Safety first: Allow the system to cool before adjusting valves or pumps.

Check all circulators: Verify that both primary boiler pump and secondary system pumps are operating when there is a call for heat. Listen and feel for flow.
Inspect balancing and mixing valves: Ensure any thermostatic mixing valves or balancing valves are not nearly closed, which can cause high return temperatures.
Verify primary/secondary piping: Confirm that closely spaced tees are installed correctly and that there are no closed valves between primary and secondary loops.
Bleed air: Air pockets can reduce flow. Bleed air from high points, radiators, and air separators.
Reset boiler: After improving flow, reset the boiler and monitor supply/return temperature difference. Lochinvar condensing boilers typically operate best with a 20–30°F delta-T.

Professional help: If piping or sizing issues are suspected, a hydronic specialist should evaluate the system design.

Boiler shows F.12 and shuts down, often associated with very hot return water

Possible Causes