Written by Leo Sharkey, owner
How Heat Moves
First, let’s talk about how heat moves between objects or environments. We know from experience that heat moves from a hotter object to a colder object and not the other way. That is, if you put a cup of hot coffee on a cold granite countertop, the countertop gets warmer, and the coffee gets cooler. It just doesn’t work the other way where the counter gets colder and the coffee boils.
Photo by NordWood Themes on Unsplash
A heat pump is a device that uses energy to force heat to move the opposite direction of its natural flow. A heat pump placed between the coffee cup and the countertop would pull heat from the countertop, making it get colder, and cause the coffee to boil. Technically, an air conditioner is a heat pump because it pulls heat from your warm house and pushes it to an even hotter outdoor environment. In the HVAC industry, we refer to heat pumps as systems that cool in the summer and can reverse and provide heat in the winter – taking heat from outside and putting it into your even warmer house.
At this point, we understand what a heat pump does. It makes heat flow from a colder object to a hotter object, which is the opposite of what would happen naturally. Now, let’s start to examine efficiency.
Efficiency
Continuing with our coffee cup analogy, think of the difference in temperature between the countertop and the coffee as a hill. The lower the temperature difference, the smaller the hill. If your coffee was lukewarm and the counter was room temperature, the difference or hill is small and making the heat flow up the hill isn’t as hard. The larger the gap, the more work needs to be done to get over the hill. The smaller the hill, the less work, therefore the higher efficiency we can get. In terms of temperature, the lower the difference in temperatures, the higher efficiency we can get from our heat pumps. This means that when the outside temperature is closer to the inside temperature of a home, the more efficient a heat pump will be.
Let’s start to get specific about efficiency. In heating and air conditioning systems, we use electricity to power the heat pump to add or remove heat from our homes. This brings up the obvious question of “why don’t we just use electric resistance heat instead of a heat pump?” An electric resistance heater is a very simple device that uses electricity to make heat by passing a current through a resister. If you put 1,000 watts of energy into an electric resistance heater, then you will get 1,000 watts of heat out. This is a constant and never changes. Due to some complicated physics which we won’t get into, if you put 1,000 watts into a heat pump you will get a varying amount of heat out. For example, a Mitsubishi heat pump operating in heat mode at -5 degrees Fahrenheit outdoor temperature will provide 2,000 watts of heat output for the 1,000 watt electric input! At -5 F, the heat pump is 2X as efficient as an electric resistance heater. That multiple is known as the Coefficient of Performance or COP. As the temperature warms up outside the COP increases.
Compared to electric resistance heat, even at -5 F, a heat pump puts out 2x more heat than a resistance heater using the same amount of energy input, or it is 2X as efficient. As the temperature outside rises, that multiple also increases as you can see in the table above. So, you absolutely want to run a heat pump when it is mild outside and then switch to a fossil fuel at some point where the heat pump efficiency drops below the cost of running the fossil fuel. All of this depends on your exact cost of electricity and the cost of the fossil fuel.
To read the table, here are a few examples:
- An electric resistance heater will cost you $74 per million BTU’s.
- A heat pump has a variable cost depending on the COP or outside temperature. At worst (-5 F), a heat pump is 2X as efficient as electric resistance and on a warm day, COP of 4, it is almost 5X as efficient.
- Propane is very expensive fuel and natural gas is very affordable.
Based on the table above, we can draw the following conclusions:
- If you use oil for your primary fuel, a heat pump is as efficient or better than your oil system.
- If you use natural gas, you should run your heat pump until it is roughly 35 F outdoors.
- If you use propane, you should turn off your system and always use a heat pump.
- If you use electric resistance heat, you should always run a heat pump instead.
Conclusion
In conclusion, heat pumps are extremely efficient for heating and cooling your home. Your exact savings depend on several factors including the fuel you use now and its cost, the cost of your electricity and the outdoor temperature.
Please let us know if you have any questions and thank you for your time and attention.
This is the clearest explanation I’ve found for using a heat pump for house HVAC. In my co-working space, they have an insulation “earth room” in the basement, which reduces the temperature differential with buried underground tubes, and now I understand why. It’s to make their heat pump more efficient to warm the building. But they call it a heat exchanger, so another lesson for another time. Thanks for the lesson.
Hi Paul, thank you for taking the time to leave a comment! We’re thrilled to hear that you’ve found our post helpful!
Great article! I appreciated your tips for maximizing efficiency, such as regular maintenance and choosing a system with a high HSPF rating. Overall, I found your post to be a great resource for anyone interested in heat pumps. Thanks for sharing your knowledge and expertise!
I am looking at installing a 4 ton HP SEER 16 HSPF 10. I understand a HP is more efficient at higher temperatures. I am trying to understand the likely hourly electrical demand of the unit as the temperature drops say from 45F to 40F and 35F. I am trying to determine the economic best temp to use my standby gas furnace. We are in DoEvzone 4 and get 90 to 200:hours per year with temps below 32F. Large house current furnace 80000 btu/hr output.
Appreciate any thoughts.
Hello Don,
We appreciate you taking the time to read our article!
A heat pump has a variable efficiency based on the temperature difference between the hot and cold environments. This variable efficiency is summed up as the COP – Coefficient of Performance for a heat pump. COP is the ratio of the heat (energy) moved divided by the energy input. I.e. if you use 1,000 watts of electricity to move 2,000 watts of heat, the COP = 2.0.
To calculate the optimal temperature to switch from a heat pump to a fuel (gas or oil) one would have to know the cost of electricity per kW/Hr, the cost of gas per therm (propane or oil per gallon) and the COP of the heat pump system at various temperatures. Without knowing your electric rate, gas price and exact system COP ratings, there isn’t enough information for us to provide you with a reliable answer. But, we hope this helps you know what questions to ask and what information to gather. Your HCVAC installer should be able to help you with this calculation!
Hi! Quick question….
Do heat pumps produce varying temperatures, or should they be constant?
The temps on my heat pump are all over the place depending on where I set the temp.
Thank you
Hi Kevin,
Thanks for reading! In regards to you question, The short answer is “yes, but it depends.” Depending on the brand and the control scheme, most heat pumps will turn down, or run at a lower, but constant output when the heating demand is not high. High demand occurs when the heat pump needs to run harder to provide more heat, which can occur when it is colder outside or if the set-point is much higher than the actual room temperature. When most heat pumps turn down to a low fraction of the total rated output, generally the discharge air temperature is lower than if the unit was in a high demand situation.
I hope this helps to answer your question!
Best,
Leo Sharkey, President