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:

Circuit diagram

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.

  • 2
    \$\begingroup\$ You can use a current-sensing relay (easily-available example to sense whether the main element is being powered, and adjust the sensitivity to whatever level makes sense in your situation. Then use that relay to control the larger contactor which switches the other heating element. That current-sense relay is just an example - you should preferably find one that's 'legal' to use in your part of the world. \$\endgroup\$
    – brhans
    Commented May 19, 2023 at 23:40
  • \$\begingroup\$ Thank you very much for that suggestion. It is an ideal solution. That Amazon one is not available but they are available on eBay for under $19. eg. ebay.com.au/itm/334824074811 It would in turn activate a contactor which would switch power to the small HWS. \$\endgroup\$ Commented May 20, 2023 at 9:38

1 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.

enter image description here 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).

enter image description here

  • \$\begingroup\$ It is not possible to run another pipe unfortunately. The kitchen is a long way from the HWS and it is impossible to run a water pipe to where the small tank will be. I like the concept of connecting the HWS elements in series to get lower power. That would be a good idea if one was using a relay and timer to power the HWS but with a diverter it is not of benefit. I like the last idea however. \$\endgroup\$ Commented May 21, 2023 at 0:19
  • \$\begingroup\$ Just realised that that solution is not viable for safety and regulatory reasons.The HWS would have to have to be rewired in a totally non-standard way with two independent power feeds. That breaches our local wiring codes. It also means there would have to be 2 circuit breakers in the meter box for the HWS. I have a very full meter board and no room for another breaker. I believe that the current sensing relay idea mentioned above is the most practical option. No change to the wiring in the HWS. No additional circuit breaker. Just install the new relay in the middle of the HWS cable somewhere \$\endgroup\$ Commented May 21, 2023 at 10:03
  • \$\begingroup\$ I can think of two possible solutions. First is to implement a recirculating system. This is common in high end houses and motels. Requires extra plumbing. The other is something like a Zip hot water unit. These can be rather expensive to purchase, but can be got second hand much cheaper ( a friend got one for free recently!). These run from a gpo and require minimal plumbing. \$\endgroup\$
    – Kartman
    Commented May 21, 2023 at 12:25
  • \$\begingroup\$ New plumbing is not possible. A Zip heater uses peak power. I want to use solar only if possible. \$\endgroup\$ Commented May 22, 2023 at 10:32
  • \$\begingroup\$ If your solar system is producing enough power when you fire up the Zip, then you’ll be using solar, otherwise, you’ll draw from the grid. As for ‘not possible’, I think you really mean that it is not something you want to do. Clearly if the plumbing was required, it could be done. You seem to be bound by too many constraints. \$\endgroup\$
    – Kartman
    Commented May 22, 2023 at 13:23

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