Ladder Programming

[1stwave]

sorry I'll try to explain better.

These are not wall a/c units.
I have two 2 1/2 ton HVAC outside units feeding the same ventilation duct work inside the room. We are right now just turning on and off circuit breakers to the units. We have two wall thermostats for each unit.

 

[MarkTTU]

We have two wall thermostats for each unit.

Do you mean you have two thermostats per unit (total of four) or that you have one thermostat per unit (total of two)?

I'm assuming you meant to say you have one for each unit.

If that is the case I am curious why you do not just set one thermostat at 75 and the other at 76. This would mean that one compressor would do most of the work and that the second compressor would only come on if the first compressor failed to keep the room under 75 degrees. In this setup the first system set at 75 degrees would also be the one to controll the air blower fan; the second comporessor would cut in only if the first was running anyway, and would cut out before the first cut out.

Now if you're trying to run the compressors equally for even wear then I do understand the PLC.

 

[1stwave]

yes, I meant to say one thermostat per unit. Also yes I want to evenly run compressors so when we need to run the unit it will be ready. Another point is we will be able to monitor this thru RSView(HMI).

 

[TConnolly]

Is there a reason that you cannot use programmable thermostats and program one thermostat to lead (72) on Monday, Wed, Fri, Sun, with the other lagging (75) and then switch which one leads/lags for Tues, Thur, Sat? This would be the simplest solution that would still keep each compressor running about the same amount of time over all. I know its not as cool as a PLC but its a lot easier and then anyone can maintain it in the future.

 

Day  Tstat1  Tstat2

Mon    72      75

Tue    75      72

Wed    72      75

Thu    75      72

Fri    72      75

Sat    75      72

Sun AM 72      75

Sun PM 75      72

 

[1stwave]

I am already using a PLC for this and plant monitoring.
Just need programming help to get this to work right.

 

[Lancie1]

1stwave,

The progrmming is the easy part. You have not gave enough details to know what brand and model thermostats that you are using, but the following would apply in most cases.

To control two HVAC units from a PLC using only the thermostat circuits requires modifying the wiring of the thermostats. Think about it: The PLC must have an input from each thermostat to tell it when the thermostat is trying to run the HVAC unit. Then The PLC must have two or three outputs (Heat, Cool, Fan) to each thermostat to interrupt the normal thermostat functions and instead allow the PLC to determine when each HVAC unit will actually run. All this must be accomplished without interferring with the normal thermostat functions. If your PLC has 24 VDC inputs and relay outputs, then maybe it will be easier. Otherwise you will need 2 input relays and 2 output relays.

I have attached an Excel spreadsheet file which illustrates the problems you are facing.