Heat Buildup in Complete Insulation Box

[CWatters]

The spec says: "A standard transformer with 220°C insulation and a 150°C temperature rise, will be rated to run full load in an average 30°C ambient environment over 24 hours with a maximum 40°C ambient temperature.".

So if the load is only one half.. and it is not as hot as full load. Let's say the surface temperature of the transformer is 50 Celsius. If the ambient temperature is 42 Celsius. Can the 50 Celsius heak sink into the 42 Celsius ambient temperature?
Or maybe the rule is.. as long as the temperature of the ambient air is less than the transformer.. it will heat sink into the colder environment.. isn't it.

Yes. Heat always flows from hot to cold never the other way.

Now it's clear you are really talking about a power supplies...

PC Power supplies are designed so that when operated in the rated environment (40C) the temperature of their internal parts does not exceed a higher temperature. Typically the die of a semiconductor must not exceed 125C and to meet that the case of the semiconductor must not exceed 60-70C. That is usually measured where the case of the semiconductor bolts to the heatsink. The size of the heatsink, method of mounting, and airflow through the power supply are designed to keep everything to these sort of temperature limits. They are tested in special environmental chambers where temperature and humidity can be controlled. I've actually done this.

If you want to use a power supply in an environment that is outside it's specification (eg 40-50C) you need to de-rate as you suggest. Eg run it at less than full load to keep the temperature of the internal parts within specification. Unfortunately PC PSU manufacturers rarely provide the information needed to work out how much to de-rate their products to achieve this. I don't recommend putting temperature probes into the power supply yourself because of the dangerous voltages inside.

One thing to know is the effect of heat on electrolytic capacitors. As I recall every 10C increase in temperature reduces their expected life by a factor of 2. So when choosing a power supply look at reviews online to see if it uses high temperature capacitors. Typically these are rated to 105C I think.

Edit: to clarify. It's possible that a power supply operating in an office at 20C will live four times as long as a power supply operating in an office at 40C (all the time). But I can't tell you if that's 20 years instead of 5 years or 4 years instead of 1 year. That depends how good the designer was. Some design engineers have got this horribly wrong in the past and large numbers of their products have failed within the 1 year warranty!