ECN Forum Forums General Topics of the Trade General Discussion Area Solid state relay or not

Hi
We have a storage water heater (2kW) and I believe it will work well with a timer, and just heating during the night when the power is less expensive. (The power company has automatic reading of the meter every single hour) The power is less ekspensive from midnight to about 6 a.m.
I want to build a unit controlled by a clock and an extra thermo-relay to override the clock if the temperature falls to much.

I know this is more or less common in the USA so your experience is important for my project here in Norway. Pretty much are equal, the system here is nominal 230V yours are 240, it is single phase, and the frequency does not matter as long as I do not use a timer with a frequency depended motor.

I do not trust regular timer with a mechanical switch even when it is rated for 16A so does a solid state relay or a mechanical industrial relay be the most trust-able?
Or what would you use in your house if you should build it and not buy it?
Should I put in more functions? what and why?

I would run some numbers first to see if the saving are worth the work.

Even if the water heater was running continuously you would only be able to save 12kWH.
Over here that would be around $1.20; and could be a lot less if the water heater cycled during operation as most storage water heaters do.

To answer the original question there are very capable SSRs I am running Opto22 45a SSRs on my spa heater that is basically a 5500w water heater element . They can be driven directly from CMOS but I usually put a little silicon transistor in there (2n2222 sort of thing)
You can do a lot of cool things with SSRs and they fit nicely in the square sided handy boxes.

Thank you, both of you. The calculations are difficult but I have a lot of parts in the basement, parts I actually have no other use for so this will not be a fair calculation. To get confirmed that a SSR is good, for this is a positive feedback for the possibility to use a smaller cabinet. My main concern is to not increase the risk of failure. This setup will result in longer periods of full load, and if we have a weak point, that may be a risk of fire.

Our codes has been changed due to observations of hot connections, the old 16Amp (schuko) are not allowed for continuous loads as 2 kW water heaters in new buildings. In my house, this has worked for 30 years with no problems, putting in this modification, it will probably be vise to change that to a CEE 17 system, this is also used in USA.
http://internationalconfig.com/prod_shot/888-2126-NS.jpg

I remember those from the building of the Oakland bay bridge project. The only US job I ever has been involved in.

The only thing I will caution you about is a SSR will need a heat sink if you are running 10a through it. Plan on shedding about 7 watts. This is not a huge heat sink and the cabinet may be plenty. On my spa controller I am dealing with more like 11kw of load and that is around 16-17 watts each for 2 SSRs. They are screwed tight to the cabinet with heat sink goo and a finned heat sink is on the other side with the goo under it. The thing never gets more than slightly warm (35c or so in a 20c ambient)