I wish to find out a workable means for controlling the temperature at different times inside the drying chamber of a gas fired oven.I have thought about using a solenoid operated valve connnected to two thermostats.But I am not really sure how this will work

Can I ask a few questions:

1. What is the BTU/hr ration of the oven?
2. Input pipe size?
3. Temperature range involved and typical tempertures?
4. # of temperatures?
5. What kind of sensor is used to measure the temperature now?
6. How do you propose to change the teperatures? PLC?

The solution I may propose may not be cheap.

If you/we can't find a suitable combustion control then using a mass flow controller would definitely work combined with a PLC. See: Modulating Gas Valve (http://www.homebrewtalk.com/f11/modulating-gas-valve-53837/)

This is easy enough for bang/bang control. http://customer.honeywell.com/techlit/pdf/PackedLit/68-0039.pdf

Proportional control would be harder.

Here is another idea, but they failed to mention what the PWM signal characteristics should be.