My first thread - this site has helped me over the past few years in leading the design and install of the Geo system on my new ICF home. Diagnosis of maintenance issues is now in my lap... (We have very few Geothermal experts here within the city limits of Chicago. As to not offend - we'll just say you can count them on one hand... and you don't need close to all the fingers on that hand to do the job.) Can someone assist me with what I should expect to see for a typical pressure fluctuation on a closed loop system (with proper functioning expansion tank) when in operation? I'm pretty sure I have an expansion tank issue. I have a two stage Carrier (Climatemaster) Water-to-Air system (with built-in expansion tank for the well) that has been getting high pressure lock-outs periods of during high cooling demand since it was installed 2 years ago. (Since the pressure sensors for the refrigerant and well pressures are hooked up in series and throw the same code - that was a fun one to diagnose and narrow down...) Within a given summer day, (like we are experiencing now) my pressures are fluctuating by 120+ psi. After the system has rested for a while, it may start cooling in the morning at a well pressure of 40 psi and then it will shoot up to 160 psi where we get the lockout. In the cooler spring season, the pressures never get high enough to trigger the lockout. My HVAC company has limited experience with Geo systems and swears that they followed the commissioning instructions at the install. They are hesitant to load up their geo tools and make the trip out to us (to depress the system, check the expansion tank, and re-press the system) so any advice you all can provide will help me expedite this next step.