For many reasons I have a long run of garden hose exposed to the sun. The water source is always on but the outlet opens and closes intermittently. Some times the water doesn't flow for many hours. On really bright hot days the hose sometimes bursts. Is there an inline pressure relief valve I can put into the hose to prevent the raised pressure from bursting the line? This has happened several times to date.
Summing up some comments so this question has an answer.
If the hose valve is always on there's no way for pressure to build up inside the hose. If there was an increase in pressure, water would back up into the house plumbing to equalize the pressure. If there was a backflow preventing valve an increase in pressure would cause that valve to drain water until the pressure equalized.
So, the water increasing in pressure is not the issue. The hose material heating up and becoming more flexible is probably the cause. If that is the cause, installing a pressure regulator on the hose could help because it would reduce the starting pressure of the water. In other words, if your hose pressure is at 50psi and that is fine for a cold hose but causes fatigue in a hot hose, then a regulator could bring that pressure down to 30psi. Maybe a hot hose can deal with 30psi just fine.
This would mean less pressure at the point of use and probably less water flow but you didn't state what the hose is for. The best solution is going to be to replace the hose with something more suited to heat and sunlight like an outdoor PEX or UV safe PVC. Something like irrigation pipe is not that expensive.