Oversizing & Heat Dump Loops

If you want to oversize the collectors for your consumption, (and many people do to get the most from the system) you will need to consider a heat dump loop.
This can be a buried pipe, towel radiator or radiator in the loft as examples.

What you do is very simple; connect a three port valve probably 3/4" in the return to the collector but below the pump (ensure the valve orientation is correct, see valve body to make sure the normally closed port is the one leading from the radiator). Then use the additional thermostat function in the BS/3 or BS/4 controller to switch the flow to this circuit when the tank is complete, but the collectors are still collecting heat.
This way the system will not overheat, and allow you flexibility in the design.

Pressurized solar systems are normally designed for the peak months of solar radiation, in the northern hemisphere this is June / July. If you oversize the collectors you would collect too much heat during this period.
This would normally mean that your cylinder can get up to temperature early and the pump stops. Your solar collectors will continue to collect heat (you can not turn them off) so the system will get hotter and hotter, this is called stagnation.
The system components including the collectors are designed for this condition stagnation and will not damage them.
The controllers have some control options to aid this, for example tanking the tank to it's maximum temperature, or using a cooling function where the pump will circulate the solar fluid around the circuit when it is cool (evening / night) to cool the tank, but the last line of defense is the pressure relief valve which will lift and dump pressure should the system pressure get too high. This normally calls for maintenance intervention to recharge some of the system fluid after the event.

The most effective method of combating stagnation is to add a heat dump loop.
By adding this loop you can purposefully add additional collectors to gain a much wider peak heat collection (get 100% of free heat for a longer period).

Please note that for standard systems, TheSolarShop size its kits using 10 year averages of solar radiation, information supplied by NASA. Standard kits are designed for the peak period and should have no over heating problems.