Quality digital thermostats can help HVAC systems both maintain a comfortable temperature differential and maximize their performance while prolonging equipment life.

By Tim Burke

Homeowners expect their HVAC systems to first and foremost provide comfort. Studies show that consumers value comfort, ease of use and energy savings above all other HVAC benefits. The critical role thermostats play in the performance of an HVAC system is often overlooked. Thermostats are typically undervalued—even though they are the link between the HVAC system and the homeowner’s comfort.

Over the years, the market has moved to more feature-rich digital thermostats to meet consumer demand for greater comfort control. The human body can feel uncomfortable with temperature changes greater than ±2° F, but with today’s digital thermostats a consistent temperature can be maintained throughout the home.

Thermostat accuracy equates to comfort
Thermostat accuracy also is known as operating differential [“differential”] which is the difference between the cut-in and cut-out points as measured at the thermostat under specified operating conditions. In NEMA testing, different heating-/cooling-load conditions are applied to measure differential. Differential is sometimes referred to as “swing.”

Of particular importance is the relationship between the differential and cycle time. As the graph illustrates, narrower differential results in shorter equipment-cycle times, while wider differential time results in longer equipment-cycle times.

The right balance
The ideal temperature differential for personal comfort is zero. However, a zero differential is neither practical nor achievable. Zero differentials require rapid short cycles that sacrifice efficiency and equipment life. For increased efficiency and longer equipment life, long cycles are perfect. Long cycles however, unfortunately are not practical for personal comfort.

Since the differential must be wider than zero, the key is finding a differential that does not cause homeowner discomfort while ensuring optimum equipment cycling. Since most occupants will notice temperature changes of as little as ±1°F and feel discomfort at ±2°F or more, the maximum differential should always be less than ±2°F.

The cooling differential for a compressor is typically a minimum of 10 minutes constant run time to ensure proper mechanical lubrication. If it cycles too quickly, compressor life will be reduced. Additionally, cooling equipment has another design consideration—humidity removal. In cooling, longer cycles (wider differentials) more effectively remove humidity.

Heating equipment does not have the same compressor minimum run-time constraints. Therefore, heating equipment run times can be shorter—up to the point where rapid cycling results in excessive wear—with or without a compressor.

Because cooling equipment has longer minimum run-time requirements than heating equipment, the minimum differential for cooling should be greater than the minimum differential for heating. High-quality digital thermostats typically have two differentials, one for cooling and one for heating. To optimize both temperature- and humidity-comfort control, and to protect against excessive equipment cycling, the cooling differential should be between 0.8°F and 2°F; the heating differential should be between 0.5°F and 1°F. A heating differential below 0.5°F is too narrow and causes excessive cycles.

Droop: a drag on performance
Another leading contributor to wide differentials is a condition called “droop,” which is more prevalent in mechanical thermostats due to the heat created by the heating or cooling anticipators. Under high-load conditions, heat from the anticipator builds up within the thermostat, causing it to cut out and cut in at lower temperatures than under lighter loads. The lower cut-out and cut-in points result in a lower control point known as droop.

Droop also can occur in digital thermostats, but it is typically less pronounced than that found in mechanical units. Digital thermostats that use triacs instead of relays to energize heating and/or cooling are more susceptible to droop. Triacs generate heat inside the thermostat, and like anticipators, the heat builds up on longer cycles, leading to greater droop and less accuracy.

The easy way to identify digital thermostats that use triacs is the absence of an audible “click” from a relay when turning on heating or cooling. Relays minimize heat buildup and are a more accurate, but more expensive, way to minimize droop.

Maximizing HVAC performance/comfort
Heating and cooling differentials are critical to the comfort of a homeowner. Even the benefits of the best designed system can be negated by using an inferior digital thermostat because of excessive differentials and droop. That is why it is important that quality digital thermostats are automatically included on a contractor’s punch list. A quality digital thermostat will help to maximize the performance of the HVAC system and increase homeowner comfort.

Tim Burke is the Education and Learning Manager at Emerson Climate Technologies. He has 30 years developing educational solutions for the HVAC industry on behalf of Emerson Climate Technologies and can be reached at Tim.Burke@emerson.com.