PLC help

I am working on a HVAC project. I have existing wiring diagrams of a relay based control in Air Conditioners. I need to rethink the control using PLCs.
what would a standard PLC replace in the AC circuit? Can anyone provide some insight or references? Am new to this... Actual PLC programming will be mostly outsourced to the company which will be providing the PLCs I guess. thank you.

Dynamcheat,

You need to define your problem more clearly. Are you trying to control a large HVAC system or a single unit, home HVAC. If its a single unit, the control is very simple. I don't know why you would replace the standard thermostat control in the building, or the very little bit of logic control in the unit. If its a larger system, no-one is going to be able to give you any help unless you define it clearly.

PLChelp

If you have an older system, the existing relay drawings will be similar to what you need in a PLC program. Ladder logic is set up and functions the same as a ladder diagram. The size and type of the PLC will be determined by the number of inputs and outputs as well as if they are analog or discrete(digital). The installing vendor will be able to help with this.

What would a standard PLC replace in the AC circuit?

Click to expand...

Basically the only thing a PLC needs to replace in a typical Air Conditioner circuit would be the thermostat control.

Here is an example of a Heating/Cooling unit with a Honeywell T8112C thermostat, and partially replacing the thermostat with a small Omron ZEN. The circuit uses 2 relays in fail-safe mode, so that if anything goes wrong, the PLC can be removed and the old thermostat will still work. In this particular project for a large church complex, the ZEN controlled 11 HVAC units. The ZEN only controlled when each unit could start (demand charge reduction), with the old thermostat still regulating the temperature.

What about fan speed control, heating/cooling, de-icing, time control ....

You can make it as crazy as you want.

In our main office we have a heat pump taking water from a large body of water. We have passive cooling (just using the actual water temperature), active cooling (using the heat pump to cool down the water) and heating (using the heat pump to heat up water). Connected are various speed drives, control valves and so on.

All this for a simple air conditioning..

I dont have multiple units.. I have a standalone unit.. I realize that evaporator and condenser sections will remain unchanged.. In the control part i have defrost control board, relay logic board, indoor blower relay, etc.. I.m basically talking about a single unit which might have dehumidifier or humidifier later... Can someone draw me an example?? Just a simple block diagram also would help.. Thanks a lot guys..

Just a simple block diagram also would help.. Thanks a lot guys..

Click to expand...

See the connection diagram in Post #4. I hope you realize that almost every HVAC system is slightly different, so no one can draw a diagram without first seeing a diagram from your HVAC manuals and instruction sheets. At least give the brand and model number for your unit. With that, it is maybe possible to find the wiring diagram on the Internet somewhere.

A single AC is not worthwhile using PLC, the included control board would work better and interface with the thermostat without the usage of any signal conditioner. On multiple room unit with central air supply use VAV damper unit.

give us your diagrams and we will give you solution for it.

(btw think of how to measure the temperature as this will smooth operation)