2x4.8kw hot water with 1x4.8kw source

[jpsauer]

I have a 4.8kw solar diverter with 2 x 4.8kw hot water systems. Is there a switch that will only heat one hot water system at a time and switch over? So basically, 1 x 4.8kw line that goes to two hot water systems that only heats one at a time and switches over to the next one when one is done, when there both done just switches off and when one needs power again it switches to that one, but never both at the same time.

 

[littlespark]

It can probably be done using a complex system of high and low temperature thermostats, controlling a contractor to switch on the heating load.

Ie
Low thermostat on A calls for heat, turns on heater A, but also turns off the power to B completely. When A reaches high thermostat temperature, it cuts off A and turns the B system back on.
The B system has an identical control, which would turn A off while B was heating.

There may also be safety stats beyond these controlling stats.


There may also be an off-the-shelf solution…. A black box of electronics that just works.

 

[bill01803]

It would be quite simple to do with a relay off the first stat that when satisfied then closes a contactor to run the second heater.If the first then calls again the relay would open reverting it back to the first heater.

 

[Avo Mk8]

Could you use a non-priority shower board?
Assuming each of your 4.8kW systems already have thermostats to turn off when target temp. reached, this might work "out of the box"?
e.g:

https://www.meteorelectrical.com/pub/media/catalog/product/file/Shower%20Boards_1452600737.pdf

 

[timhoward]

We’ve talked about only heating one at a time.
Is any thought needed for where the hot water comes from, i.e. the cylinder that is actually hot?
In other words are they designed to both be on or does one feed into the other.

 

[Lucien Nunes]

As above, to decide the best interlocking configuration, we need to understand the priority requirements e.g.

• Cylinder A is always more important and must heat whenever there is both supply and demand, including cutting off cylinder B while it is heating.
• Any cylinder that is heating should be allowed to get fully hot before starting on the other.
• Both cylinders should be given equal time-shares of the supply on average.
• When the supply becomes available, if both need heat, the colder cylinder should be heated first.

 

[jpsauer]

Hello all and thanks for the suggestions
The diverter will switch to off-peak power at off-peak time to heat up any system that didnt get enough solar during the day. Priority is no real issue, as they both get to heat during the night, but excess solar will be diverted to them to reduce off-peak use.
I looked up the shower board - thanks for that, looks promising but not available in Australia.
So priority isnt an issue, getting equal amounts not an issue either, requirements:
1. Only one system heating at a time
2. When one turns off the other is heated this is due to the diverter having a 4.8kw limit.

As far as the diverter is concerned it should feel like its heating a very large 4.8kw hot water system as opposed to two different ones.

 

[davesparks]

If the only concern is ensuring that only one tank heats at a time then this can be achieved with a simple thermostat and contactor arrangement.

Do you have details of the heaters? If they are anything like common immersion heaters in the UK you may be able to use the thermostat already in them to trigger the contactor.

 

[Simon47]

From the description, it's fairly easy - harder to describe than to doodle really.

Get two contactors and an interlocking kit - you can do it without, but the interlocking kit mechanically prevents both closing under any fault conditions (or even someone poking in the armature with a screwdriver) and protects your solar kit. If that's not an issue (e.g. the solar system can cope with a momentary overload, or you are happy that it's not a big risk) then you could use a couple of relays - but by the time you get to 20A, relays are likely to cost as much as contactors anyway.

Wire the solar output to the input of both contactors in parallel.
Wire one heating load to the output of each contactor - so contactor A powers heater A, contactor B powers heater B.
You now have a pair of contactors that can connect no more than one load to the solar kit.

Each contactor needs to have 1off normally closed (N/C) contact. Many do, some don't and you'll need an auxiliary contact or it may be included in the interlocking kit - devices vary.

Do the control wiring as :
Supply to stat A common
Stat A demand (N/C) via contactor B N/C contact to contactor A coil
Stat A satisfied (N/O, normally open) to stat B common
Stat B demand (N/C) via contactor A N/C to contactor B coil

If tank A is under temp, then the stat will route power to the contactor for heater A - via the N/C contact on contactor B which is off.
Only when tank A is hot will it's stat change over, power is then routed to stat B.
Assuming tank B is under temp, then initially contactor B will not pull in. As stat A changes over, there's a lag until contactor A drops out - very short, but there's a small lag. Only when contactor A has dropped out will it's N/C contact close, and then contactor B can pull in powering up heater B.
Similarly, if tank A drops below temp, then it's contactor can't pull in until contactor B has dropped out and it's N/C contact closes.

It should be fairly obvious that this naturally gives tank A priority.

If you don't want that, then it can be changed. Wire a N/O contact on contactor B between the common and satisfied terminals of stat A (i.e. in parallel with the contacts that close when tank A is satisfied). This will then hold in contactor B until it's stat is satisfied and opens.
A still has priority at startup, but after that, whichever tank is heating will continue to heat until it's satisfied and then the other tank can have a go until it too is satisfied.

Just a quick doodle just using the basic library parts from Frizting, so used relays for the contactors etc (ignore the part numbers !). Should give you enough to work the rest out.

One thing to consider though is how well the solar kit will cope with this. I would hope it should be fine if it's designed to drive something like an immersion heater with a stat. When it turns on it's output then there will be no load - and a fraction of a second later the load will be applied. Similarly, when stats change, the load will be dumped, and if that results in the other heater turning on, then the load will disappear for a fraction of a second before being re-applied.

Also, I've not come across a standard contactor that doesn't hum. I used to work in an office with night storage heaters controlled by teleswitches (remotely switched by radio by the supplier for night rate). Occasionally they'd turn on during the day - we'd know this by the "clack" of the contactor pulling in and the hum that follows (and a similar clack when turned off again later).
I have no experience with them, but I believe there may be quiet versions, or it's possible to use a DC coil with a diode - perhaps someone else can expand on this.

 

[brianmoooore]

Supply wired directly to one immersion. Second immersion connected to supply through a relay/contactor. Coil of contactor connected between supply live and the first immersion after the 'stat.
While the first immersion is heating, coil will have 230V on both terminals, so second immersion will be off.
'Stat turns off first immersion when water is up to temp. Coil will have 230V on one terminal and 0V on the other, connected to neutral through the first immersion element. Contactor applies power to second immersion.