How to Install Underfloor Heating: A Step-by-Step Guide for Hydronic Systems

Learning how to install underfloor heating correctly is what separates a system that delivers consistent warmth for decades from one that underperforms or develops problems that are expensive to fix once the floor is closed up. This guide covers the full hydronic installation process using WBI’s above-floor panel systems: subfloor preparation, panel installation, PEX routing, manifold connection, pressure testing, and commissioning.

Before You Start: What You Need in Place

A hydronic underfloor heating installation has two components: the floor assembly, which most builders and contractors can handle, and the mechanical system, which requires a licensed plumbing or mechanical contractor. Before any floor work begins, the mechanical design must be complete.

Pre-Installation Checklist

• Room-by-room heat loss calculation completed by a qualified designer
• Heat source confirmed: boiler or air-to-water heat pump specified and sized
• Manifold location determined and wall mounting point confirmed
• Zone count and loop plan finalised
• WBI CAD panel layout received: room-by-room floor plans showing loop routing and connection points
• All materials on site before installation begins

Tools and Materials Needed

• WBI Ecowarm RadiantBoard or ThermalBoard panels (per WBI layout)
• Half-inch PEX tubing (ASTM F876/F877 rated)
• Low-VOC construction adhesive and screws or cross-staples
• Manifold, circulator pump, and mixing valve (mechanical contractor supplies)
• Pressure test pump and gauge
• Tape measure, chalk line, circular saw or jigsaw
• Perimeter expansion foam strip

Step 1: Prepare the Subfloor

The subfloor must be clean, dry, structurally sound, and flat before any panel goes down. Sweep and vacuum thoroughly. Check for protruding fasteners or warped boards and correct them.

• Flatness tolerance: 3/16 inch over a 10-foot span. Correct anything beyond this with self-leveling compound (concrete) or sistered framing (wood subfloors).
• Install perimeter expansion foam strip at the base of every wall before the first panel goes down.
• Confirm subfloor is dry. A moisture meter reading above 12 percent on wood subfloors requires remediation before panels are installed.

Step 2: Install the Radiant Panels

Lay WBI panels following the CAD layout exactly, starting from the manifold connection wall and working outward.

• Fasten each panel with low-VOC construction adhesive on the underside plus screws or cross-staples at the eight fastening points in the installation manual.
• Cross-stapling: two staples at opposing 45-degree angles at each point. Combined with adhesive, this creates a squeak-resistant bond through years of thermal cycling.
• Cut panels on a workbench, not over the subfloor. Dry-fit around obstacles before fastening.
• Mark each panel’s loop assignment on the face before tubing is installed. This simplifies labelling at the manifold later.

Step 3: Install the PEX Tubing

With all panels fastened, run PEX tubing following the loop routing in the WBI layout. Each loop starts and ends at the manifold.

• Walk tubing into panel grooves by stepping firmly. RadiantBoard’s overbite groove locks tubing without clips. ThermalBoard’s 8-inch grooves hold tubing securely once pressed in.
• Minimum bend radius for half-inch PEX is approximately three inches. Use combo panels at turns that cannot achieve this radius naturally. Never kink.
• Label each loop at the manifold end before connecting, for example “L1-Master Bath Supply”. This prevents connection errors when multiple loops look identical.
• Maximum loop length: 250 to 300 feet for half-inch PEX including home run.

Step 4: Connect to the Manifold

Manifold connection and heat source hookup must be completed by a licensed plumbing or mechanical contractor.

• Connect PEX loop ends using compression or crimp fittings rated for the system’s operating pressure and temperature.
• WBI RadiantBoard: supply water 85-105°F. ThermalBoard: effective at the lower end of this range, maximising heat pump efficiency.

Step 5: Pressure Test Before Covering

Pressure testing is the most critical verification step. Every loop must be tested before finished flooring goes down.

Step 6: Install Finished Flooring

• Tile: install cementitious backer board with a plastic slip sheet between panel and backer. Use flexible polymer-modified thinset rated for radiant heat.
• Floating floors (engineered hardwood, laminate, LVP): verify max surface temperature per manufacturer, typically 80-82°F. Do not exceed.
• For all wood flooring: the heating system must have been running for at least two weeks before wood delivery, and wood must acclimate on-site for 7-14 days before installation.

Step 7: Commission and Balance the System

• Purge each loop individually to remove air: isolate all others, open fill and drain valves, flush until water flows without bubbles.
• Balance manifold flow meters to match the design flow rate in GPM for each loop.
• Start system at 85°F supply water. Increase by no more than 5°F per day for 48-72 hours before reaching full operating temperature.

Frequently Asked Questions

Key Takeaways

• Complete the mechanical design and obtain the WBI CAD layout before any installation work begins.
• Prepare the subfloor thoroughly: clean, flat within 3/16″, structurally sound, with perimeter foam installed before the first panel.
• Fasten WBI panels with construction adhesive plus cross-staples at all eight attachment points.
• Walk PEX into groove channels following the WBI layout. Label all loops. Keep each loop within 250-300 feet including home run.
• Pressure test every loop to 60 PSI for at least 30 minutes before laying finished flooring. Document the result.
• Commission by purging air, balancing manifold flow rates, and warming up gradually at no more than 5°F per day.