Wiring thermostat to DHW tank immersion.

It sounds like you've already gone down a road so I guess you've previously ruled out adding a dedicated immersion lower down the tank solely powered by the solar?

Yes but as the cylinder is only about 250mm diameter I would need both to fit an essex flange for the immersion, which is beyond my competence, and I haven't seen an immersion less than 11" long.

My solar diverter has been on a shoe string, mostly using electrical bits I had.

The willis was brilliant while it worked but the combination of the orientation and hard water seemed to cause failure.

I'm guessing that's a top-mounted element. If it was side-mounted it wouldn't matter where the thermostat is along it, I'd have thought.

Or just replacing the element with one with a longer thermostat, assuming such as available?

Theo

That's right

I have not found one.

I could fit an even longer element and thermostat rated at <1500W if one were available but in the mean while the pomp works, I just need to automate it.

In general you shouldn't switch the neutral, as it lead to the load being "off" but at mains voltage relative to earth - which means you can get a shock from something that's not switched on. Of course good practice is to isolate everything before doing any work, but people don't always follow good practice.

Why not just use a changeover relay in parallel with the element? Connect the pump to the NC contacts and it will turn on when the element is off.

Mike

Yes, it isnt safe for a start, and it is a poor solution. Get an external thermostat on the side of the tanks, position it wherever you want, you can then use that as a switch to turn on/off any device attached to it. That then leaves the immersion stat to do its thing, the temperature on it can be turned up if there is a chance that the internal water is higher than the stat requires, but there is always the cut out if the water gets too hot.

Most (all) of the off the shelf thermostats designed for fitting external to the tank will not handle the current for the immersion element so if it is to control the immersion element some form of 230V higher current relay is also required.

An external thermostat may/should be adequate for controlling/switching a pump but as the OP mentioned he doesn't want it running continuously and end up with a tank of luke warm water. Some form of re-triggering timer is required. The pump should also be disabled if there is no PV supply.

Yes, rely on the external stat to set the water temperature and leave the exiting one but set to a higher temperature.

A question: how are you heating from solar? Are you switching the element manually or do you have a solar diverter?

Solar diverters tend to be designed to vary the heating load to match the excess generation. ie if you have 550W excess they'll shape the load to be

Interposing anything between the diverter and the element could be troublesome, since what goes in there could be DC or pulsed AC.

Does the diverter have any inputs by which you can control it, separate to switching the mains?

Theo

Thanks. Yes this looks like the sensible and safe way and only requires a 240V low power relay with the coil connected across the element. The pump and element are then mutually exclusive and both get their supply from the solar PV diverter.

It's a sort of non proportional power diverter; a voltage controlled dry contact on my battery charger closes for 10 minutes once the battery reaches 54V, which can be anywhere over 75% capacity depending on solar power or battery discharge.

This then switches a 12V coil supplied by a wallwart and this powers the element for 10 minutes for a total of ~200Wh. Then depending on sunshine the thing stays on or the battery recharges to 54V.

Plus they cost a lot and are prone to break

Yes, sensing battery voltage then time on.

The main thing is it cost very little, I had a 12V mains relay which I boxed and made a lead with 16A commando plugs which I inserted into the immersion feed, thus retaining the two pole switch. When I next get an electrician here I will move the relay to the consumer unit and save running the 12V control wire upstairs.