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I recently replaced my water heater with a tankless unit (Rheem). After installation I noticed that whenever I turned on the hot water; or when my central heating unit would turn on, the tankless unit would make 3-4 loud knocking/popping sounds. I asked the plumber who installed it and he said "no idea" [gee thanks pal].

Today I replaced the gas pressure regulator thinking that might be it. The old one was rated for 100k BTU/hr while the heater needs 180k BUT/hr [you'd think the plumber would have done this]. The new regulator is rated for 200k BTU/hr, but the water heater is still making those sounds!

I don't want to open up the unit because I really would have no idea what I could do. Does it have to do with the water line? If so, why does my central heating unit make the water heater start knocking? Does this sound familiar to anyone?

EDIT: Here is a recording of the water heater after the faucet has been turned on.

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It may be sometime as simple as air in the line. Check the Rheem manual for bleeding procedure? – Matthew Feb 5 at 23:13
@MatthewPK, PK, Do you mean air in the gas line, or air in the water line? – Nicholas Mancuso Feb 5 at 23:27
Air in the water line. – ChrisF♦ Feb 5 at 23:35
How long after the water is turned on do you hear the noise? Do you hear the noise before or after the burner fires up? Would it be possible to record the sound, post it on YouTube (or similar service), and add a link to the sound in the question? – Tester101♦ Feb 6 at 12:31
@Tester101, a few seconds after turning on hot water the knocking starts. Maybe 4-5 seconds? I will record when I get home tonight and post somewhere. It doesn't sound like banging pipes, but rather popping/clacking. – Nicholas Mancuso Feb 6 at 15:33
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