Can the drain pipe from a water heater's temperature / pressure relief valve be plumbed such that some part of it can be higher than the T&P valve itself? Or must the piping always remain below the level of the valve?
T&P valves should always be piped as to not allow any build up of water or debris to occur. They should also be piped so if there is a minor failure/leak it can be observed and replaced, as well as piped to be close enough to the floor as to prevent scalding if the relief should open under high temp or pressure conditions. Check with local codes for minimum clearance between discharge pipe and floor. Some jurisdictions allow for piping to floor drain to help prevent damages due to failure.