Can I turn off the AC in my schools during the summer to save energy?

During my thirteen-plus years as the Director of Facilities and Transportation for a School District in Southwest VA, I often asked myself the same question. When energy costs began to increase and school funding decreased in the early 1990’s, I really needed to reduce costs. One option open to me involved turning off the air conditioning (AC) system during the summer months in selected areas or in an entire school.

Why not? We turned off selected AC systems and watched our energy bill drop. But I wasn’t really saving money. Over the summer we also cleaned several carpets in the schools, so we turned on the AC long enough to dry the cleaned carpets and then shut off the AC as planned. The rooms were clean and dry and ready for use. Or so we thought.

A couple of weeks before school began, the areas were opened by the custodial staff and to their surprise, mold was everywhere – on the walls, in the carpets, and on the books and desks. The AC had chilled the wet carpet and produced condensation, which we had then shut into the room without ventilation. Think about what happens when you leave a chilled bottle of soda sitting on a table over night. In the morning, the bottle is the temperature of the room but there’s a lot of moisture on the glass, which is an indication that the temperature of the bottle was at or below the temperature of the room. Condensation, caused when the carpet warmed to room temperature, had allowed mold to run rampant.

I’m sorry to say that this kind of situation is a common occurrence in schools today. We all want to reduce energy costs and run our schools more efficiently, but turning off the ventilation in the building is not the way to go, especially in areas of the country with both high temperatures and high humidity in the summer. When the AC system is not allowed to operate during the non-school session time, mold can result.

You can still save money on energy over the summer, though, by setting the AC a little warmer than you would if children were in the building. Even four or five degrees warmer than the usual set point can show up as an energy savings.

What Does AC Do?
Air conditioning systems are designed to cool a building during the worst conditions of the year, which statistically might occur during a very small segment of the school year. The sizing of your AC system may be too large. Air conditioning performs two functions. It reduces the temperature of the air, and it wrings the water out of the air, effectively lowering the dew point. It takes more time to reduce the water in the air than it does to reduce the temperature. With an AC system too large, the temperature is satisfied before the moisture is removed. As you know, the AC shuts itself off when it reaches its appointed temperature, but it may not have done its job of getting the mold-loving moisture out of the air.

If buildings are closed at the end of the school year and the AC is turned off, the result will probably be mold. Do the math. You’ll find that the cost of running the AC over the summer is still less than the cost of the remediation of mold problems you’ll create and the probable bad press you’ll receive. You’re not saving anything by turning off the AC.

Mold 101
Note that mold needs moisture and the proper medium to grow. Schools in general are likely to provide mold with exactly the diet it desires. The key to controlling mold is controlling moisture levels.
Moisture can be introduced in many different ways:
Leaky plumbing and leaks in the building envelope (roofs)
Condensation and wet spots, for instance in the ceiling
Heating, ventilation, and air conditioning (HVAC) drip pan
Poor drainage and slope of the ground around the foundation.

To reduce the probability of mold forming during the summer break:
Operate AC systems on a normal schedule during the summer break but at a temperature setting about 4-5 degrees higher than during the regular school year
Use cleaning methods that will not introduce more moisture into the space or allow for moisture being trapped in difficult to dry areas
Determine if current AC system can be operated in stages to allow for longer operation of the system to satisfy temperature demands but long enough to wring out the moisture

If you’re designing a new school:
Require the AC system be design for optimization of temperature and humidity control
Size it right
Consider installation of a two-staged system – one for day and the other for night operation to maximize temperature and humidity control
Consider variable speed motors for blower motors to match compressor operation
Condition ventilation air to remove moisture from incoming outside air


If you have any questions dealing with energy efficient construction and operation and maintenance of your schools, do not hesitate to contact editor@Schoolfacilties.com, Subject: Ask the Expert.

Future topics may include:

• Value of Dark Schools concept to energy savings and overall operations of your school You don’t know how much energy is being wasted behind closed doors
• Impact of plug loads on energy use in your schools
• Better exterior lighting while saving over 70% of current costs
• What is the largest manageable element of a school budget?

If you have any questions please contact Lorenz (Larry) Schoff at lschoff@rev.net.