I did it by bringing rents to market value, reducing delinquency by improving tenant performance, developing and implementing a water conservation program, installing temperature limiting thermostats, developing a a/c condenser maintenance program, installing solar screens on all windows to reduce heat input into units, renegotiated service contracts to reduce cost, and on and on.....  

Adding a whole new AC unit seems expensive, I could add a window AC unit, maybe have the temperature set at a certain degree? 

Utilities are included in the rent & have run between $800 to $1000 depending upon the outside temperature.6 units have been redone with six that have yet to be completed.  

Typically there are two scenarios with cold rooms and forced air heat. 1) One room never gets enough air to heat it (or cool it in the summer) and so is always a different temperature or 2) One room warms up (or cools down) the same as the other rooms but once the system shuts off, changes temperature more quickly than the others.The solution to #2 is the most difficult to fix because it is usually a symptom of building construction rather than HVAC system issues.

Dallas is hot, I am not talking about real estate market, I am talking temperature wise.

Or sometimes its just temperature changes.if you were in bad foundation areas like texas maybe that would be different.

Vinyl siding is extremely sensitive to temperature.

so... a furnace built in the 60's is going to be about 60% efgicient. a modern furnace will be in the 90% effecient range. many cities and counties offer rebates to help pay for the upgrades.if you cant get the billing individually assigned to the tennants, i would look into this.i would also consider removing the old school thermostats and replacing with an automation system where you can limit their temperatures a little bit...

It was stated that brick aids in temperature regulation for a home.

The landlord has to, by law, provide a heating source that would be able to heat the unit at x height to x temperature.