Heat/Cool PID block

I'm working on a project with a Micrologix 1400 with a temperature control PID block. The system has both heating and cooling, but the tricky part is that the cooling is not always enabled. What we have done in the past is rescale the PID so that when cooling is disabled 0-100% is heating and when cooling is disabled 0-50% is cooling and 50%-100% is heating. I'm not sure this is the best way to approach it and was thinking about having two PID blocks. One for heating and one for cooling. Anyone have any experience with this type of setup?

A lot depends on whether you are cooling above ambient temperature. Normally cooling isn't required above ambient because the system will eventually cool to ambient anyway. Obviously you need to cool below ambient. Cooling above ambient is optional.

You should have different gains for heating and cooling. If you have two PIDs then each can have its own gain. Then it should be easy to switch between the two depending on whether you are heating or cooling.

I would use two PIDs if you need to use cooling.

A little more info.

The system is batch processing in the 1700 F range. Sometimes at the end of the batch, the furnace is cooled to 1550 F. What typically happens is the cooling is enabled to get the furnace and load down to 1550 F and then the heating comes back on to hold at that temperature.

We usually have the same gains for heating and cooling, since cooling doesn't do much control. Once it has cooled to setpoint, the heating takes over. We do have to double the gain when we enable cooling so the system output acts the same when it is scaled 50-100% instead of 0-100%.

That works so why bother to change?

The way you are doing it seems simple enough.