Over the last few years, HVAC equipment manufacturers in the North America have become increasingly aware of the advantages of having two or more stages of heating and/or cooling in their products. Rising fuel costs, federal, state and local energy efficiency guidelines, along with increased competition have driven manufacturers to introduce better system for their customers. The efficiencies gained by staging vary with the type of equipment, the installed application and the geographic region where the equipment is installed.
HVAC equipment manufacturers have developed 2-stage systems to meet market demands. Yet, in many cases, the manufacturers do not build their own thermostats for use with the equipment. This is due to manufacturing economies of scale and a different technical mindset than those specialized in controls. These manufacturers must rely on thermostat and zone control manufacturers to deliver multiple stage thermostats for their equipment, and in many cases they have no control over which thermostat the installing dealer provides.
This short article provides insight into how staging works and the best methods for staging.
‘Setpoint Staging’ is the term used to describe an ‘up-stage’ or a ‘down-stage’ of a thermostat based on the difference between the room temperature and the thermostat setpoint. With this type of thermostat, staging occurs at a fixed interval of degrees from the setpoint. Using this technique, if a setpoint is less than two (2) degrees from setpoint, only the first stage will be engaged, but if the temperature is two degrees or more from the setpoint, the second stage will also be energized until the room temperature is within two degrees. At this point, the thermostat “down-stages” to only one stage energized and continues to drive toward the desired setpoint.
While this basic method may actually control the equipment to use both stages, it is also the least efficient method. This type of control is not recommended for environments that use an overnight setback for unoccupied periods for energy savings. In this situation, the thermostat will engage both stages, possibly unnecessarily, when the setpoints are adjusted to return to normal occupied settings, causing energy waste. Most of the lesser expensive thermostats on the market use this type of temperature-based logic due to a lower cost of implementation.
‘Time-Based Staging’ is the term used to describe thermostat staging based on time. With this type of control, staging occurs at a fixed interval of time after firing on the first stage. While this method is more efficient than ‘Setpoint Staging’ is, it also has some deficiencies regarding control and energy savings. Many zone control systems and some commercial grade thermostats on the market use this type of time-based logic.
The basic philosophy for time-based staging is as follows. When a call for cool is needed, the first stage is energized, and then at a particular time after that (let’s say, 8 minutes) the second stage of cool is forced on by the thermostat. At the surface, this staging process may seem to present no problem, but let’s dig a little deeper by asking a few questions.
Q1: What happens if the room is cooling down with only one stage being energized and the room temperature is nearly, but not quite (say, 1 degree high) at the desired setpoint after 8 minutes?
A1: With time-based staging, the second stage of cool would be energized, bringing on a second stage or a second compressor. This additional cooling capacity will help bring the room down to the desired temperature, but it is not really needed. Furthermore, bringing on the second stage in this case will most likely overshoot the setpoint by one or more degrees. The resulting overshoot happens because there was really no need for the second stage cooling to start in the first place, as the room was nearly to setpoint already.
Q2: What else happens when the second stage gets energized at a fixed time interval?
A2: Continuing with the example above, time-based staging could result in two or three degree overshoot because condensers are designed to have a minimum run time to keep from short-cycling the condenser and damaging the unit. The minimum run-time of 4 minutes is the recommendation of many HVAC equipment manufacturers.
Q3: What happens when the setpoint is reached when two stages of cooling energized?
A3: One of two things will happen.
Option 1: A hard shut-off of both stages will occur and if the second stage condenser has gets turned off, which can cause short cycling of the condenser. This is undesirable because it shortens the life of the condenser.
Option 2: The staging algorithm employs a short cycle protection for a minimum of 4 minutes. Since setpoint was nearly achieved before the second stage was even employed, significant overshoot of the room temperature can occur, possibly two, three or even more degrees. This yields a temperature swing in the room greater than needed and desired.
‘Rate-of-Change Staging’, or ‘Dynamic Staging’ is the term used to describe an ‘up-stage’ or a ‘down-stage’ of thermostat based on the derivative of the room temperature and the amount of time it takes to change the temperature 1 degree. Dynamic staging often employs ‘fuzzy logic’. Fuzzy logic is a very precise sub-discipline of mathematics developed in the 1960’s and is becoming more and more common in commercial and consumer applications now that sophisticated processors are available at reasonable costs. Fuzzy logic reduces a spectrum of raw numbers, such as temperature and time, into categories which enable us to simulate human thinking by quantifying concepts such as hot, cold, very far, extremely close, etc.
XCI’s thermostat control algorithm employs fuzzy logic in two different spectrums. First, the thermostat processor examines the room temperature, setpoint, and the historical time it has taken the thermostat to reach setpoint previously under similar conditions. These items are wrapped into a fuzzy logic equation that also checks for minimum run times and other system parameters. The outcome is a very sophisticated control algorithm that allows a very tight, very smooth and energy-efficient temperature control with XCI’s thermostats.
XCI Basic Control – Because XCI’s thermostats use fuzzy logic, heating or cooling may or may not come on instantaneously (as with a mechanical thermostat) with a user’s change in setpoint of one degree since the thermostat understands how long it takes to adjust one degree. As an example, if historically the room temperature drops one degree in three minutes when cooling is energized and it takes the room 15 minutes to rise one degree when the cooling is not energized, then the thermostat will wait a few seconds to several minutes before energizing cooling. The thermostat performs much more accurate temperature measurements than the displayed (one degree), and the fuzzy logic control algorithm watches for changes for temperature over time.
XCI Dynamic Staging – As stated above, XCI’s advanced algorithms examine the rate of temperature change over time in order to determine when and if a second stage is needed to bring the room temperature to the desired setpoint. The thermostat looks for a rate of change (called ‘slope’ for us techies) of 6?F over a 1-hour period when cooling or heating is energized. After twenty minutes, if the slope is not on a path of 6?F/hr or more, the second stage will be energized. The thermostat continues to monitor the rate of change and the room temperature throughout the cycle, and will downstage appropriately for the environment.
This technique delivers very smooth and energy efficient staging, while allowing the HVAC equipment to perform at its optimum levels. XCI’s thermostats are ideal for environments with unoccupied period that take advantage of energy efficient setbacks (8 – 10 degrees or more). The algorithm allows the first stage to run longer before energizing a more expensive second stage of heating or cooling, and intelligently modifies when the second stage should be energized based on learning from the room’s history. Other advantages include near elimination of overshoot and the tendency to short cycle second stage condensers.
Q1: Since XCI’s thermostats use fuzzy logic, it is stated above that the thermostat may or may not come on immediately if I raise (or lower) the temperature only one degree. Is there a way to force the thermostat to come on instantaneously?
A1: Yes. In some cases it obviously makes sense to want to have the heat come on briefly if you’re a little chilly, or visa versa when you’re a bit warm. Simply adjust the setpoint to the same as the current room temperature on the display. Let the thermostat return from setpoint adjustment mode (it does this automatically in two seconds), and then increase or decrease the setpoint (push the up or down button again) two degrees from the current room temperature. This will tell the thermostat to override the fuzzy logic efficiencies and energize the first stage immediately.
Q2: Can the second stage be energized sooner than the thermostat’s algorithm has determined? Is there a way to force the second stage to engage manually?
A2: Yes. Simply follow the two step process above to energize the first stage if it’s not already energized. Then increase or decrease the setpoint again by another two degrees (from the setpoint). This will tell the thermostat to energize the second stage immediately. Downstaging will occur at the appropriate time by using the algorithm. Alternatively, changing the setpoint back to the room temperature will de-energize the appropriate stage(s) while taking into account short cycle protection.
In conclusion, the type of staging for multi-stage thermostats ranges from the very simple temperature based controls for inexpensive thermostats to the sophisticated dynamic staging that uses fuzzy logic mathematical algorithms for smooth, efficient control. The differences in control techniques should be considered when determining which thermostat will be connected to the manufacturer’s equipment. While the more sophisticated versions can be used with nearly any system, if you’re using a simpler version, make sure that it’s compatible with the HVAC system and the discussion above is taken into consideration.