Hi.

I have a friend that I am trying to help out. He has an Astoria Argenta SAE1 Jun. It is tripping its 20 Amp circuit breaker. It doesn't happen until it is up to (or near to) operating temperature/pressure. When it happens, there is a single thump sound. It makes me think of water hammer. You can dispense water through the group (runs the pump, etc.) with no problem, but once it has been up to or near full temperature for a few minutes, it thumps and the breaker trips.

I had all the panels off, and was trying to locate the source of the thump, but it quit. The unit operated just fine for two days, then started tripping the breaker again. I'm wondering if it might be something to do with the system trying to do it's auto-fill or auto-flush cycle (whichever it is) that it does periodically.

Any suggestions about diagnostics to do, or most likely parts to replace, would be greatly appreciated.

Details would be nice. What kind of breaker is it popping? How old is that breaker? What else is plugged into that particular run of wiring?
You do know that the heater on that machine is a 1700W* (15.5A) unit, and that the pump is probably going to pull at least a couple/ two and a half at start-up. Lots of things could contribute, including the length of wire from the breaker, the diameter (gauge) of that wire, the number of connections between the machine and the breaker, and how well they are made, etc, etc, ad infinitum.
Get thee an ammeter, toss it around the cable, and see where it gets to when the breaker trips. If the ammeter does not exceed the numbers printed on the left side of the machine, then the problem is between the breaker and the machine. If the numbers *do* exceed what the machine says it will pull, then the problem is on the machine side. From your description, I'd take a SWAG and say that something ain't right in the pressure-stat, and you're getting some sort of hero-sized arc or draw when the contacts separate. Here's what it looks like. Take the plastic cover off ** and watch it while the temp cycles. Oh, BTW, at what pressure does it cycle off? You may want to consider making the top of it's temperature curve a bit sorter if you're at 1.2 Bar or near that. Definitely, if you're over that. Anyhoo, if you see big-O sparks that coincide with your breaker popping, you'll know why. AFTER UNPLUGGING THE UNIT!!! you can try to recondition the connections on the pressure relay, in almost the same manner that you'd do an old Ford's points.

*We are talking about the 110V unit, yeah?

**I'm assuming that you have some idea of what that kind of electricity can do to you, and the necessary precautions that you must take. If I assume in error, skip all this and take the machine to a qualified repair person. It's very, very hot under those panels, in more ways than one. Caveat caveat, caveat caveat caveat. Basically, don't be a dumb-ass and get yourself killed by things you have no business playing with, and don't come cryin' to me if you do. This warning is for anyone that read this post, not solely for the use of the OP, and my lawyer can beat up your lawyer.