Installing Under Floor Hydronic Heating

Intro

Hydronic tubing is relatively easy to install. The real work lies in the planning. Plan your system room by room, using grid paper to draw the tube spacing and the locations of the manifold stations (the place where each circuit begins). Have a professional design the layout based on your plans.

PEX tubing comes in different diameters. Make sure the flow of water through the pipes is sufficient for the room you're heating. For example, 1/2-inch tubing needs a manifold station every 300 feet; 5/8-inch tubing needs one every 450 feet.

Space the tubing lines more closely along exterior walls. This increases the heat output in the coldest areas of the room. Provide access panels to manifold stations so you can make repairs easily.

Prestart Checklist

Time
A day to run underfloor PEX lines

Tools
Drill, hole saw, staple gun, PEX cutter and crimping tool

Skills
Connecting PEX tubing, basic carpentry skills

Prep
Determine tubing size and arrangement under floor

Materials
PEX tubing, fittings as needed

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Hydronic thin-slab installation

Burying a hydronic system in 2 inches of concrete results in a heavier load than most wood-frame floors can handle. Even thin slabs, which are perfect for installing radiant heat in an addition to your home, may not be suitable in existing construction. They add 15 to 18 pounds per square foot. Thin slabs also raise the floor level, which requires raising base cabinets and toilet flanges, and shortens the floor-to-sill distances of windows.

Step 1

Plan the system so you can locate the holes for the PEX tubing as close as possible to the subfloor and to the end of the joists. Using a drill and hole saw or a Forstner bit 1/4 inch larger than the tubing (a spade bit will wear out quickly), drill all the holes in the joists before you begin threading the tubing.

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Step 2

Lay a coil of tubing on the floor at one end of the room. Walk the end of the tubing down the length of the room and thread the tubing through the first joist. Have a helper unwind the tubing from the coil and push the tubing through the holes as you walk it down the bays between the joists. Untwist the tubing from the roll so it does not kink as you thread it through the holes.

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Step 3

Staple the diffusion plates to the subfloor, spacing the plates and tubing as directed by the manufacturer. One-coil systems are generally centered between the joists. If you're working with a system that has a return line, label the inflow and outflow lines and connect the lines to a boiler.

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Installing an above-floor hydronic system

Cut 3/4-inch plywood or 1x sleepers to a width that will space the tubing consistent with the manufacturers specifications. Make sure the ends of the tubing will bend without crimping. Mark the locations of the sleepers on the subfloor and fasten the first two with 1-1/2-inch decking screws. Dry-lay a run of tubing to make sure it wont crimp, then fasten the remaining sleepers. If using a diffusion-plate system, snap and fasten the plates between the sleepers and press the tubing into the recesses of the plates. Cover the installation with plywood or thinset as recommended by the manufacturer.

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Using precut underlayment

Avoid having to cut and fasten sleepers by using plywood underlayment with channels precut in its surface. Fasten the panels to the subfloor, snap in the diffusion plates (if required), and press the tubing into the channels. Cover the installation with the proper substrate.

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Connect the Lines to a Boiler: Supplying hot water to hydronic systems

Although building codes in some localities may allow water heaters to be used for small hydronic systems, most systems require a hot water boiler or geothermal heat pump. Boiler manufacturers generally make models with and without a self-contained water heater for home use. When planning a hydronic system, make sure the output of the boiler or heat pump meets the heat-load requirements of the room.

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