its design output no matter how hot you run the water. If carpet is going down, it has to be in the design from the start: thinner pad, close tube spacing, and a higher design water temperature, or the room runs cold.

Design to the actual finish floor, not a generic floor. A slab designed for tile and then carpeted after the fact is a callback waiting to happen, because the R-value that nobody accounted for swallowed the output. Get the covering decided before the water temperature is set, and if it changes, the design changes with it.

Insulation under the radiant

Insulation under the tubing is not an upgrade. It is part of the system, and a radiant floor built without it leaks heat in the wrong direction and never works right. Heat radiates from the tube in all directions. The only thing that makes it go up into the room instead of down into the ground or the basement is the insulation below the tube.

Under a slab on grade, that means rigid foam under the whole slab and around the perimeter edge, because the slab edge is where heat escapes fastest to the outside. Skip the edge insulation and you heat the dirt at the foundation and watch the perimeter rooms run cold. Under a thin-slab or plate floor on a framed floor, you insulate the joist bay below. Under a staple-up, you insulate the bay under the tube so the heat is driven up through the floor and not down into the basement.

The number to carry is that without under-insulation a real chunk of the heat you paid to make goes the wrong way, and the colder the space below, the worse it is. Insulate to the design R-value for the assembly and the climate, and treat slab-edge insulation as non-negotiable on a heated slab.

How much heat can a radiant floor put out?

A radiant floor's output is capped by how warm you can make the floor surface, and the comfort and safety limit on floor surface temperature is generally about 85°F, with around 80°F the routine ceiling for living spaces. That cap, not the boiler, is the real limit on output. A floor cannot deliver more heat than a roughly 85°F surface will radiate into a 70°F room, no matter how hot the water in the tube.

The 85°F figure is human physiology, not equipment. A surface much warmer than skin-comfortable underfoot starts to feel hot and the body reads it as overheating, so the industry holds floors at or below that line. Working from the covering and the surface limit, a floor commonly delivers somewhere in the range of about 20 to 35 BTU per hour per square foot, with tile and bare slab at the top of that band and carpet at the bottom. Use the design calculation and manufacturer output charts for the real number.

Here is the design consequence people miss. If a room's heat loss per square foot is higher than the floor can deliver at its surface limit, radiant alone will not hold that room, and no amount of hotter water fixes it because the floor surface is already capped. A wall of glass, a sunroom, or a poorly insulated room can out-lose what the floor can put out, and that room needs supplemental heat or a better envelope. Check the load against the floor's capacity before you promise radiant will carry it.

Response, controls, and outdoor reset

A radiant floor is slow on purpose, and the controls have to respect the mass instead of fighting it. A heated slab can take hours to move the room temperature, so a thermostat strategy built for forced air, with deep setbacks and fast recovery, works against radiant and leaves the room cold for half the morning while the slab catches up.

Do not deep-setback a heavy radiant floor. Pulling the slab down 8°F overnight means hauling all that concrete back up in the morning, which it cannot do quickly, so you either give up the savings or sit cold waiting. A shallow setback or a steady setpoint suits the mass better. Outdoor reset is the control that makes radiant feel right: it reads outdoor temperature and slides the supply water temperature to match the heat loss, so the floor runs a gentle, near-constant warmth that tracks the weather instead of cycling hard.

Zoning and thermostats live at the manifold through the loop actuators, so a room can call for its own loops without a dedicated pump. Keep the control logic simple. A floor that runs steady on reset with modest zoning beats a floor chasing aggressive schedules every time, because the mass that makes radiant comfortable is the same mass that punishes fast changes.

Zoning the radiant floor

Radiant zones at the manifold, loop by loop or room by room, using the loop actuators driven by thermostats. A single circulator and one mixing temperature can serve many zones when each zone's loops open and close on their own call, which is cheaper and simpler than a pump per zone for most homes.

Zone to how the building is actually used, not to every room reflexively. A bedroom wing that wants to run cooler than the living space is a real zone. A bathroom that wants a warmer floor is a real zone. Splitting every room onto its own thermostat adds cost and controls complexity for comfort gains nobody notices, and it can starve the system if too few loops are ever open at once for the circulator to behave.

When you zone, the flow has to still balance across the loops that are open, which ties back to keeping loops equal and setting the flow meters at the manifold. The companion balancing guide covers proving that the flow lands where the design put it. Zoning decides which loops are live; balancing decides how much water each one gets.

Electric radiant vs hydronic

Electric radiant, a resistance heating mat or cable under the floor, is the other way to heat a floor, and it is a different tool for a different job. It heats fast, installs thin, and needs no boiler, pump, manifold, or tubing, which makes it the common pick for a single bathroom floor or a small area where running a hydronic loop is not worth it.

Where it loses is operating cost and capacity. Electric resistance heat costs what the electricity costs, with no efficiency multiplier, so heating a whole house with electric floor mats runs the meter hard compared to a hydronic floor on a boiler or heat pump. For warming a bathroom tile floor so it is not cold underfoot in the morning, electric is the right call. For heating a building, hydronic is the system, and the rest of this guide is about hydronic. Electric mat install is its own topic with its own GFCI and load rules; size and protect it per the manufacturer and the electrical code.

Snow melt: the radiant slab variant

Snow melt is radiant tubing in an exterior slab, a driveway, walk, or ramp, run to melt snow and ice instead of to heat a room. It is the same idea, tubing in concrete with warm fluid moving through it, but the design numbers are different and bigger. Snow melt slabs run higher output and higher fluid temperatures than a comfort floor, because they are fighting outdoor cold, wind, and the snow load, not holding a 70°F room.

Two things separate it from interior radiant. The fluid is a glycol antifreeze mix, not plain water, because the slab is outdoors and will freeze, and the controls usually include a slab sensor or a snow sensor so the system fires on weather rather than on a thermostat. Edge and under-slab insulation matter even more, because an exterior slab loses heat to frozen ground on every side. Size and design snow melt to its own load by topic; it is related to floor heating but it is not the same calculation.

Commissioning: purge, pressure test, and start

A radiant system lives or dies on getting the air out, and commissioning is where that happens. Air trapped in a loop blocks flow the way a vapor lock blocks a fuel line, so a loop full of air passes water nowhere and the room over it stays cold while every other room heats. The first job at startup is purging each loop, one at a time, forcing water through it with a purge cart or the fill valves at enough velocity to carry the air out to the manifold and the air separator.

Pressure test before you ever heat it, and on an in-slab job, before the pour and through it. Hold the loops at a test pressure and watch the gauge: a drop means a leak, and you find it while it is fixable. After the pour, retest before covering anything. Once the system holds pressure and the air is purged, set the flow at the manifold so each loop gets its design share, which is the balancing work in the companion guide, then bring the water temperature up gradually rather than slamming a cold slab with hot water.

Start it slow and watch the surface come up over hours, not minutes. Check that every loop warms, that no room lags, and that the mixing control is holding the supply at setpoint and keeping the boiler return where it belongs. A radiant system that was purged, tested, balanced, and started gently runs quietly for decades. One that was filled and fired in an afternoon generates the callbacks.

Common mistakes

• Using non-barrier PEX on a closed system, which lets oxygen rust the pump, boiler, and steel fittings into sludge.
• Loops too long past the tube's limit, so the tail end runs cool and the room underheats.
• Unequal loop lengths on one manifold, so water takes the short loops and the long loop starves.
• Setting the water temperature too high, scorching a finish floor or running the surface past the comfort limit.
• No under-slab or under-floor insulation, so a large share of the heat goes down instead of up.
• Ignoring the floor covering's R-value, then carpeting a floor designed for tile and watching it run cold.
• Deep thermostat setbacks on a heavy slab that cannot recover fast enough, leaving the room cold for hours.
• Firing the system without purging the air, so an air-locked loop leaves one room cold while the rest heat.
• No mixing or reset, so boiler-temperature water hits the floor and the boiler return runs cold enough to condense.

Field checklist

What to document

The radiant record is what lets the next person warm a cold room or troubleshoot a leak without tearing the floor up guessing. A buried system you cannot see is only as good as the drawing of where the tube went and what it was set to.

Capture the install method, the tubing type and size, each loop's length and the room it serves, the manifold location and loop labeling, the design and balanced flow per loop, the design supply water temperature and the target floor surface temperature, the mixing and reset settings, the floor covering each zone was designed for, the under-insulation R-value, and the pressure-test and purge records. A loop map tied to the manifold ports is the single most useful page in the file.

Large-area and commercial radiant

Radiant scales up well to big open floors, and warehouses, shops, gyms, and aircraft hangars are good fits because the slab is already there and the ceilings are high. In a tall space, forced air dumps heat at the ceiling where nobody is; radiant heats the floor and the people and equipment on it, which is where the heat is wanted, and it does it without blowing dust around an open floor.

The design is the same physics at a larger scale, with more loops, more manifolds, and more zones, and the loop length and equal-loop rules still govern. A large slab gets divided into many equal loops fed from several manifolds, each manifold balanced like its own system. Data centers and other process spaces sometimes use radiant or in-slab loops as part of the thermal design, but those are specialized loads with their own constraints; design large-area and process radiant to the project engineering, not to a residential rule of thumb.

Standards and references

Radiant design draws on a few bodies, and the right one depends on the question. The Radiant Professionals Alliance and IAPMO publish radiant design and installation guidance, including the loop-length and design practices the trade uses. ASHRAE handbooks cover the heat-transfer and comfort side, including floor surface temperature limits and the radiant design method. The tubing standards, commonly ASTM F876 and F877 for PEX, govern the pipe itself, with the oxygen-barrier performance tied to standards such as DIN 4726.

The numbers in this guide are the common design figures, not mandates: the roughly 300 ft max loop for 1/2 in tube, the 90 to 120°F supply range, the 80 to 85°F floor surface limit, the output band per square foot. The tubing and manifold manufacturer's tables and the project's heat-loss calculation control the actual values, and they vary with the product, the covering, the spacing, and the climate. Where a boiler return temperature, a glycol concentration, or a slab detail is involved, follow the boiler, fluid, and concrete manufacturer instructions by topic.

And the install has to meet the adopted mechanical and plumbing code edition with any local amendments, which covers the pressure testing, the materials, and the heat-source connection. Confirm the adopted edition and the manufacturer listing before you commit a number to a submittal.

Units and terms

Radiant work mixes plumbing, heating, and concrete vocabulary, so the same part can carry different names across a drawing set and a manufacturer sheet.

Supply and return water temperatures are given in °F in North American work and °C in metric sources. Output is BTU per hour per square foot, sometimes written BTU/hr/ft2, or watts per square meter in metric. Tube size is the nominal inside diameter in inches, 3/8, 1/2, 5/8. Flow per loop is gallons per minute, GPM. The oxygen barrier is the EVOH layer in the pipe wall.

Oxygen-barrier PEX

PEX tubing with an EVOH layer that blocks oxygen diffusion, required on closed systems to protect ferrous parts from rust

Loop / circuit

One continuous run of tube from the manifold supply, through the floor, and back to the manifold return

Manifold

The supply and return assembly where all loops in a zone connect, carrying flow meters and actuators

Supply water temperature

The temperature of the water sent to the loops, commonly 90 to 120°F for radiant, set by the design

Mixing / injection

Blending hot source water with cooler loop return to hold the low radiant supply temperature

Outdoor reset

Control that lowers the supply water temperature as outdoor temperature rises, so the floor runs steady

Floor surface temperature

The temperature of the finished floor surface, held to about 80 to 85°F for comfort, which caps output

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