The minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated.Heat Source- RequirementsR303.9 Required heating.When the winter design temperature in Table R301.2(1) is below 60°F (16°C), every dwelling unit shall be provided with heating facilities capable of maintaining a minimum room temperature of 68°F (20°C) at a point 3 feet (914 mm) above the floor and 2 feet (610 mm) from exterior walls in all habitable rooms at the design temperature.

These would be finding people who haven't listed their property but likely just need the easy button and a nudge from you to make their decision to sell.Moving outside the city to some surrounding counties can help as well.2 good targets for you to Consider: Absentee Owners with Equity and Seniors with 15+ year ownership.Here are some suggestions for your area counties,Absentee Owners: 13,256 Home value up to $300,000, single and multifamily, high equity Adams 1970   Douglas 122 Arapahoe 1162   Jefferson 1703 Boulder 863   Larimer 2643 Broomfield  49   Weld 2737 Denver 2007    Senior Homeowners: 9881 availableOwner Occupied, age 60-89, single family homes, High equity, home value up to $350,000, 15+ year Ownership AND Low Financial Stability Scores (FSS) Adams 2710   Douglas 38 Arapahoe 1211   Jefferson 771 Boulder 343   Larimer 523 Broomfield  29   Weld 1205 Denver 3051

However, it's super expensive for a 12 unit building and we have the sprinkler system which I believe would get set-off in the high temperature.  

Find a local metal shop (preferably one who specialized in wrought iron) and have them weld on the extra needed material, then have the railings powder sandblasted and powdercoated.

This would give her the ability to change the temperature while she is out of town.Then she could just text you if there is a package on the porch that needs to be put in the house.

welding? 

Its a concrete tilt up construction and it has a 30 ft. clear ceiling height.What is the best way to reduce the temperature?

This means around the 1st of the year there is a temporary boom of unqualified renters entering the market and to make matters worse the temperature normally drops tremendously which increases the danger of frozen pipes or high utility bills (choose one or the other) insurance agents tend to frown upon vacancies... 

What was the temperature that day?  

If it has centralized heat already, why not set the heat at a constant temperature, say 65 in the winter, and install baseboard heaters in each unit so the tenants can up the heat to their liking, on their own dime.