Temperature Controller Function

Hi, can anyone help me in understanding how this controller functions. I'm trying to mimic the control of this temperature controller with ladder logic using RsLogix 500 CPU Slc 5/04, 1746-ow16, 1762-nr4. I'm using this switch for heating application
Here’s how I understand this:
If the temperature set-point on the dial is set to 45 deg. Fahrenheit and the temperature begins to fall below the set point then MAKE R-W on right switch.
I’m not quite sure about the Interstage Differential and how it functions, but here is how I think of it. So if the SP= 45Deg F and the Interstage differential =10 Deg F then the R-W contacts would break at 55 Deg F. And again make at 45 Deg.
I don’t understand what sets the left switch differential?

Any help greatly appreciated.

Here is one more

I don’t understand what sets the left switch differential?

Click to expand...

After going thru the spec sheet I'm assuming that the offset between the right switch and the left switch is fixed at either 3F or 3.9F but I'm still confused about the operation of the Left switch

The device is an on-off controller with an alarm, or an on-off controller with 2 stages, either controlled from a single input variable.

Let's call it a controller with an alarm.
The controller, the right switch, uses the setpoint of 45°F. Above 45°F, the right switch makes. If it cools to 42°F the right switch resets (opens).

The alarm is the left switch. The alarm's setpoint is the "interstage differential" above 45°F. That value is adjusted by star wheel. If the differential is 10°F, then the alarm, the left switch trips (makes) at 55°F (45 +10) and resets at 52°F. If the differential is 5°, then the left switch trips (makes) at 50° (45 + 5), resets when it cools to 47°.

The alternative is to think of it as 2 stages. Substitute the stage A for 'controller' and stage B for 'alarm'.

Dan thanks for a swift response, the temperature controller is in fact as you specified, used in a two stage heating application.
The controller is used in a air supply unit, that supplies fresh air to our chemical storage room, since the fresh air supply has to be continues, in winter we have to pre-treat the air to above freezing temperature in order not to freeze the chemicals. However at times the storage room gets quite cold or hot in winter time, and the only temperature control we have is the controller described above that is located inside the unit on the roof.
My boss has asked me to figure out a way to somewhat control the temperature in the room, as well some alarms, and status of the unit. I thought I could mimic the temperature controller function and write code that could let us have a handle on the temperature in the room , but as I’m digging deeper in to this, I’m starting to think that for one I’m inexperience in this field and two that the existing controller is the only option we have. Therefore I’m in need of your help. We are supplying fresh air ambient temperature to the room 24/7 all the controls of the unit are off or on we only have control of the 2 stage heat and that is with the controllers dial. Without any variables such as the blower speed of flame modulation what would be solution to this? Any help.

Seriously, I think you need a couple of HVAC vendors to come in, survey what you have, listen to what you want to accomplish and then propose whatever as a solution.

Those people do it for a living and can do the calcs.

I didn't mean to blow it off.

You can hang a temperature sensor and use it as an input to make a control decision call for heat with either a thermostat or a PLC, but whether a pre-heater has sufficient capacity to maintain temperature does not depend on the controls, it depends on the heater.

You didn't say anything about air conditioning. If the idea is to control a temperature, in this mid-west climate, AC is usually needed for 1/3 of the year.

Recommendations for a higher capacity heater and an A/C unit with whatever their controls are, is what HVAC guys do.

You are supplying fresh air 24/7. 2 stage heater. No provision for cooling.
Use two thermocouples, one for alarm. Use PID block, but set P only.
Don't see where your existing switch will control both heat stages. I assume the heater itself has internal controls for activating the second (boost) heater.
Agree with Dan that you should consult someone to accurately define current operation. You can't automate anything until you know exactly how the existing system works.

It looks to me like replacing the existing thermostat with some PLC logic and I/O is very easy, but making it outperform the existing thermostat...well that's is where I would agree with Keith and Dan.

If you have no cooling capability, you will not be able to reduce the room temperature below the outside air temp.

I rewrote some logic in a King Air system with both heating and cooling for a very large room. The SLC logic that was supplied by the OEM was very weak, and it's operation didn't even follow the description in their own manual.

At first, I was just going to scale the values (they were all raw numbers in the code) and try to figure out if I could close the fresh air dampers when there was a burner controller failure and the outside temp was below freezing. but I found so many problems, I decided to start from scratch and make the thing behave like the book said it was supposed to behave.

At first I thought a PID would be nice and simplify the logic. In the end, there were too many "what-ifs" and disturbances. I would have to keep the gains quite low, and then when they switch setpoints every night for cleaning, it was taking too long to swing the CV around the other way to fully engage the burners. After strapping a shedload of rungs on top the PID to switch modes and manipulate the output, I finally just did away with the PID instruction and just used common sense.

Now the room stays within about 0.5F of the setpoint except in the summer when the system simply cannot keep the room at 43.0F when it is over 100 outside. The best it can do is about 45F. This goes to the point Dan made that you can make the switching smarter but you can't make the heater exceed it's design capacity simply by controlling the start/stop logic.