its took me a while but i am in the middle of installing the heat dump having fitted the solenoid valve downstairs i was just about to run the cable downstairs when you said there was a better way of doing this were the solenoid would not be on all the time.. but you were a bit worried about me installing is there any chance you could tell me i might give it a try.. many thanks mate much appreciated
It must be the long spells of sunshine that has galvanised you into action! I'd forgotten all about this.
The first method i described using the diagram posted back in sept 2012. Here the thermostat was switching the power connection between the PSU and the solenoid, and so there was no messing with mains voltage wiring. If you were using a 12v battery instead of a PSU, or you were not competent at mains wiring, this was a reasonable approach.
With a PSU wired to power the solenoid via the stat however, the PSU would be on all the time with the (normally open switch of the) stat controlling whether the power output of the PSU was connected to the solenoid. It was not the case that the solenoid would be on all the time, rather that the PSU, even if not delivering power to the solenoid (the normal state), would use a small amount of standby power consumption arising from the PSU simply being on all the time, albeit without a load.
If you are competent with mains wiring, a technically better solution would be to feed the output of the PSU directly to the solenoid, i.e., a single bit of flex from the PSU to the solenoid, and not having the break in the flex shown in Sep12 diagram. Instead make a break in the live mains cable which powers the PSU and connect both sides of the broken mains cable to terminals 1 & 3 of the thermostat.
Now if your PSU is 'built in' to a 3 pin plug affair, then you will have to use a cheap plug board and put your switching break in the red cable inside the flex to the plugboard, so that the stat switches the power to the plugboard instead. Also, always do your switching on the cable that will be live (red or brown), rather than a neutral cable (black or blue).
Safety note: Please don't attempt any mains wiring if you do not feel competent to do so. Never perform wiring, cut mains voltage cables, or touch uninsulated mains voltage wiring without first disconnecting the power source from the cabling you are about to work on. If in any doubt, throw the switch to OFF in your consumer unit first. Then, after the wiring is completed, test for correct operation of your wiring when powered up. Use a mains voltage tester, preferably one of those that detects power when the tester is beside something that's live (rather than touching live). First test the tester itself, so you know it can detect live! Then use the tester to ensure that nothing that is not meant to be live is live (pipes, 12v cables, tank, cables that are not supposed to be powered up, etc). Be very careful testing a tank or pipe thermostat that is switching a live mains feed. Set the stat to the lowest trigger temperature possible, then use immersion, boiler or solar to bring the tank stat to the trigger temperature, rather than take risks such as manhandling the stat, with live cables, up to a boiling kettle! Before all of the above, it is essential to ensure that all your metal pipework has an earth connection including your hot water tank. If your consumer unit does not already provide RCD protection, use a plug-in RCD (£5 ebay) before the plugboard.
Apologies if the above is too Dick and Dora, I just don't want to make anyone reading this feel they can attempt mains wiring without the prior experience and competence to do so.