So this is a new one for me - on expansion tanks our local Authority Having Jurisdiction is apparently requiring Pressure Relief Valves between the isolation valve and tank. Apparently if there is a fire or high heat source near by and the tank is isolated the expansion tank can blow up. Is this code in your area? I'm interested in what you know or have heard.
Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
Never heard of that requirement. Seems to me because the hydronic expansion tank is connected to the system, at the point of no pressure change, any significant increase the system pressure would set the units PRV off.