I have a gravity recirculation loop which worked for ~18 years. The recirc loop has never worked since replacing the Buderus LT160 water heater a few days ago. I made no changes to the loop, just reconnected it to the recirc port on the new heater.

The loop has a 3/4" swet swing check valve at the recirc port. I opened the valve top & the gate is working i.e. not stuck. It has never had a hole in the gate like I see recommended.

I have two questions.

Is the most likely cause of no circulating water that the loop is air-bound? I don't recall bleeding air out of the loop when it was installed the first time. Also, the loop worked instantly 18 years ago.

What is the purpose of the small brass screw which screws into the valve housing near the gate? This screw has always been fully screwed in.

Thanks,
GB

That is where the gate's pin is inserted. A proper gravity loop should NOT need bleeding, because if it did, it would eventually stop working as air built up in the "high spot". There can be several reasons for it not working and a photo might help us decide what the problem is.

You replaced a conventional "tank" water heater with the LT 160?