Powering two hot water services from solar diverter

Question

I am shortly installing a 10.1kW solar system. I will be installing a solar diverter from powerdiverter.com.au to feed excess solar to our 315L twin 3.6kW element hot water tank. It will be switchable between the solar and off peak power overnight.

Our HWS is a long way from the kitchen and my wife complains about how long it takes for the hot water to arrive, and the obvious cost. I therefore intend to install a 25L tank directly outside the kitchen. I would install a 2-way tap under the sink so that the sink can be fed from either the new small tank (most of the time) or from the existing big tank pipe (if the small tank runs cold). Question is how best to also power the new tank using solar.

The diverter can supply 4.8kW so I could change the element in the new tank to a 1.2kW element and just power them both directly, but there are a number of issues with that. Firstly I want the main tank heated first since the smaller tank running out is not an issue. Secondly a 1.2kW element is non-standard and hence expensive. Thirdly I would prefer to retain the 3.6kW element in the small unit so it heats quickly at times when there is intermittent full sun and cloud.

A simple solution which would work fine if the input supply was a constant 240V would be to wire the coil of a contactor across the main HWS thermostat. When the main HWS is heating, the thermostat would be closed and hence the contactor would be shorted out and off. When the main tank reaches temperature the thermostat would open, allowing the contactor to activate and connecting the diverter output to the small tank:

Diagram is also here: https://www.dropbox.com/s/o87w2n9xl1fhtob/Twotanks.png?dl=0

That circuit will not work however when the diverter is in PWM mode, effectively supplying a lower AC voltage. It will likely be insufficient to activate the contactor coil or cause chatter of the contacts.

Can anyone think of a similar simple method of switching power to the second tank once the main tank thermostat opens please? It can either activate based on no current flowing to the main tank, or based on the main thermostat being open. I would prefer a simple non-electronic solution similar to my diagram above, not one that involves Arduino or the like.

Answer 1

First, consider a simple hydraulic solution: connect hot output of 315L tank to the input of small 25L tank, and the output of small tank to the kitchen sink. The main cold water will be input to the main tank, only a little volume of cold water (in the pipe between both tanks) will be feed to the small tank when you use hot water. If temperature of main tank is a little over the second, most of time only the main tank will heat.

About a new 1.2kW heater, if the 315L have 2x3.6kW elements, you can connect these resistances in serial and you will get 2x0.9kW=1.8 kW. Using a switch, you can short one resistance and get 3.6 kW, or with a doble pole switch you can get 2x3.6kW if necessary.

And it possible to wire the thermostast of main HSW to a relay, supplied directly from the grid, without the issue of PWM from diverter. Use a normally open contact of this relay to supply the heater of the main tank, and a normally close for the small tank (in serial with the tank thermostat).