Evacuated solar hot water system Question about Pressure Gauge Dropping

Im a bit confused as to why you would add some Air? This looks like a fluid drop to me. I would check each of yuor connections and see if it is leaking anywhere. it only looks to be a small amount lost. when this has happened with mine i simply added more fluid. I got a cheap pump of amazon

Looks like you have a small leak on the fitting in the picture. A small amount of glycol was probably lost. It's a good idea to check the condition of the glycol every five years as the high temperatures associated with solar thermal break the glycol down into an acidic, sometimes sludge if it is neglected too long. Do yourself a favor and test the fluid for acidity, and freeze/boil protection. Replace if necessary.

Most solar systems are sealed loops with an expansion tank. To set them up, there is usually a three way valve and makeup assembly. One valve isolates the loop and on either side of the isolation valve are two tees with valves and hose connections. The isolation valve is closed and a hose is run to a bucket from up stream of the valve on the discharge side of the permanent circulator pump. A temporary pump is connected to the other hose fitting on the other side of the isolation valve. The temporary pump draws from the same bucket. The bucket is half filled with a safe glycol mix and the pump is turned on. Fluid is pumped until all air bubbles are out of the system and then the hose upstream of the isolation valve is closed. The pressure in the system is watched until meets the setpoint. Once its at setpoint, the valve on the pump side is closed and the isolation valve is opened.

The setpoint varies by system, its usually set a few psi higher than the pressure required to pump to the roof. The expansion tank bladder pressure is set to match the system pressure. BTW bladder tanks have a finite life and it seems to be shorter than if used on water. In my experience systems will leak down over the years despite no visible leaks.

Installers rarely left the charge pump kit and if the system needed charging it was a service call. Most installers are long gone. I use three washing machine hoses and a Shurflo positive displacement pump with a built in pressure regulator. I make sure the pressure regulator is set above system pressure. Shurflo pumps are great for clean liquids, the wear out quickly if used on water with abrasives. I only use mine for clean liquids.