Heater causes the pool pump to lose prime or shows air bubbles in the return when heater runs.

Safety first: Turn off power to both pump and heater before adjusting plumbing.

• Check heater elevation: If the Thermeau heater is significantly above water level, ensure a check valve is installed to prevent backflow and air entry.
• Inspect bypass configuration: Verify that any manual or automatic bypass is plumbed according to Thermeau diagrams to avoid excessive restriction.
• Look for suction leaks: Check pump lid o-ring, unions, and suction fittings for air leaks that become more apparent when backpressure increases.
• Evaluate exchanger restriction: If pressure rises sharply when the heater is in line, internal fouling may be restricting flow and require professional cleaning or exchanger replacement.