Meant to say, the pop was more I guess like a bang. A loud bang. Like, metal pipe slamming into something. Not a pop like.. something inflated and popped..

My guess is water hammer, and that the expansion tank could be to blame, either has failed or lost its prime. The pressuretrol you pictured is an inline valve that controls your water main water pressure. That is why you dont see an inlet and outlet. If you took off the insulation you will see it is connected to the valve. Does the pressure indicator fluctuate? Perhaps the pressuretrol has gone bad if it fluctuates a lot.

The honeywell device is the pressure switch that controls your (jet) pump outside. The tank is the pressure tank that maintains the water pressure inside the house. As the tank drains the pressure drops. When it reaches a low limit, it activates the pump which pumps water back into the tank. When it reaches a high limit, the pump shuts off. Note the shutoff valve to the right of the honeywell device. This should shut off the water flow from the pump which you would need to do in order to service the pump, replace the tank or whatever.

Is that yellow valve at the bottom by a drain?

Well, the water comes in from the left of the honewell, then down and also to the right where you see the cutoff valve, but that only goes to the water spigot on the house. So that valve looks like it just cuts of water to that spigot. I assume it come in from the left that is, I see no other pipes.

I don't understand how this tank can work at all unless a second pipe coming out of it goes into the ground under it, and that only one is going through the top because it has to hit the honeywell. It is up on bricks, I could probably try to pull back the vapor barrier and look under to see if i spot anything.

See my poorly attached images. The one with the star goes to the water spigot.

As for the bottom one that you asked, is it a drain, ... I think so. It is in the closed position and nothing is attached to it. So I assume if I open it, my crawlspace would flood.

The other one in front of it - closer to the camera, is the city water.

I appreciate your help, any insight is good.

Test the charge pressure for the expansion tank. Here is an in depth guide, it discusses a water heater, but same theory is implied. You will know whether the diaphragm in the tank has broke or not. If it has broke you need a new tank, if not then you need to charge the pressure again.

http://www.stevejenkins.com/blog/2013/12/testing-and-replacing-a-hot-water-expansion-tank/

Thanks for that link - very helpful read. So, I'm still trying to wrap my brain around all this before I go to far. Let me explain.

First, I'm trying just to understand how this works. I understand it in theory and in that article but, in our setup, it looks like all it is really servicing is the water spigot on the side of the house. The water comes in from the left, down into the tank and also to the right through the honey well then just... on to the water spigot that comes off the house. That's it.

Now, I CAN simply unthread the pipe a bit further down, and then into another pipe right by it, to hook the house to the well water in an emergency - as it was shown and explained to me by the seller. So, is it to keep and regulate pressure to the house, its there just in case of that?

That might all make sense to me except, why does it make the bang sound when I use water out in the yard from a different spigot. I can't seem to understand how it is even involved there. I see the one pipe go into it. That's it.

So anyway, on to what the article says, and others after I've googled a bunch. So, banging on the side of it, it does sorta sound more thud ish than hollow, so I do wonder if it is water logged.

I understand if it is water logged the pump will turn on immediately any time you use the water. That is the case here.

It does have the air valve at the top, and air does come out - not water. So that is good.

I read I need it to be pre charged to the psi (minus 2) of what the pressure needs to be for the pump to turn on. Or two less than the .. in (something). I can't find the exact term again as I type this but it was in-something.

SO now what I'm trying to figure out. How do I find out what that number is, I don't see anything on the pump. Am I to put the pressure valve on the drain at the bottom and see what it is? If it's over pressured or under pressured wouldn't that give a bad reading?

Is that what the honeywell is for? It says 50psi It looks like on the right. I don't know what the "diff" is on the left. So, should I turn off the pump, drain everything, drain the tank, then pressurize (or depressurize) the air to 48 psi?

Everything I see says this is a simple task and I feel like I'm almost there, almost understanding it... just not quite there yet. One thing I do know for sure, it's a good sign water didn't come out of the air inlet. ;)

The jet pump supplies water at pressure to the pressure tank. The tank acts like a balloon that you are filling with water at pressure. When the pump shuts down, there is a check valve that keeps the pressurized water from flowing backwards towards the pump. The tank then supplies pressurized water to the pipe system (through the same pipe it was filled with) which in your case is the outside spigot. The Honeywell pump switch monitors the pressure in the pipe and reactives the pump when the pressure drops below a fixed level.

If you want to check the pressure in the tank, you need to drain the pipes. Thats why I asked you about the drain below the tank. Attach a hose and drain it to the sump pump or outdoors, first turning off the pump and closing the cutoff valve toward the pump. You might have to open a spigot for the water to displace itself.

Afterwards, measure the pressure in the tank with a decent tire pressure gauge, noting if any water spews out the nipple. The pressure, from the picture you have above, should be 40psi. Measure it and lets see what it is.

Usually pump switches have a high and low setting. Yours is different. Without documentation I can only guess that the Main setting is more like the average pressure (50psi) and the DIFF is the high and low from the average. But it might be something completely different.

There is no shut off valve toward the pump. There is one to the right toward the spigot but not left toward the pump that is out in the yard. However in the encloser where the pump is there are two drain point he showed me to drain when I unplug it for the winter.. one thst goes up toward garden spigots and one that goes to the house.

So.. I guess I can unplug it.. drain eeverything... then close the valves back up for all but leave pump unplugged... then drain remainder from tank... close that valve back up as well.... then check the air pressure.

The way you describe sounds like it can supply water all the way back toward the pump and around the corner to other spigots.. ? Or is it pretty strictly for the house?

Anyway sounds like worse case at this point I drain it all check pressure and plug pump back up and it'll all refill as it should and that's that. But.. do I need to measure the water pressure itself from the drain spot on the tank first like shown in that article or is that differnt since it's not constant pressure city water were talking about here?

Appreciate your (everyone's!) help, I am glad to be learning about this system! I will Google the honeywell to see if I can figure out that puzzle piece during work tomorrow before getting to this task.

Meant to say in the enclosure with the pump and drain points there is a open/close valve on one of the pvc pipes. Perhaps this would be a good time to test and see if that is a shutoff valve to the tank. I can't imagine what else it would be.

This enclosure is in the yard on top of the underground tank that is catching the spring water.. btw.

https://heatinghelp.com/systems-help-center/banging-zone-valves#sthash.jgIfsSYr.dpuf