Instructables user tikka308 writes:

Because HVAC units use a lot of energy (particularly when in ‘air conditioning’ mode during hot summer months) and renters do not have the ability to easily implement energy star (i.e. more efficient) units or to regulate their tempature, I wanted to find a way, without making permanent changes, to control an HVAC unit like a thermostat! Implementing this device can not only save you money, but can help maintain a more steady-temperature in your apartment, reduce energy consumption and help reduce the strain placed on our nations power grid during the hot summer months!