How to prevent overheating?

[J.P.M.]

Best option would be an electronic temperature sensor board and an electric water valve.

Are you talking about a solenoid valve and a high limit shutoff ?

If so, most differential controllers have high limit switches that turn pumps off at maybe 160 F.
Sounds like maybe the OP's isn't set right, has a failed sensor or two or the whole controller is toast.
My controller has a high limit control that's adjustable between 120 and 200 F.
I've also got a solenoid valve that's closed if the pump is not activated but since check valves don't work in any real sense for more than a short time, its primary function is to act in place of a check valve to prevent thermosiphoning.

[Guido in Boston]

Welcome to the forum.
I know nothing about solar water heating but it seems a very simple remedy would be to cover part of the panel. If it works you could experiment with more or less coverage to suit your needs.

and
Best option would be an electronic temperature sensor board and an electric water valve.

Thanks for the suggestions.
Panel is on inaccessible roof. House is in hurricane zone. Electronics are not good in extremely rural and salt-air zones.

[PNPmacnab]

Best option would be an electronic temperature sensor board and an electric water valve.

[J.P.M.]

1.) Get a larger water tank, or,

2.) Get a second (larger) water tank and plumb it between the existing water tank and the collector(s). Make that the solar loop and feed the existing 30 gal. tank from the other tank, or,

3.) Reorientate the collector(s) so that the tilt is somewhere between 45 and 60 degrees to the horizontal. That'll decrease the intercepted irradiance during the summer and increase it during the winter. Result: reduced summer max. water temp., increased winter max. water temp.

4.) Cheapest, and easiest but probably the most visually offensive and involves a bit of work:
Go to big box hardware and get the necessary number of corrugated fiberglass roofing panels. Mine are gray and probably the mostly visually unobtrusive, but the color won't affect the performance curtailment.
Cut them to the size of the collector(s) and the strap them to the collector(s) with (black) bungee cords in a way that makes for easy removal/reinstall of the corrugated panel(s) 2X/year.
Store the fiberglass panels in a shady spot during the winter.
The fiberglass panels last about 5 years, the bungee cords about 2.

I too have a direct system.
This is what I do from about early March until maybe early November or so.
The method is fit for purpose and gets me a solar thermal water heating fraction of ~ > 0.95 or so without summer overheating, but my solar thermal water system is relatively accessible.

[littleharbor2]

Welcome to the forum.
I know nothing about solar water heating but it seems a very simple remedy would be to cover part of the panel. If it works you could experiment with more or less coverage to suit your needs.

[Guido in Boston]

In the summer especially (S. FLA), my solar thermal hot water panel (roof mounted, not easily accessible) heats the water in my 30 gallon tank (direct, not antifreeze/heat exchanger) too much. If I don't dump hot water back into the rain water cistern every hour or 2 it will exceed 200F and blow the T&P. I want to install a second "system comfort/safety" thermal relief valve set at 150F or so but I can't find one to buy, or an adjustable one. Does anyone have a solution to this problem?