Your thermostat maintains comfort by automatically adjusting how hard your HVAC system works and for how long it operates. Your monthly summary report displays the average demand for each component in your system. Demand is a number from 0 - 100%, where 100% is the maximum heating or cooling your system can provide. For example, a system running at 50% cooling demand is delivering 1/2 (or 50%) of its maximum cooling capacity.
Demand provides better insight than runtime alone because it factors in what level of heating or cooling the system was operating. This is important for multi-stage and variable capacity systems that use multiple levels of heating or cooling to save energy. These efficient systems operate at the lowest level of heating or cooling whenever possible and only use the higher levels on those really hot or cold days. Using lower levels of heating or cooling can result in longer runtimes, but it is more efficient.
Heating demand will increase as the outdoor temperature drops. As you transition from fall into winter, you will typically see your heating demand increase. Similarly, in the spring heating demand will decrease as the weather warms up. Your report includes the average outdoor temperature for each month, enabling you to compare your change in heating demand to the change in temperature.
Cooling demand will increase as the outdoor temperature increases. Cooling demand will be the highest in the peak of summer and drop off as temperatures decrease. Increasing cooling demand without a corresponding increase in outdoor temperature may indicate a problem with your system.
Examples:
Single-Stage
Single-stage systems have only one operating capacity, on at 100%. Therefore, the thermostat simply adjusts demand by changing runtime. So, a single-stage system operating at 50% demand, runs 50% of the time.
Two-Stage
Two-stage systems have two operating capacities, typically 50% and 100%. The thermostat will first attempt to maintain comfort by operating only the lower (50%) stage, since it is more efficient. When the system cannot maintain comfort with the lower stage, it will begin to operate the second stage.
• A two-stage system operating at 50% demand runs first stage 100% of the time. Runtime would be 100% for this case.
• A two-stage system operating at 75% demand run first stage 50% of the time and second stage 50% of the time. Runtime would be 100% for this case.
• A two-stage system operating at 100% demand runs second stage 100% of the time. Runtime would be 100% for this case.
Variable Capacity
Variable capacity systems continuously adjust the heating or cooling capacity from 40%-100% of maximum capacity. The thermostat will maintain comfort by continuously adjusting the capacity of the system. When the demand is below the minimum capacity of the variable capacity system, the system will cycle on and off while running at the minimum capacity.
• A variable capacity system operating at 40% demand will run continuously at 40%. Runtime would be 100% for this case.
• A variable capacity system operating at 20% demand will run at the minimum capacity of 40% for 50% of the time. Runtime would be 50% for this case.
