I had ordered 3 SSR right before CNY.  Then covid happened, and I didn't think I was going to see my SSRs any time soon.  So I made alternate plans, and I actually like this better than the original plan.

I use one high quality, Carlo Gavazzi, SSR to high speed switch all my heaters with PID control.  But I get independent heater selection by also using a 4 relay module.

But you can't really use PID control with relays!  That's right, you definitely shouldn't, as mechanical relays don't switch quickly, and wear poorly if switched often.  But, here's how I get around that.  The SSR controls the power going through the relay module, and the mechanical relays are never switched on or off with power applied.

Here's the process for start up:

1. Turn off the SSR.  Power is removed from the high voltage side of the relay.
2. Configure the 4 relay module for desired heaters.  Wait one second for contacts to make.
3. Enable the SSR for PID control of the enabled heaters.  The 4 relay module doesn't switch, but does have the power switched by the SSR flowing through.

If heaters need to be added or removed, the same process is followed. 

This configuration is very protective of the mechanical relays, and an argument could be made that the mechanical relays will last a very long time, much longer than usual, in this configuration.  Most of the wear on mechanical relays is when arcing occurs during the make and break process when power is applied.  This should not ever happen because make and break only occurs when the power is removed by the SSR.

Here you can see the black wires coming off the T1 terminal of the SSR, going down to the terminal block and feeding the common terminals of the mechanical relays.  White wires come out of the mechanical relays and connect to the heaters.  Each heater is under 5a which is half of the 10a rating of each individual mechanical relay, if you believe the mechanical relay ratings.  Running all four heaters is under 20a, and my Carlo Gavazzi relay is rated for 25a with a heat sink, and I absolutely believe the ratings on the SSR.  You can't really see the heat sink under the SSR, but it's a large one.

I used the terminal block because I don't really trust the screw down blocks on the relay module.  I'm actually nearly certain that the wires will loosen and disconnect if the wires move at all.  That's not an option when dealing with line voltage.