Hi there,
We are in the US. An electric water heater is run off a 240V system with three wires (hot-N-hot). Currently, there is a magnetic contactor relay on only one (!) of the hot wires. when that contactor gets triggered by a 110V signal, the coil closes the contact and the water heater starts. The other hot is permanently connected to the heater. I don't know who installed it that way, that's how it was when I found it.

I want to replace the magnetic contactor with a solid state relay (SSR). My hunch is that a regular 1-wire 240V SSR -- and thus a 1-to-1 replacement -- is not going to work, am I right? I have a hunch that most of the SSRs can't deal with switching one hot of a two hot system, as I suspect the other hot will send some signal up the other leg or something ...

Help!

--Thor

 

Your contactor now needs 120V to turn on. Most SSRs have a low voltage input. I see some will work with 120V.
You need to decide what voltage will drive the SSR.
You need to select an "AC" relay. Or AC,DC relay. Not a DC!
I see no problem with switching only one side of the 220V. It will work. I like switching both sides just to keep me safe but many times I switch only one side.

 

Since there's no connection from the heater to the neutral, switching only one of the hots in the 220V connection is not a problem.
Basically the relay is just in series with the heater element and the two 110V lines.

 

aha - but so far the SSR would only be on one hot, not the two 110V lines (which I don't have a relay for, but I have plenty of SSR with a single channel to switch). Does that still work?

 

I'm used to double-pole contactors or relays used on 240VAC motors and heaters.
You don't want the thing hazardous live- yet it appears to be off.

Note that SSR's dissipate about 1W per amp. Not sure what your heater rating is. 5kW at 240VAC is around 21W of heat which needs a large heatsink. You don't want to use a cheap SSR because these fail short-circuit and the tank could overheat. In all of this there are safety limit switches you want to preserve their function obviously.
At higher power levels a relay ends up a better solution due to the losses in an SSR and their high cost.

 

Just to be clear, when the relay is open, you should not touch either side of the heater. It will bite you. I would put a label on both wires. "When the relay is off both wires are hot." Maybe add that the breaker should be off before touching any of this.

 

but so far the SSR would only be on one hot, not the two 110V lines

But the two 110V lines are in series through the heater so the SSR stops the current from both lines.
It's the same as if there were one 220V line and neutral.

 

I'm used to double-pole contactors or relays used on 240VAC motors and heaters.
You don't want the thing hazardous live- yet it appears to be off.

Note that SSR's dissipate about 1W per amp. Not sure what your heater rating is. 5kW at 240VAC is around 21W of heat which needs a large heatsink. You don't want to use a cheap SSR because these fail short-circuit and the tank could overheat. In all of this there are safety limit switches you want to preserve their function obviously.
At higher power levels a relay ends up a better solution due to the losses in an SSR and their high cost.

ALL triac based devices will fail short-circuit in the event of an overload. You need a simple bimetallic thermostat set at 80°C as protection, as they rarely weld their contacts closed, especially if they don't have to do many switching cycles. It will also protect in case your temperature controller fails (e.g. temperature sensor loses contact with the tank)

 

I'm used to double-pole contactors or relays used on 240VAC motors and heaters.
You don't want the thing hazardous live- yet it appears to be off.

yes, I have a 120V idiot light when the other unswitched hot is .. hot

The SSR i have at hand has a huge heat sink, rated for 60Amps. But it’s single pole (can switch one line with 90-600V). I connected it to try it out and it doesn’t work: it does see the input signal and “switches” but the heater isn’t turning on. I’ll get a two-line SSR instead. Or any silent Contactor that doesn’t constantly click on and off (the heater is working against a cooler, long story, it’s a fluid dynamics system for a very large industrial test bench)

— Thor

 

I connected it to try it out and it doesn’t work:

There's no reason that should not work. You don't need two SSRs.
If one won't work, then two are also unlikely to work.

How exactly was it connected?
Draw us a diagram of the complete circuit.

 

How exactly was it connected?
Draw us a diagram of the complete circuit.

And the specs for the SSR. You could put any number of contacts in series, but why. For safety, you want the bimetallic as previously mentioned, but for on/off, a single contact works. L1 and L2 are always hot, switched or not, and neutral is provided for control. Don’t forget a solid ground to activate OC protection.

 

aha - but so far the SSR would only be on one hot, not the two 110V lines (which I don't have a relay for, but I have plenty of SSR with a single channel to switch). Does that still work?

On most electric water heaters, the heating element operates on the 240 volt circuit and the neutral is only used to power the control contactor. Consider that if a single switch is doing the control now, a single ADEQUATELY RATED SSR will work quite well.
CAUTION!!! Be certain to switch off the power before starting any work on the wires. AND a second caution is to be sure that any SSR you install is rated for the operating temperature, and has an adequate heat sink attached.

Question: WHY do yo want to replace the contactor with a SSR.

 

There's no reason that should not work. You don't need two SSRs.
If one won't work, then two are also unlikely to work.

How exactly was it connected?
Draw us a diagram of the complete circuit.

 

 

Certainly DANKO has provided a correct version that is probably close to the original.
NOTE the size of the heat sink!

The water heater system that I repaired a few months ago was similar, except that it had two 2400 watt heaters controlled by two separate thermostats, both fed from the 240 volt mains. The failure in that application was that both sets of thermostat contacts had welded closed. The safety over-temperature devices did not open. Fortunately the safety pressure/temperature valve did operate and so instead of a boiling water steam explosion, water and steam were pouring out onto the floor.
My repair was to replace the failed thermostat switch and rewire the two heater elements and thermostat in series to the 240 volt mains. So now the power input is 1200 watts instead of 4800 watts.
In this system there was no neutral connection available, because the code did not require it.

 

View attachment 352234

If it is actually wired that way, then if it doesn't work, there's a problem with the SSR.

 

The SSR control voltage will need to be compatible with the control system in the water heater. I am guessing that circuit was 120 volts AC. It should be the same as the contactor being replaced.