I have a 10 year old WaterFunace Premier2 unit (model Model: ATH028D110NLS) with horizontal loops, and the water heater circulation pump got a pin hole in a weld this summer, so the unit shut down. I wasn't here, but the local small-town HVAC people (who don't know how to work on WaterFurnace stuff) said the pin hole was spraying water on the circuit box and shut the unit down. They removed the pump (and let the unit run with out the hot water heater bypass), and after a month of them not fixing it (but billing me for a complete job), I called a company in the big city with lots of WaterFurnace experience. They replaced the pump and everything was fine. Just a few weeks ago (after about 3 months with no problems), the unit shut off in the middle of a cold night. I checked the error lights on the unit and the 'air flow' light was blinking red. I change the return filters every 3 months and it was only 2 days until they were due for a change, so I changed them. Then I turned the circuit-breakers off to the unit and heat strips for 30 seconds and restarted the unit. It ran fine. I also changed the electrostatic filter in the plenum later that afternoon. The unit has shut off 3 more times with the same error ('air flow' light was blinking red), mostly when it was cold outside. I cycled the power as before and everything is fine. Before I call the people in the big city (1.5 hrs away) and pay for a big service visit, do you have any advice?