Can you change a PRV by isolating the radiator at both ends?

A hot water heating system has to have enough water pressure in the heating system (boiler, piping, and radiation) to keep the water up to the highest point in the system. So, for each pound of water pressure in the system, the water will rise up in the piping a height of 28" or 2.31feet. For example, if the height of the highest point in the heating system from an accurate pressure gauge on the boiler was 20 feet, the boiler static pressure or cold fill would have to be 8.66# (20 divided by 2.31). Most PRV settings are 12psi for a 1 story house and 15 psi for a 2 story house. So, if you remove the TRV there is pressure in the system that is being supplied by a PRV set at at least 12 psi If you want to remove any of the isolation valves you should reduce the pressure to a point that the water will not flow out of that pipe. MYself, I would drain the system. Hope this answers your question.

I can not answer your question with a definite answer since neither you nor I know if the gauge is reading correctly. Theoretically, if the gauge is reading correctly their should be no water in the system but we know better. The pressure could be at zero and still have water in the system. The zero reading could be generated by a vacuum above the water line sufficient to reduce the gauge reading to zero. If you open a vent at the top of the system and allow air in, the water will seek a level and again show a pressure on the gauge, equal to the height of the water above the gauge. The only thing you could do would to be to open a vent, allow the water to seek a level, then loosen a fitting at the item you want to replace and see if water comes out after the sucking sound goes away. If not, the water is probably below that point. This is the best explanation I can give. Remember , 1 foot of water = .433 pounds of pressure or 1 pound of pressure = 2.31 feet of water elevation. Hope this helps (sorry I answered twice; 1 answer and 1 comment