Clever Switching for water heating.

[SquirrellCook]

I know it doesn't look like it, but trying to keep things simple the water heater will be directly powered from the solid state relay. This relay is controlled from the Cerbo. As it is there is no easy way to turn the water heater off! So a single pole switch is required in the relay signal wire to disconnect it. It also would be nice to be able to override the Cerbo and manually turn on the water heater. Here we have the danger of leaving it on consuming the batteries whilst unattended. So this override switch needs a timer in it. This is where Wildebus's timer jumps in. https://amzn.to/3hrtILx

Have I over simplified this or will it work?

 

[wildebus]

I know it doesn't look like it, but trying to keep things simple the water heater will be directly powered from the solid state relay. This relay is controlled from the Cerbo. As it is there is no easy way to turn the water heater off! So a single pole switch is required in the relay signal wire to disconnect it. It also would be nice to be able to override the Cerbo and manually turn on the water heater. Here we have the danger of leaving it on consuming the batteries whilst unattended. So this override switch needs a timer in it. This is where Wildebus's timer jumps in. https://amzn.to/3hrtILx

Have I over simplified this or will it work?

Those timers are very neat.
Word of note .... When I was rewiring mine to go with the new layout board I noticed the action of tightening the wires on the 2-pin connectors exposed a dry joint on the PCB (pretty typical actually with the through-pin 2 pin connectors due to the twist-stress). I just resoldered all the joints on the connectors and would recommended you do the same.

But yes, it should work fine and just as you want. You can set the timer from between 1 minute and 90 minutes. I find my 10L tank on AC takes around 50 minutes before it cuts out with the thermostat and set my timer for 60 minutes so will give a full duration heat before cutting off (it does take a fair lump out the battery of course!)
There are a couple of different ways to wire it up but I think the way I wired it up works best for our kind of setups ... link the two pins in "KEY" together, and then connect the PCBs +ve to the switch. This kicks off the timer as soon as you give it power.
This I think is better than having it powered up all the time and using the "KEY" to start it, as it means the timer is only on and consuming power when you are using it.

 

[SquirrellCook]

So this answers another question about turning it off. With your switch in position (1) the timer will run it's course and then turn it all off. To reset it it will require the switch moved away from position (1) and then back again.
To recap.
(1) Timed and then all off.
(0) Cerbo control.
(2) On all the time.

So just three wires to the switch.

 

[wildebus]

So this answers another question about turning it off. With your switch in position (1) the timer will run it's course and then turn it all off. To reset it it will require the switch moved away from position (1) and then back again.
To recap.
(1) Timed and then all off.
(0) Cerbo control.
(2) On all the time.

So just three wires to the switch.

Almost - one slight amendment ... When the switch is ON, the timer PCB is powered up, the Timer counts down and the output relay is activated, then once the timer is reached zero the Timer counter resets back to 60 (or whatever you set) and the relay turns off, but the actual PCB is still powered up (so to turn off the PCB itself will need the switch back to off).
And as you say, to start the timer again, switch off and back on again and the cycle repeats.

 

[SquirrellCook]

The down side of turning it back to off, is that the Cerbo has control. So again another switch. Unless position (2) can be all off. I don't use EHU's so not a lot of point.

Got it! DPDT and use the other pole of the switch to disconnect from the SSR.

 

[wildebus]

The down side of turning it back to off, is that the Cerbo has control. So again another switch. Unless position (2) can be all off. I don't use EHU's so not a lot of point.

Got it! DPDT and use the other pole of the switch to disconnect from the SSR.

I don't understand. Surely the Cerbo having control by default is what you want?
You program the Cerbo Relay 1 to work how you want
e.g. In my case, the Relay will activate when the Battery SOC is lower than a set value or if the AC Load is over a certain value (if I am using the Microwave and this prevents the water boiler coming on and causing Inverter overload). It will also deactivate whenever EHU is present. And the SSR is driven by the Relays NC terminal, not the NO (this is a key thing).
So you can set it up pretty well just as you like. so not sure where is the downside?

 

[SquirrellCook]

I seldom use EHU's so not worried about having hot water day and night, remember you have a fridge on the same circuit. I don't.
When solar is enough and I'm in attendance I don't mind having the Cerbo in control.
I might want hot water when the SOC is not wonderful, believing that the sun will come out later or we're taking to the road. So this is a good time to use the timed circuit. Batteries can be charged then via the alternator and the water is heated via the engine cooling circuit.
So having an (off) position will be good when not in use.

I could be getting it all horribly wrong, so thank you for questioning me.

 

[wildebus]

I seldom use EHU's so not worried about having hot water day and night, remember you have a fridge on the same circuit. I don't.
When solar is enough and I'm in attendance I don't mind having the Cerbo in control.
I might want hot water when the SOC is not wonderful, believing that the sun will come out later or we're taking to the road. So this is a good time to use the timed circuit. Batteries can be charged then via the alternator and the water is heated via the engine cooling circuit.
So having an (off) position will be good when not in use.

I could be getting it all horribly wrong, so thank you for questioning me.

After changing to a 240V Compressor Fridge I did need to seperate the circuits as the fridge needs a supply 24/7. I only had the fridge and boiler together when they were both AC AND Gas powered.

Remember also that with the Cerbo and when you have Internet in the van and VRM you have remote control of the Relay Function.

Having said that, having a master OFF switch is not a bad idea and in my setup as well as having the original switch in the 13A socket the heater is plugged into (hidden under the cooker!), I have an easily accessible On/Off switch on a fused spur so whatever the cerbo and/or timer is doing, if that switch is off, AC power will never make it to the Boiler (and that spur tends to be in the off position).

 

[SquirrellCook]

I had considered a mains switch, but it would require heavy wire in the wrong direction. My calorifier is behind a rear wheel under the toilet.