I'd like to follow this guide to create a reflow oven controller: https://apollo.open-resource.org/mission:resources:picoreflow
But I am confused on the zero-crossing SSR.
Does it need to be zero-crossing to control the heating element?
I already have a very old SSR for which there isn't even a data sheet, so I am unable to tell whether it has zero-crossing.
The oven itself won't care where in the power cycle the heater is switched on and off. The time constant of the heater should be many times the period of a power line cycle, so timing details at the level of power line cycles don't matter.
However, the rest of the world might care. Switching at the zero crossings reduces the voltage and current transients. That in turn reduces conducted and emitted interference.
If I were doing this, I'd try to switch at the zero crossings. If this is a commercial product, you may need to switch at the zero crossings to pass EMI (electromagnetic interference) requirements.
The design/guide is for using the oven AND a fan controlled from seperate zero-crossing SSRs. The oven is just a resistive heating element and could be controlled from a non-zero-crossing switch but it's more likely that the fan would be better served with a zero-crossing current detecting SSR as they show: -
The element won't care, and if the control cycle is slow (on-off time), it doesn't matter. Your electric stove has mechanical simmerstats which are not zero crossing.
Zero crossing is the right thing to do, if you were buying a new SSR.
I made one from a fan type toaster oven, and originally intended to make it programmable, IOT, green-tech, burn unicorns, etc.
I simply discovered that it heated up 2x as fast as the reflow profile, so I wired the two elements (top and bottom) in series, reducing the power, and giving a perfectly acceptable temperature ramp. No controller needed.
Popping the door 1" open gave the correct ramp down.
You need a LOUD end temperature alarm, or much better, use a self energising relay and a thermocouple sensor set so that the heater power is turned off when peak temp is reached.
You can also just use a stove simmerstat to set the heater power, and therefore the ramp. (but just control the heater elements not fan too)
