Help -Is my syste Solar Syphoning?

[djahawes]

I've had a 30tube collector installed in a secondary coil into an unvented cylinder.

The collector is higher than the cylinder and the solar circuit has a non-return (flappy brass type) valve in the return leg of piping after the cylinder but before the pump.

This has been operating brilliantly for about 2 years now.

Just recently however (in the last 2 weeks) I have been noticing the system has been circulating at night. In fact the 'night circulation' alert was only the tip of the iceberg. On further investigation I have discovered that whenever the boiler is employed (more often over the past 2 weeks) to heat a cylinder of water the collector sensor is indicating a rising temperature. So much so that the temperature difference between this and the tank becomes enough to trigger the controller to circulate in the solar circuit. This continues then as long as the boiler is heating water which in some circumstances (depending on outside temperatures) continues indefinitely, dissipating my precios boiler generated heat out in the night (or day) air via the solar controller.

My first thoughts are that the non-return valve is failing to prevent the solar syphoning effect once the boiler brings water in the tank up to temperature.

I dont think there can be other factors or problems as this system has being running very effectively and within parameters for 2 years now.

Any other thoughts appreciated. Paritcularly if there are other ways that I can determine that the above is actually an accurate diagnosis

Thanks
David

[wookey]

Loks like you have thermosiphoning/circulation of some sort. It could be going the 'right' way round and thus not being stopped by the flap valve, or the flap could have got stuck (usually something stuck in it with those).

A plumbing diagram might help. Is your boiler coil half-way up the tank? Is the bottom of the tank getting hot too? (why/how?) Can you feel any of the pipe to find out which ones are warm (especially when solar pipe starts to get hot)?

I take this didn;t happen last year? Or might you just not have noticed?

[micko]

I would change the none return valve first, that sounds like the problem.