This sounds like a us installation.
In the uk we use 240 v phase and routinely switch one side except for isolating switches.
Contacter would be the normal route with ssr normally only used if power control was needed.
Yeah it is a US installation. And I have been looking into a Contacter relay. But they are not as easily switched with arduino low voltages. It could be done though, so we'll see.
Water heater elements are 5000W and they are not both on at once. So you really only have 21A x 1.5V (spec on the SSR) which is 33 watts. If you mount it on a piece of 3x5x1/4 aluminum and mount that to the water heater (I assume you will use the top by the input j-box) you will not completely waste the heat since it goes back to heating the water heater. Maybe buy a plastic box to go over the components.
That's an interesting idea to 'sink' the heat from the relay back into the water heater. I gotta admit I'm not sure I'm on board with that. It seems that it would cool far less efficiently, since it would be cooling into a warm tank, in stead of room temp air. I guess if the tank, even at max temperature, is still less than what the SSR would heat up to, it would still 'send' heat in the right direction.
Just to be overly cautious, I'm pretty sure I'll just heat sink the relay and attach it somewhere nearby, but not on the tank. Unless someone feels pretty strongly that using the water heater as a heat sink is a great solution.
I'm an electrician.
You need to use a double pole relay, it doesn't have to be three phase, but it can be.
I guess I don't know what to respond to that. I totally see where the advice comes from. I just don't know that using a double pole relay is necessarily that much better than two single relays. The arduino would switch both on well within a few seconds of eachother. And as others have said, you could potentially just get away with one anyway. But I do appreciate the thought and will probably price some double pole relays just to see if it would be worth it.
just a point of correcton. on a US water heater, the thermostat breaks both sides of the line. it is a double pole relay that is activated by water temperature.
my first choice would be the pair of SSR's.
second would be a contractor that was rated for twice the load. hard to find with a low voltage DC coil.
Yeah I'm kind of slightly leaning towards two SSRs right now. Don't know why, just 'feels' right. But I haven't decided for sure yet.
I don't know about whether my water heater breaks both sides or not. I would think that would depend on brand as well, not just country (US). But maybe there are US regulations that say they all need to be the same. I'll have to see if I can find more detailed information about this old water heater and what it does specifically.
I would also install a water shut off valve with alarm.
I gotta admit, I'm not seeing the connection to this with my current set up. As a piece of 'general advice' it sounds great. But I'm not seeing what it has to do with switching it on/off with a relay. But as a general advice, yeah it seems great. I actually have a cheap version of something that alerts me to spills in their already. Nothing fancy like you linked, but a small little alarm device I just put on the floor.
Just a point of correction: The thermostat does NOT break both sides of the line. The Upper-Limit protection does, but the lower element switches only ONE line. Just a point of correction: It's not a RELAY either, it's just a bi-metallic switch. The circuit is already protected by the circuit breaker/disconnect so no one will be unsafe but it is not necessary to switch both lines since the manufacturer doesn't either. I also stand by my original post wherein I stated an HVAC compressor only switches one side.
In this instance, switching both sides of the line doubles the heat dissipation.
I'm just going to have to see if I can figure out for sure what my thermostate/water heater does.
But you do raise a valid point about switching both sides leading to double the heat dissipation. Since I haven't tested this yet, I don't know if that really will be an issue. Don't know how hot they will get. But I do have to fairly decent sized heat sinks for both my relays. So I'm happy to hook both up to a heat sink, if I end up going with both.
In the end, it sounds like any of these ideas will work (two relays, one relay, double pole/contactor switching, 3 phase, etc). I'm kind of being left with the impression that as long as I wire it safely, make sure I dissipate heat, do my due diligence, etc., that I'll be okay with any option.
The debate in the thread kind of leads me to infer that there is no 'one right way only' and that I should just pick one, monitor it a bit, and see how it goes.
I do appreciate everyone's replies though.