A plumber is servicing a radiant floor heating system that uses PEX tubing embedded in a concrete slab. The homeowner reports inconsistent heating, with some zones remaining cold. After checking the zone valves and thermostats, they seem to be functioning correctly. What is a common issue that could cause this in a PEX radiant system?

Question: A plumber is servicing a radiant floor heating system that uses PEX tubing embedded in a concrete slab. The homeowner reports inconsistent heating, with some zones remaining cold. After checking the zone valves and thermostats, they seem to be functioning correctly. What is a common issue that could cause this in a PEX radiant system?

Answer options: ✅ Air-bound loops.

• Over-pressurization of the system.
• Excessive flow rate to all loops.
• A faulty expansion tank.

Correct answer: Air-bound loops.

Explanation: Air accumulation within the PEX tubing loops is a very common problem in hydronic radiant floor systems, causing reduced or no flow to certain zones and resulting in inconsistent heating. Over-pressurization or a faulty expansion tank would likely manifest as relief valve discharge or system pressure fluctuations rather than inconsistent heating in specific zones. Excessive flow to all loops wouldn't cause some zones to be cold.