Disclaimer

Before diving into details, I acknowledge that various users report differing performance levels with similar heat pumps. Factors like home size, building materials, ductwork, air handling, and insulation all influence heating efficiency. For instance, my heat pump's performance may be constrained by duct diameter relative to home size, and upgrading may not be feasible.

This article focuses solely on heat pumps, assuming the other components of your heating system are satisfactory. An HVAC professional can provide a comprehensive evaluation.

The insights and data presented here are specific to my home! Each system behaves differently, and small variations can lead to significant differences in results.

Traditional Heating Dynamics

Conventional heating systems operate consistently under varying conditions. My furnace, for example, raises indoor temperatures by approximately 11°F (6°C) each hour, irrespective of external conditions.

In homes with traditional heating, costs escalate during colder months due to increased heat loss through walls. This is unrelated to the heating equipment itself; rather, lower outdoor temperatures necessitate more frequent heating to compensate for the enhanced heat loss (illustrated in Figure 2).

Heat loss can be described using Newton’s Law of Cooling (NLoC). To simplify:

To reduce heating expenses, one can either enhance insulation to minimize heat loss or lower the thermostat, thereby narrowing the temperature gap between indoors and outdoors.

I attempted to quantify my home's temperature-dependent heat loss, but the data was too noisy. It’s not surprising that the 1%/1°F guideline from the DOE was derived in a controlled experimental setting.

For some calculations in this article, I relied on the DOE’s published findings and my own limited observations to estimate temperature drops related to NLoC.

Heat Pump Dynamics

Heat pumps experience diminished heat transfer capabilities as the difference between indoor and outdoor temperatures increases. Consequently, homes with heat pumps not only lose heat more rapidly but also generate heat more slowly (see Figure 3).

To quantify this reduced heat generation, we reference Carnot’s Rule:

It’s often stated that heat pumps function best in moderate temperatures. I mentioned this myself earlier! However, it’s more precise to state that heat pumps are most efficient when outdoor temperatures are closer to indoor levels.

Real-World Performance Analysis

While reading a thermodynamics textbook can be enlightening, practical application is essential. Utilizing my heat pump, thermostat data logs, and a background in Statistical Process Control, I quantified savings directly.

During the cold months, I set my thermostat to 65°F (18.3°C) during the day and 60°F (15.6°C) at night. After analyzing data from the winter of 2022-23, Figure 4 presents a comparison of heat pump performance during the day and night.

The electricity usage of my heat pump is constant (2.4 kW). By analyzing the relationship between indoor and outdoor temperatures and factoring in estimated heat loss, I created a model to compare operating expenses at various indoor temperatures (see Figure 5).

On average, I save around 40% on heating costs by lowering the thermostat by 5°F (3.3°C), which equates to approximately 7% savings per degree (assuming exponential savings).

One important note: my home may not be a scientifically controlled environment. The data presented here is based on heat pump performance in Figure 4, supplemented by estimated heat loss corrections. Given the data's variability, some error may exist.

Overall, I am confident that lowering indoor temperatures leads to substantial heat pump cost reductions. All reasonable estimates indicate significant savings. The 40% for a 5°F reduction (or 7% per degree) is my best estimate, but real savings may vary.

Even with my conservative thermostat settings, heating expenses comprise nearly half of my overall utility bills. Any reduction in these costs significantly affects my home's energy consumption.

Heat Pump Efficiency and Thermostat Settings

Observations regarding heat pump performance are specific to individual homes. However, if my findings hold true for others, they could have far-reaching implications.

For heat pump users, even a slight decrease in indoor temperature can yield substantial savings. Based on my calculations, lowering the thermostat from 72°F to the DOE-recommended 68°F (20°C) could reduce heating costs by 25% or more, translating to hundreds of dollars annually. This conclusion was drawn from months of data collection, analysis, adjustments, and verification. I understand if you prefer not to repeat this process.

Even without personalized verification, I can assert that any reduction in indoor temperature for a heat pump-equipped home will significantly lower heating expenses, far exceeding the often-cited 1% rule for traditional heating systems.

Fewer Emissions with Reduced Furnace Usage

Operating a heat pump at 60°F dramatically affects the lowest outdoor temperature at which it remains functional. Currently, I lock out my heat pump below 30°F (?1°C) based on its performance at 65°F.

As illustrated in Figure 6, when the indoor temperature is set to 60°F, the heat pump remains effective down to 20°F (?7°C). Last year, around 30% of my furnace usage occurred at night when outdoor temperatures hovered between 20–30°F while my thermostat was set to 60°F.

This winter, I plan to test a lockout at 20°F (?7°C) during nighttime. I could potentially save hundreds of pounds of CO? emissions annually while likely reducing my utility costs as well.

Personal Comfort and Its Impact on Savings

In my earlier disclaimer, I noted several overlooked factors that significantly influence heating expenses in an HVAC system.

Placing indoor temperature at the top of that list is crucial.

Your comfort level regarding indoor warmth can determine whether a heat pump effectively halves your heating bill or proves less efficient than a furnace. If someone were to occupy my home and insist on maintaining a temperature of 72°F (22°C), they would likely find minimal cost benefits from the heat pump compared to solely using the furnace. Again, this varies by home, yet heat pumps still offer substantial emissions reductions compared to combustion heating.

When discussing heat pump effectiveness (as seen in comments, for instance), it is essential to specify typical indoor temperatures alongside home size and minimum outdoor temperatures as key factors influencing efficiency.

Final Thoughts and Future Plans

With emerging heating technologies, we must reevaluate established guidelines and preferences in light of new capabilities. Indoor temperature plays a pivotal role in heat pump efficiency, more so than in traditional heating systems.

To maximize the benefits of a heat pump installation, I recommend:

• Lowering your indoor temperature by a few degrees and monitoring whether this results in reduced heating cycle durations. Share your findings in the comments!
• If you are comfortable working with data and have access to thermostat logs, I would be interested in assisting with a similar analysis. This could provide you with practical insights on thermostat settings and help validate my findings.

Personally, I have follow-up tests planned for this fall and winter:

• Assess heating costs at warmer indoor temperatures (70°F versus 72°F)
• Investigate lower heat pump lockout temperatures during nighttime (25°F or even 20°F)

Those who follow my work can expect updates on these investigations when they are complete.

It appears that individuals who prefer or can tolerate cooler indoor temperatures will enjoy significantly lower costs and emissions, while those who set their thermostats higher may only see marginal benefits.

If you found this article insightful, please consider giving it a clap. Your support helps sustain the author's work.

I welcome any questions in the comments. Additionally, feel free to explore my other articles on heat pumps and HVAC systems.