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If I use a PID and a SSR to control a resistance heating element can I utilize a silicon controlled rectifier voltage regulator wired between the solid state relay and the heating element? I want to control the over all temp with the PID and limit the voltage to the heating element at the same time. The SCR has a up and down push button control, (no potentiometer) will the control reset to zero voltage every time the power is interrupted?

You have three units here, PID, SSR and SCR-VR which are each in themselves systems rather than components. It is possible that your SCR-VR can be set to reset to zero or not but you need the manual on it to check this out. A word of caution - the PID will suffer from different heating and cooling time-constants because heating is controlled by the power you put in while cooling is determined by the heat loss.

So, I'm thinking that the PID=proportional-integral-derivative, will keep the temp at a relative constant as long as I don't modulate the power. I want to use a 4500 watt heating element but limit power to 4000 watt or equivalent voltage with the SCR to extend element life. The element operates on 240 volts so I can use two separate solid state relays, one relay on each 120 volt leg = two heat sinks and a separate heat sink on the SCR with a cooling fan. I just wanted to know if there might be a problem with the SSR sending power through a SCR.

Presumably your SCR is not a single discrete SCR but is some sort of module containing an SCR because you talk of it having buttons. What is its circuit? What sort of memory does it have? If you are not going to use the PID to modulate/control the heater power, then why have it? If you want to restrict the heater to 4000 Watts then why not use a lower voltage?

I have to say this is all new to me. As far as I know the SCR is faze angle control but instead of having a dial potentiometer it has a small three digit led display panel with power up and down buttons the panel is attached to the SCR module with a mini plug and small cable for easy mounting. My whole idea with this project is a heat controller for a alcohol distillation vessel. I originally used open flame heat witch is unsafe and hard to control. I am more of a novice electrician than anything else with some but little electronics understanding. What I want to do is make a electric heat controller, I know a PID controller is much better than a water heater thermostat but I don't like the 100% ON or 100% off cycle because not only does this shorten element life but the element running full power can cause scorching of the corn mash and this would ruin the final product. So I want the temp control of a PID but I also want to reduce the power to the heating element to say 75% power for obvious reasons, even using a low watt density element will not give me what I want. My understanding is that a 240 element has two 120 volt legs and one neutral and one ground, usually on this type of circuit there is only one solid state relay on one 120 volt leg of the 240 volts circuit to control power. These SSR get hot so they have aluminum heat sinks. I haven't tested this out, but my theory is that if I put a SSR ON EACH 120 volt leg of the 240 circuit each with it's own separate heat sink this will cut the SSR heat in half, at least I think it will. I would like to hear your thoughts.

The heat in an SSR is current x volts drop. If you use two, then each will pass the current so both get too hot. Sorree! mike

PS - Why not split the heater into two parts, one continuous, say 2500W, and one switched by SSR, say 1500W? Then the switching current will be less and the heat won't be on/off, but 2500W/4000W.

Yes, that's a good solution. Tell me, on a PID controller when it sends a low voltage signal to the solid state relay it is either 100% on or 100% off and it controls the heat by the % of time the heating element is turned on and then when it reaches the set temp it shuts off but it doesn't work like a regular thermostat it cycles on and off much faster to keep a set value?

It depends on the amount of hysteresis, ie max and min temps.