Standard PVC is not intended to carry heat, but does have a rating based on operating temperatures. The base optimal temperature for PVC is 73 degrees Fahrenheit, at which the pressure rating of the pipe is equal to what it can truly carry. At 140 degrees, PVC is unstable and unreliable for carrying pressure. At higher temperatures, use the rating chart via the first link in the References section. Multiply the pressure rating for the pipe you are using by the "De-Rating" factor in the row of the temperature you will be operating at. The number obtained is the maximum pressure allowable for that application.
For situations where heat needs to be carried through the pipes, such as a water supply to a faucet, CPVC may be used in place of PVC. CPVC is rated for heat and is slightly smaller in diameter. This pipe may be used with temperatures up to 200 degrees Fahrenheit. Use the chart found via the second link in the References section and multiply the pressure rating of your pipe by the "De-Rating Factor" next to the operating temperature you will be using.
Solvent vs. Threads
Threaded PVC connections are not as strong and therefore cannot handle the same amount of pressure as a solvent-welded connection. According to Harvel, a PVC manufacturer, threaded PVC connections have half the pressure resistance of solvent-welded connections at the base operating temperature of 73 degrees, and should not be used with a a temperature of 110 degrees Fahrenheit or greater. Harvel recommends flanged unions and fittings for higher temperature joints that must be replaceable.
While the heat capacity is not based on pressure, the amount of pressure handled by PVC is determined by the temperature. Using PVC with a pressure below the listed capacity can still be dangerous and cause pipe failure with increased temperatures. As the temperature of the pipe goes up, the amount of pressure supported goes down and vice versa. Check the listing for your specific pipe and fittings for pressure ratings and temperature requirements.