One of the first questions people ask when planning a radiant floor heating system is surprisingly simple: How much PEX tubing do I need?
The quick answer is that the amount of PEX depends largely on your heated square footage and the distance between each run of tubing.
For many hydronics projects, a good starting point is:
This means a 1,000 sq. ft. concrete slab with PEX spaced 12 inches apart will require roughly 1,000 feet of active tubing before accounting for the additional tubing needed to travel from the heated area back to the mechanical system.
A 1,000 sq. ft. wood framed floor using 8 inch spacing would require approximately 1,500 sq. ft. Easy enough?
Mostly! There are a few important details that can change those numbers considerably.
Why does PEX Spacing Matter?
PEX spacing refers to the disatnce from the center of one tubing run to the center of the next. You'll usually see it written as "on center" or "O.C.".
So:
12" OC means approximately one tubing run every foot.
8" OC means the runs are closer together.
6" OC puts twice as much tubing into the same floor area as 12-inch spacing.
Closer spacing allows heat to be distributed more evenly across the surface and can allow a system to deliver the required heat using lower water temperatures. Industry suggests that decreasing tubing spacing in a concrete radiant slab can substantially reduce the water temperature required to produce the same BTU output.
But, closer spacing also means more tubing.
That's why the first question shouldn't be simply "How big is my floor?" but instead should be "What kind of floor am I heating?".
How Much PEX Do You Need in a Concrete Slab?
For a typical hydronics concrete installation, we generally work with 12 inch OC PEX spacing. That gives us a wonderfully simple rule of thumb.
Industry guidance also commonly treats 12 inches as an upper-end spacing for heated concrete slabs. They recommend that tubing spacing in a heated slab not exceed 12 inches in applications designed for good low-temperature performance.
Why does concrete allow for a wider spacing compared to other floor choices?
Concrete is very good at spreading heat. As warm water travels through the PEX, the surrounding slab absorbs that energy and distributes it outward. Instead of only heating directly above the tubing, the concrete itself becomes a large radiant heating surface.
The thermal mass is one of the reasons radiant concrete floors work so well. It also means you usually don't have to place tubing as closely together as you would in some wood-floor applications.
How much PEX do you need under a wood floor?
Once you change to wood, the equation changes too.
Unlike concrete, a wood-framed floor doesn't provide a large thermal mass to spread the heat. Instead, a properly designed underfloor system often uses aluminum heat-transfer plates to spread the heat from the PEX across a larger portion of the floor.
For HUG installations, we typically use 8 inch OC. That transfers to 1,000 sq. ft. x 1.5 = 1,500 feet of PEX.
It's recommended to place tubing no more than 8 inches for underfloor tube and plate systems intended to work effectively with lower water temperatures. This is also where insulation and heat-transfer plates become extremly important.
The goal isn't merely to make the PEX hot. It's to encourage as much of that heat as possible to travel up through the floor and into your room.
Active PEX Isn't the Same as Total PEX
The previous calculations estimate the tubing actually covering the heated area. Your tubing still needs to get to and from that area. Imagine a radiant loop covering a bedroom at the far end of your house. The tubing doesn't magically start at the bedroom door. Nope, it must travel from the distribution system (your HUG Hydronics Tank, for instance), make it's heating pattern, and then return. Those supply and return portions are sometimes referred to as leaders.
Industry guidance specifically instructs designers to calculate the active tubing based on spacing and then add the leader distances to determine total loop length, and, in turn, total PEX needed for the project.
What About Snowmelt?
Now things get interesting.
A driveway may look like a giant radiant concrete floor, a big ol' heat sink, but heating a driveway and heating your living room are very different jobs, indeed.
Inside your house, radiant heat is generally trying to replace heat that slowly escapes from an insulated building. A snowmelt system may be asked to take an outdoor slab that's below freezing, warm its surface, melt any falling snow, and counteract cold air, wind, frozen ground, and moisture, all at once, without much insulation to help.
The required heat output can be far higher than ordinary indoor floor heating. For that reason, snowmelt tubing is commonly installed anywhere from six to 12 inches on center, depending on the design load and required performance (which is mostly determined by climate zone and size of the project). Industry recommends spacing withing six to twelve inch range and notes that critical areas such as steps and emergency access areas should not exceed six inch spacing.
Why Not Just Put PEX Every Six Inches Everywhere?
If closer tubing spacing can improve heat distribution, why not simply install twice as much tubing in every radiant project?
Well, it's a bit of a balancing act.
More tubing adds materials, installation time, loops, fittings, pumps, and overall system complexity. If a properly insulated concrete slab can satisfy the building's heating load with 12 inch spacing, doubling the amount of PEX doesn't automatically make the house twice as comfortable.
There can, however, be applications where closer spacing makes sense. Higher heat-loss spaces, unusual floor coverings, low-temperature heat sources, thin slabs, or specialized applications may benefit from different spacing.
That's why the "one foot of PEX per square foot" is a useful estimating shortcut, but not a universal designing rule.