Radiant Floor Heating for Flooring Installers and Carpenters

Why this matters

A heated floor is the one floor where the fastener, the grinder and the tile chisel can all reach the heating system. Hydronic tubing sits a fraction of an inch to a couple of inches from where flooring crews work, and an electric heating mat sits in the thinset under the tile you are taking up. A cleat through a tube does not leak dramatically. It leaks slowly into the subfloor or the slab, the boiler quietly refills itself, and months later somebody is chasing a wet ceiling, a cold zone or a corroded boiler. This card is how to tell a floor is heated before you start, where the heat lives in each kind of floor, and what the boiler gauge is telling you afterward.

How to tell a floor carries radiant heat

Look before you fasten anything, and ask the homeowner directly: "Is this floor heated?" Then confirm, because homeowners who bought the house with it often do not know.

• A manifold. A pair of horizontal pipes with several smaller pipes coming off each, often with small flow gauges or caps on top, in the basement, a mechanical room or a cabinet in a wall. Each pair of small pipes is a loop running through a floor somewhere. Labels on the loops ("kitchen", "bath 2") tell you which rooms.
• A boiler, or a water heater serving heat. Radiant floors are usually fed hot water by a boiler, sometimes by a water heater or heat pump. A boiler with no baseboard radiators or cast-iron radiators anywhere in the house is feeding something you cannot see.
• A thermostat with a floor sensor. Thermostats for heated floors often show a floor temperature or have a "floor" setting. Electric floor-heat thermostats often carry a built-in GFCI test button.
• Tubing in an unfinished ceiling below. In a basement or crawlspace under the room, PEX tubing stapled to the underside of the subfloor between joists, often in aluminum plates, often with insulation under it.
• A warm floor. In heating season, a floor that is warm to the hand in a band pattern is a heated floor. An infrared camera on a running system shows the loop layout directly.

No sign, and still a slab-on-grade or a tiled bathroom in a house built with radiant elsewhere? Assume heat until someone who knows the house says otherwise.

Where the heat lives in each kind of floor

SLAB (tubing in concrete)
  finish floor / tile ..................
  concrete slab   o   o   o   o   o
                  (PEX tied to mesh, depth varies)
  insulation board under slab

STAPLE-UP (tubing under the subfloor)
  finish floor ..........................
  subfloor  ===========================
            ==[plate o]====[plate o]==  <- tube stapled
  joist |   insulation      insulation  | joist
        |   (in each joist bay)        |

ELECTRIC MAT UNDER TILE
  tile        [][][][][][][][][][][]
  thinset     ~ ~ cable ~ ~ cable ~ ~  <- mat in thinset
  backer or subfloor ==================
  floor sensor wire runs between two cables

Slab. Tubing is tied to wire mesh or rebar and poured in. Its depth depends on how the mesh was supported during the pour, which is why nobody can tell you from the surface. Grinding, scarifying, drilling for tack strip, a bathroom toilet flange relocation and a concrete anchor are the ways flooring work finds it. For carpenters, the classic hit is a new wall's bottom plate shot to the slab with powder-actuated pins every couple of feet, or anchors drilled for a stair or a kitchen island; on a staple-up floor it is a saw cutting a new opening for a stair, a floor register or a chase.

Thin slab or overpour. A thin layer of concrete or gypsum poured over a wood subfloor with tubing in it, often around an inch and a half thick. Tubing here is close to the surface by construction.

Staple-up. Tubing stapled to the underside of the subfloor, usually two runs per joist bay, often pressed into aluminum heat transfer plates. Any fastener that exits the bottom of the subfloor can reach it.

Panel systems. Grooved panels laid over the subfloor with tubing in the grooves under the finish floor. Fasteners into the floor surface land directly in the tubing layer; these floors are usually meant to be floated or glued, and the panel maker's instructions say which.

Electric mats and cables. Resistance cable woven into mesh or laid in a membrane, bonded in thinset under tile or stone. Taking up the tile with a chisel, a rotary hammer or a floor scraper cuts the cable. There is also a floor sensor wire between two runs of cable, which the thermostat needs. Electric floor heat is a circuit under NEC Article 424, as adopted, and repairing or extending it is an electrician's or the mat maker's certified installer's job, not the tile setter's improvisation.

What a cut or punctured loop looks like

Hydronic. A loop under pressure that has been punctured leaks wherever the water can go: into the subfloor and down into the joist bay insulation on a staple-up, into the slab and the soil under it on a slab. Signs come days later: a damp spot in the basement ceiling under the room, a cold strip on the floor, a manifold loop that stays cold, and the boiler pressure.

Do not pull a fastener you suspect hit a tube. A nail or cleat through a pipe often plugs most of its own hole; pulling it opens a free leak. Leave it in, mark it, and call the trade that owns the heating system.

Electric. A cut cable means the floor does not heat. The thermostat may show an error or the GFCI may trip. The owning trade checks the cable's resistance against the label the installer was supposed to leave; you do not.

Reading the boiler gauge after your work

A hydronic system is a closed loop held at a pressure set for the house. The boiler carries a gauge, often a combined pressure and temperature gauge. A common residential cold fill is somewhere around 12 to 15 psi; the relief valve on most residential boilers is set at 30 psi. Read the actual gauge, not the typical figure.

Two things make the gauge honest:

• Compare readings at the same condition. Water expands as it heats, so pressure rises when the system is hot. A gauge read cold in the morning and hot in the afternoon will differ with no leak at all. Read it before work and after work with the boiler in the same state, or read the temperature beside it and note both.
• Know whether the boiler refills itself. Most systems have a feed valve (a pressure-reducing valve) that adds water from the house supply when pressure drops. With it open, a leak does not show on the gauge at all; the system just keeps adding fresh water, which brings in oxygen that corrodes iron parts over time. With the feed valve closed, a leak shows as falling pressure.

So a pressure drop after flooring work, at the same temperature, with the feed valve closed, means the system lost water. The same gauge holding steady with the feed valve open proves nothing.

Worked example: hardwood over a staple-up floor

A flooring crew is installing 3/4 in solid hardwood, 2-1/4 in wide, in a living room 12 ft by 15 ft. The boards run the 15 ft length, across the joists. Under the room, in the unfinished basement, the crew lead sees PEX tubing stapled to the underside of the 3/4 in subfloor in aluminum plates, two runs per bay, joists at 16 in on center. The manifold label says "living room".

How much of the floor sits over tubing. Along a 15 ft (180 in) board, joists at 16 in on center give 180 / 16 = 11.25, so 11 full joist bays, and 2 runs each makes 22 tube runs crossed. With a tube outside diameter of 5/8 in (0.625 in), that is 22 x 0.625 = 13.75 in of every 180 in board directly over tubing, or 13.75 / 180 = 7.6 percent of the board's length.

How many fasteners that is. The room is 144 in wide, so 144 / 2.25 = 64 rows of boards. Fastening every 8 in along each 180 in row is 180 / 8 = 22.5, call it 22 fasteners per row, so about 64 x 22 = 1,408 fasteners. At 7.6 percent, about 107 of them land over a tube's footprint. Checked another way: 22 runs times 64 rows is 1,408 crossings, and each crossing has a 0.625 / 8 = 7.8 percent chance of catching a fastener, about 110. Either way, on the order of a hundred fasteners sit above tubing, not two or three, and that is before counting the aluminum plates around each tube.

Whether they reach. That depends on fastener length, drive angle and where it enters, so the 107 is an exposure count, not a hit count. What is fixed is that the tube sits right under 3/4 in flooring plus 3/4 in subfloor, 1.5 in from the surface, and any fastener path that exits the underside of the subfloor reaches the tube's plane.

The call. The crew does not choose the fastener and hope. The crew lead tells the homeowner the floor carries staple-up radiant and asks for the heating contractor's input before starting. The heating contractor, together with the hardwood maker's instructions for radiant subfloors, decides: a fastener length that stops in the subfloor, a glue-down, or a floated floor over an underlayment. The crew installs to that decision and writes it on the ticket.

The gauge, before and after. Before work, with the boiler cold at 7:30, the gauge reads 14 psi and the feed valve is open. The crew lead asks the homeowner, who asks the heating contractor, whether they want it closed during the job to make a leak visible; the contractor closes it. The next morning at 7:30, cold again, it reads 14 psi. Same condition, no water lost, feed valve still closed until the contractor opens it.

What would change the answer.

• The next morning's cold reading is 9 psi. A 5 psi loss at the same temperature with the feed closed means water left the loop. The crew stops fastening, checks the basement ceiling under the day's work for damp insulation, marks the spot, leaves any suspect fastener in place, and the heating contractor takes it from there.
• A slab floor instead of staple-up. The fasteners are tack strip pins or concrete anchors, and the exposure is anything that drills or grinds. Nobody can see the tubing; it takes the drawings, an infrared scan on a running system, or the heating contractor marking it.
• Tile coming up over an electric mat. There is no gauge. A homeowner keeping the mat expects it to work afterward, which usually means the tile comes up by a method chosen with the mat's installer, or the mat is accepted as lost and replaced as part of the job.

References

• NFPA 70, National Electrical Code, Article 424 (fixed electric space-heating equipment, including floor heating cables), as adopted by your jurisdiction
• The hardwood or flooring manufacturer's installation instructions for radiant subfloors
• The heating system installer's layout drawings and the boiler manufacturer's documentation
• See related: Expansion Tanks and Pressure Valves for Non-Plumbers
• See related: How to Scan a Wall Before You Cut or Drill