I have been reading on this device lately and I’m very impressed at what it’s capable of doing. I’d like to control my Immersion Heater over internet and would like some help as to which parts I need.
The device uses around 3KW/h on initial heating, and is plugged to a 240v 16A double pole switch at the moment. I want to be able to switch this on/off over the internet.
Which parts do I need to get please?
Not sure is this helps 
I use a Arduino relay shield from ebay to control 3 2Kw heaters, most of them works with 3.3V signals to. The most of them manage 10A, but you might be able to find on for 16A to. Simple and cheap solution if you don’t want/have time to make a relay board or hook up one your self.
To be honest I saw that link already but I was unsure about the amperage, as well as the way it’s wired (switching from 4mm wires to those thin ones to control the heater).
Also, bit of a newbie here, when you mentioned the 3.3v signals, what do you mean exactly?
Can an arduino relay 10a+ be used with the spark core directly?
You drive the relays with a voltage signal, which can be either HIGH or LOW. Different relays have different requirements. Some relays need to get 3.3V to operate, while others need 5V. Since the Spark Core operates on 3.3V, you’ll need to find a relay which is compatible with that, or you’ll have to figure out a way to get 5V to it (shouldn’t be that hard if you can’t find a 3.3V).
The amperage in relation with the Spark doesn’t matter, since the current will only run through the relay. The relay will have to be able to handle the current that is necessary for your heater. The only thing your Spark has to do is supply a high enough voltage to switch your relay.
Depending on what voltage characteristics said arduino shield has, you may, or may not, be able to use it with the Spark directly. As far as connecting the two goes, you could use some wires (rather messy), or you could use the Shield Shield, which is basically an arduino shield adapter for the Spark Core.
To recap; the two most important things to watch out for are current ratings of the relay, and the required voltage to switch the relay. The current would be the most important, since it could cause hazardous situations if it’s not cabable of handling those currents.
My general rule is to use devices rated at 2 x the current I need to switch. This has been reliable over the years as I have a 20A SSR switching a 1.6KW water heater for a few years now.
Take the KW rating of your heater and divide this by the voltage and this is the current you need to switch. Then find a relay or SSR rated 2 times this.
My heater is 1600W / 220V so I have max around 7.3 amps when the heater is on. The 20A SSR easily handles this and I also mounted this inside an aluminium enclosure (with a good ground connection) and this helps to dissipate the heat. Not that there is much though.
Taking the suggestions of @v8dave into account, you would need a 2*(3000/240)= 25A relay. A quick Ebay search will show you plenty of those, at nice prices (as low as $3.50 from china). You just need to make sure that you get one with a 3V input, so you’ll be able to switch it directly from your Spark. Not that I don’t trust the Chinese, but their products sometimes lack a bit in quality, which they compensate for in the price. However, on a project like this, I’d rather spend a few bucks extra to know my switch is of decent build quality, and therefor won’t burn down my house. The choice is yours.
Thanks for all your help guys! Will do some homework on these relays and hopefully build the thing