Evac tube solar water system not working

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Puerto Rico
Hi everyone,

We purchased an evac tube based solar water heating system 5 years ago. It's a 50 gallon (I think) system with 24 evac tubes. Rather satisfied with it until now. A few months ago, we noticed one of the evac tubes was cracked. Didn't think much of it until last week when we decided to replace it when the water wasn't hot enough anymore. The 'repairman' claimed that a second evac tube was also cracked and that the filter was clogged (we didn't know the filter had to be changed every 6 months or so). But after replacing the evac tubes and filter, the system only heats water during the day. After 5pm or so (we live in Puerto Rico) the water starts to cool down all the way to room temperature. A second 'repairman' was not able to fix it either. Here's what we can tell:

- Water pressure coming out of the serpentine seems fine, so no apparent blockage.
- Evac tubes will have hot water inside if removed during the day. Since we live in the tropics, unsure if our evac tubes have any liquids inside them like they do in cooler locations.
- Evac tubes show only very slight sediment (slight white stains).
- This is a natural circulation system. Unsure if water is properly circulating.
- Water temp becomes fairly hot during the day. Not as much as in the past but quite a bit. Then around 5pm water starts to cool off all the way to room temperature. Honestly, we are not sure if this change in temp during the day was occurring before changing the filter since we weren't really paying attention to water temp during the day.
- Could air bubbles be the issue? Circulation? Faulty serpentine? Loss insulation in tank? Loss of vacuum in the evac tube?

Any tips appreciated. Neither hiring more 'repairmen' or buying a new system sound appealing at this stage. Thanks,
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