PID for Resident Heating Loops

J

Thread Starter

Joe Kempin

I manage several Central Plants and have a PID control system in all. Unfortunately, it works to maintain the heating supply temperature at
a constant, when what I want is a constant return temperature.

Anyone have any experience with a closed loop building heating system controlled by PID?

Thanks, Joe
 
R

Robert Scott

Since the PID loop works by comparing the setpoint to the sensor value, the loop will control whatever parameter is attached to the sensor. If you want to control return temperature, then make sure your PID loop is reading the return temperature sensor instead of supply temperature.

Of course the return temperature will probably have a considerably longer lag than the supply temperature, so your loop tuning may have to be changed to accommodate the additional lag.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
 
H
Joe,

First and foremost, are you controlling with an outdoor temperature reset schedule? To control heating by return is difficult at best even with a contant volume heating loop given the time it takes to see the change in the system temperatures. If the controlled area is warm, reset the setpoint downwards and vice versa. I have done this a lot of times and even look at the surface temp of the windows to decide if I can use the solar gain as a source of heat.

Later
 
I think it would be prudent to explain the exact conditions in more detail. If this is a forced air heating system, perhaps you need to use a Master/submaster loop control. Control the space temperature with a sensor and bring on the heat accordingly, the closer you get to setpoint the less the PID will adjust the heating capacity. Usr the Supply air conrol of the loop to maintain or achieve a not to exceed temperature (limit the supply air and adjust it according to Space temperature reset).

Hope this helps.
 
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