Flyback diode mesa 7i95t

Then you need arc suppressors across the relay contacts.

Possibly something like this:

www.digikey.com/en/products/detail/bourn...OV-10D201KTR/2408223

Note that it looks like the relays have their own drivers, so the 7I95T does not need
any flyback diodes.

The issue here is that the relays are switching a 120VAC solenoid.

Because the solenoid is an inductive load, this will cause massive EMI unless
there is contact arc suppression across the relay contacts. This can be done
with a bipolar TVS diode, a MOV or an RC arc suppressor.

Some do some don't, you'll need to check the data sheets.
The role of the diode is to protect the driving circuit from the reverse voltage generated from the collapse of the magnetic coil. No flyback will eventually cause component failure, a far bigger issue than EMI.
I'd take PCW's advice.
Another thing I would look at is the routing of your wiring and ensuring physical seperation of control and AC, if you haven't already taken care of this issue.
If you do use optos make sure that the input side and output side are powered by seperate PSUs and the ground are not connected, it aggravates me when I see opto circuits with the input and output sides connected to a common ground.

wrote:

A really big relay should not need arc suppression.

Please do some research before making such statements, especially after it has been answered by a VERY experienced member of this forum.
And who asked about MosFET's ?

unknown wrote:

... it aggravates me when I see opto circuits with the input and output sides connected to a common ground.

+1

Using the 12v rail from your PC negates the protection the optos are used for.

I suggest to forget all the BS you've learnt from the Hipster Maker crowd and read & learn by someone that knows his stuff.

This is a good start, written by the guy that designed the TEC-1 sbc back in '83 or '84:
www.talkingelectronics.com/te_interactive_index.html