When it comes to heating your home, there are plenty of options to choose from. Not only are there various brands, sizes, and efficiencies for you to juggle, but the equipment itself may also vary depending on your specific heating needs. For such a significant purchase, it’s important to know what all of your options are.
For starters, most people in North America have a furnace as their primary home heating system, while boilers are more common across the ocean. While a furnace can certainly do the trick, in certain situations, it might be advisable to include a heat pump into your heating system, as well. In fact, sometimes a heat pump is all you need!
To see exactly how a heat pump can help, let’s explore how both heat pumps and furnaces work. First, it’s important to understand some basic principles about how heat, energy, and materials interact.
Temperature Control - 101
Lesson 1: What is Temperature, Really? What we commonly call temperature is actually a measure of the average kinetic energy of molecules of a substance. The higher the temperature of a substance, the faster its molecules move and the higher the average kinetic energy. As the molecules move around within the substance, they transfer energy/heat between each other.
Imagine a pool player breaking a rack. At first only the cue ball is moving. Once it strikes the racked balls, the cue ball is slowed and much of its energy is distributed to the other balls. This leads to a situation where all the balls are moving roughly the same speed.
In the same way when fast moving, high energy/temperature molecules (the cue ball) come into contact with slower moving, low energy/temperature molecules (the racked balls), energy is transferred to the slower moving molecules. In the same way that a pool ball at rest will never cause one in motion to speed up, energy and heat will always move from fast moving, high temperature regions to slower moving, lower temperature regions.
Lesson 2: Energy Phases
If enough heat is added to a substance, it will eventually reach a point where its molecules cannot travel any faster. This is called a phase boundary and additional heat added to the substance at this point will not increase the temperature. Instead this energy will change the state of the substance to a higher energy.
This is can be seen in a pot of boiling water. No matter how long the pot is left to boil, the liquid water will never get hotter than 212 F. Instead the water will be converted to steam and escape from the pot. The energy required to convert a substance in one state to another state is called the phase change energy.
In the reverse process, if water vapor loses energy to a cold substance, it will return to the liquid phase. This is called condensation and can be observed on the outside of a cold glass on a humid summer day.
Lesson 3: Phase Control
Liquid is considered to be saturated with vapor when the rate at which molecules are evaporating equals the rate at which molecules are condensing. Let’s imagine, then, that a liquid saturated in vapor was in a contained space, like a tube. If you were to squeeze (or compress) the tube, the same number of molecules is suddenly in a much smaller space. This means that there’s more chance for the vapor molecules to hit the surface of the liquid and turn into liquid themselves.
The opposite would be true if we decompressed the tube and allowed more space – with fewer molecules hitting the surface of the water, there suddenly become more molecules evaporating than condensing. This means we can influence the overall phase-change direction just by adjusting the pressure in the tube.
Lesson 4: Phase Control = Temperature Control
When a gas is compressed, molecules are moving around in a smaller space, and therefore are more likely to hit each other (imagine a bunch of people running to try to get onto a bus all at once).
The increased hitting means more kinetic energy and leads to the gas having a higher temperature, and this principle is referred to as Gay-Lussac’s law. The opposite is true as well – expanding the gas leads to lower pressure and lower temperature.
How This All Applies to a Heat Pump
The heating process in a heat pump begins with liquid refrigerant passing through an expansion valve to reduce its pressure. This causes it to become a cold saturated liquid (lesson 4).
This saturated liquid then passes through the outdoor coil. The outside coil acts as an evaporator, where the cold refrigerant absorbs heat from the less-cold outside air (lesson 1) and subsequently enters an entirely vapor state (lessons 2, 3).
This vapor then enters the compressor, where it is compressed and its temperature is increased (lesson 4).
Finally this super-heated vapor enters the coil in the indoor air handler. The indoor coil acts as a condenser and the refrigerant releases its heat to the indoor air and becomes a liquid once again (lessons 1, 2).
While this refrigerant heads back to the expansion valve to begin the cycle again, the air it has heated is distributed through the home.
How This Saves Energy
The important point to understand about a heat pump is that it does not use electricity or burn fuel to generate heat (although some electricity is used to run the compressor). Rather, it is able to draw heat from the outdoors and transfer it into the home through the manipulation of refrigerant states. The benefit of this is that the heat pump is able pump 3-4 times more heat energy into the home than the amount of electrical energy it uses.
A furnace also heats up air by transferring heat, but it needs to create it’s own heat to transfer. The furnace creates heat using a burner, which burns a fuel source, usually either oil or gas. The hot combustion gasses created from the burning are sent to the heat exchanger, which then gets quite hot itself. Cool air from the house is drawn into the furnace through return ducts due to negative pressure created by the furnace blower. The furnace blower then blows this air over the heat exchanger, where heat is spontaneously transferred to the air. The blower then continues to push this warm air through supply ducts and, eventually, back into the house.
Because a heat pump is able to pump more heat into the home than energy it consumes, it is often a more cost efficient heating source than a furnace. This is especially true in areas with low electrical prices or ones that rely on propane or natural gas as a fuel source. In addition to the likely energy savings, the heat pump has the added benefit of acting as an air conditioner during the summer months. It accomplishes this by reversing the heating cycle and pumping heat from inside the home to the outdoors.
A Heat Pump's Biggest Weakness
However, like many things, a heat pump’s biggest strength – relying on outside air as a heat source – is also its biggest weakness.
As it gets colder outside, there is less heat available for the heat pump to use, making the heat pump less efficient and unable to supply as much heat as before. Eventually, the heat pump will reach a point where it is unable to supply enough heat to maintain a comfortable temperature inside the home. This typically happens at an outdoor temperature of about 40° Fahrenheit, but there are a wide number of factors that will influence the exact temperature. A heat pump is a great solution for all climates, but your specific conditions will dictate how best to integrate into your homes heating system.
In warmer climates where the temperature doesn’t get very low, a heat pump can be a complete home comfort solution, since it can provide air conditioning when it’s hot and will stay efficient in the less-warm months. In case of emergency, you can always add electric heat using something like this electric boiler by Electro Industries.
Incorporating Heat Pumps in Colder Climates
In areas where the temperature can drop quite a bit, it still makes sense to use a furnace, but the addition of a heat pump can still reduce your energy costs during those times when heat is needed and the temperature isn’t too low yet. This can be automated by a thermostat that senses the outdoor temperature and makes a decision about which heat source to use.
Plus, since a heat pump can work as an air conditioner, you can get rid of your current air conditioner and not worry about having much more equipment than you had before (this is great if you’re already looking to get a new air conditioner, too). If you’re looking to get a new furnace as well, there are some great packages available, including Goodman.
Hopefully, now you can make a more educated decision about your home heating system. As heating season is rapidly approaching, consider updating your heating system now, before you become dependent on it for comfort.