Relay/TRIAC for water heater switching

[JitterJaw]

I'm designing a remote controlled switch for my water heater and I am bothered by the following questions:

1. Should the switch be DPST, or can I just switch the phase line (dangerous?) ? can I do it by using 2 SPST with the same control signal instead of a DPST?
2. Can anyone recommend a relay for switching ~2.5K Watts water heater running on 220V AC? Should I look for something specific in the datasheet of the relay?
3. There seems to be a big range of prices on relays, is there any reason to invest in a brand name, or ordering one from dx will also get the job done?
4. Is there any reason to prefer a TRIAC over a relay for this kind of project, keeping in mind that the times it will be switched is about twice a day.
5. Is zero-crossing relevant for this low number of switching, or is it just a precaution used on high switching frequency. Is an opto-isolator obligatory?
Thanks in advance

 

[barry]

1) You can use either DPST or two SPST(easier to find)
2) Since you've got a load of about 11 amps, I would use a 25 amp relays. Heaters are a resistive load, so you might see something about that in the data sheet. Personally, I'd use a solid-state relay for this. Mechanical relays wear out; an SSR could last forever.
Here's a DPST SSR-about $70 US **broken link removed**
3) You get what you pay for...but not always
4) Solid state is more reliable than mechanical
5) I don't think zero crossing is necessary for your application. Zero crossing is used for loads that have a high inrush current (e.g., a motor).
As far as opto isolation goes, that's inherent in SSRs. Its necessity all depends on how you are driving your relay.

 

[Ogu Reginald]

I am giving recommendation on 3kwatts load (to be on a safer side). For 3kwatts of load with Voltage of 220V, the current is approximately 35A.
So if you are using a relay look for one that can handle upto 35A.

The TRIAC is recommended more than the relay because: relays are electromechanical in nature so they make noise when switching, and their reliabilty can not be compared to that of semi conductors like the TRIAC.

You know water heater is a resistive load so zero crossing is not neccessary.

 

[barry]

I am giving recommendation on 3kwatts load (to be on a safer side). For 3kwatts of load with Voltage of 220V, the current is approximately 35A.
So if you are using a relay look for one that can handle upto 35A.

The TRIAC is recommended more than the relay because: relays are electromechanical in nature so they make noise when switching, and their reliabilty can not be compared to that of semi conductors like the TRIAC.

You know water heater is a resistive load so zero crossing is not neccessary.

3KW is 13.6A. P=V*I

 

[Ogu Reginald]

3KW is 13.6A. P=V*I

Pleass I am sorry for the wrong calculation, please I meant 15A. Thank you for the correction.

 

[JitterJaw]

Hi and thanks for your replay,
It seems that crydom do recommend zero crossing for resistive load, and they even claim that zero-crossing is the most common relay type for resistive heaters.
Link : **broken link removed**
Any idea why?

 

[Ogu Reginald]

All I know is that zero crossing is strongly recommended for inductive loads but can also be used for resistive loads. There is no blame or disadvantage of not using a zero crossing for resistive loads.

Please correct me if I am wrong.

 

[barry]

As I said, zero crossing is recommended for loads with large inrush current. Tungsten lamps, which are resistive, fall into that category. It won't hurt to use zero crossing. The disadvantage could be large current spikes that would occur with random firing.