How Does a Heat Pump Work and How Does Temperature Affect Efficiency?

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.  

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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.