Introduction
Heat pumps are one of the most efficient ways to heat with electricity, but they have their limitations. Learn more here.
Step by step
Some Background…
Heating systems all use some sort of fuel to produce heat. Combustible fuels such as natural gas, propane, heating oil, etc. are usually the most efficient in terms of heat produced (BTUs) per dollar. The combustion methods used (boilers, furnaces, etc.) aren’t super efficient, but typically these fuels are fairly low cost compared to electricity (on a BTU basis).
However, when these fuels aren’t available, we are usually forced to use electric heat. This can often be an expensive proposition.
The traditional method of heating was simply resistive heat. Think of resistive heat as your toaster. There are elements that gain heat through Resistance when electric current is passed through them. These can be configured in a furnace (air is blown over the elements), boiler (water is heated with the element then pumped throughout the home), or radiant (baseboard heat holds the element and heats through natural convection and radiation).
The biggest advantage to resistive heat is that it is cheap to install up front. The equipment doesn’t cost much and it’s easier to pull wire than install piping for fuel and hot water. Maintenance is also pretty low. The biggest disadvantage to resistive heat is cost. Running a bunch of toasters all the time would hit your electric bill pretty hard, and so does resistive heat.
We’ve said all this to lay some background for why electric heat pumps are so great. They allow for many of the advantages of electric heat with competitive operating cost to combustible fuels.
How a Heat Pump Works
Later in this article we’ll talk about the difference between air source, water source, and ground source heat pumps. Don’t worry about that for now though as the concept is the same for all.
Did you ever notice how hot your refrigerator gets? Maybe you’ve stood outside your air conditioning condenser on a warm Summer day and noticed that the fan is blowing hot air? In both cases this hot air is a byproduct of the cooling process.
Since energy can neither be created or destroyed, all heating and cooling processes are just transfers of energy from one place to another. This transfer takes work, which consumed energy. In a cooling process, the work is supplied through a compressor. The compressor is a key part of the vapor compression cycle that allows refrigerants to transfer heat from one area to another. In a cooling application, the goal is to remove heat from the conditioned space and dump it elsewhere. That’s why you have hot condenser coils on your refrigerator or hot air coming off you’re A/C condenser. It is all the concentrated BTUs of heat being expelled from the area you’re keeping cool to the ambient surroundings.
Now imagine you want to heat the area instead of cool it. If you take a cooling system and just reverse it, you can extract heat from the ambient area (air, water, ground, etc.) and add it to your conditioned space. This is how a heat pump works!
Heat pumps are almost always used for both heating and cooling. The process is virtually identical! All that it takes is proper design and an important component called a reversing valve. The reversing valve is what switches the cycle from cooling to heating, or vice versa, by reversing the heat transfer process. Due to this design, it is usually cheaper to install a heat pump system in homes that already need cooling than install a separate furnace/boiler. If a home has heating only, then using a heat pump is a poor proposition for a few reasons, which we’ll touch on soon.
Types of Heat Pumps
Air Source
The most common type of heat pump is an “air source” heat pump. These are the cheapest variety and can be installed easier than the others. The simplest example is the lowly window air conditioner. While some are cooling only, most that provide heating do so by incorporating a reversing valve in a heat pump design.
Air source heat pumps draw their heat from ambient air. This is a readily available source, but isn’t without its limitations. Outside air temperature impacts both efficiency and delivered air temp. Worst case scenario is that it gets too cold outside to draw any heat and the system is unable to meet set point. More on this when we cover limitations.
Ground Source
The “sexier” cousin to air source heat pumps is ground source. Once you get below the frost line, the earth temp is consistently mid-50’s Fahrenheit in most climates. While this may not sound like much, it is enough to extract plenty of heat out of.
Ground source heat pumps use a series of water filled coils buried in the ground to draw the heat needed. In cooling mode, the expel heat to the ground in a similar manner. These coils need to be buried below the frost line and are installed either in vertical wells or laid out in trenches. Depending on the size of the system, the number and depth of wells or trenches will vary. Obviously, all of this digging and piping makes these systems a lot more expensive than air source systems. However, they offer consistency of temperature that air systems cannot and the only real heat pump option for cold climates.
Water Source
Think of these like a ground source but instead of buying the coils in the ground, they are submerged in a pond or lake. If a naturally occurring water source is already in place, installation cost may not be too much, but usually a pond needs to be built just for the system. Once installed, they have similar advantages to a ground source system.
Heat Pump Limitations
We’ve alluded to a few downsides of heat pumps as we touched on the various types. Let’s dig in to these a little more.
Climate
Air source heat pumps draw heat from ambient air. When temperatures get very cold (typically below mid-30’s Fahrenheit), there just isn’t enough heat in the air to draw from and they are unable to provide adequate heat. Areas that routinely experience temperatures at or below freezing are most susceptible to this issue. Therefore, air source heat pumps are (usually) best suited for Southern climate zones that rarely drop below freezing even in winter.
Note that this points at which heat pumps require supplemental heat is based on the system as well as the home. A very well insulated and air tight home may be able to use a heat pump down to a lower outside air temperature (mid-20’s) without supplemental heat vs. a poorly insulated and drafty home. Build tight and insulate right! The benefits extend to both load and system options.
Supplemental Heat
Even in Southern climates, heating systems must be able to provide adequate heat in the event of the irregular cold snaps. In the case of heat pumps, this is usually provided by supplemental electric resistive heat.
Resistive heating elements are easy to add at low cost and provide the extra boost needed during cold periods. However, as we discussed early in this article, they are very expensive to run. That means that while they are good “insurance” for climates that have relatively infrequent freezing temps, you don’t want to use the heat pump + supplemental resistive heat in colder climates as the efficiency of the heat pump will quickly be outweighed by the inefficiency of the resistive heating elements.
Comfort
Whenever a forced air system is used, either for cooling or heating, the temperature of the space feels different based on the air movement and temperature within that space. For example, if you have a room at 72 degrees cooling and the A/C is running hard to keep it that way because the outside temperature is 95, you’ll typically have a discharge air temperature of about 55 degrees constantly circulating throughout the space. This 55 degree air moving across our skin makes 72 feel cooler than a room that is at 72 degrees with little air movement because the outside air temperature is only 75 and the A/C is hardly blowing. Ceiling fans also help with this phenomenon.
Using a similar example with heating, a room that is kept at 68 degrees heating with a lot of hot air blowing will feel different than a room at 68 with little air movement or varying discharge air temperature. The point is, the discharge air temperature of a forced air system has a large impact on comfort even at the same temperature.
So why is this important with heat pumps? A heat pump coupled to a forced air system normally operates with a discharge air temperature of about 90-100 degrees when in heating mode. This is significantly lower than the normal 130-140 degree discharge air temp found with a traditional furnace (gas, propane, etc.) or electric resistive heat. While the heat pump will keep the space at the same set point as the furnace by compensating for the lower discharge air temperature with higher air volumes, the perceived temperature is lower.
One of the chief complaints with a forced air heat pump is that they always feel “cold” or “drafty”. This is a real concern, even though they may keep the actual temperature at a perfectly reasonable range. This shouldn’t be a deal breaker, but is something to be aware of if you have some “cold blooded” members of the family.
Note that the issue above is really only a concern with forced air systems. Heat pumps, especially ground or water source, can be tied to hydronic (water) based systems like baseboard, radiant, etc. and will not give the same problem.
Heat Pump Efficiency
Early in this article we spoke about the energy advantages of heat pump systems. Let’s dig in a little more to that here as there are multiple factors that go in to the equation of what is better in the heat pump vs. standard electric vs. gas vs. propane vs. ?? debate.
Utility Rates
Gas, fuel oil, electric, and propane all have varying utility rates. Some regions are highly correlated, as the electric grid is primarily fed by gas fired generators. Others may have cheap power rates due to lots of hydro or nuclear power while having higher gas/oil/propane rates. Recent years have seen record low natural gas and oil rates, but this has also driven down electricity rates in most areas.
When comparing utility rates, it is important to remember to normalize for energy content. A CCF of natural gas is not the same as a kwh of electricity. Both must be converted to a common unit, such as BTU, in order to evaluate the cost of energy in a comparative manner.
Keep in mind that utility rates will fluctuate over time just like gas prices. Some consideration needs to be given to this or else the best solution now may not be later. For example, most areas are seeing very low natural gas prices right now. These will likely maintain for a while, but due to their current low prices it is only reasonable that they will go up. Compare that to an area with predominate hydo-electric power (Pacific Northwest, Niagara region) that gives consistently low electric rates. If a gas system proves to be the lowest cost now, some consideration should be given to what will happen if gas prices go up while electricity prices stay constant.
Climate
We spoke earlier about how different climates are better suited for air source heat pumps or not suited for them at all. Keep in mind that very cold climates can still benefit from ground source heat pumps even if air source doesn’t make sense.
If a heat pump is used in an area that it shouldn’t be, the supplemental heat will run too much and ruin overall efficiency.
System Performance vs. Cost
Comparing a 97% efficient heat pump to an 80% efficient gas furnace is only fair if the cost is the same. All equipment types have varying levels of efficiency, with high efficiency always coming at a higher initial cost.
When completing the calculation, it is best to compare a range of options against each other. For example, pick two similarly priced systems (perhaps a high efficiency gas boiler vs. a standard efficiency heat pump) and run the calculation and as a lifecycle cost for each to see which will be the lower cost over the system life. It is always wise to test this with some other options, as prices may change between system efficiencies (a high efficiency gas furnace may yield better performance with slight extra cost that swings the equation).
Building Design & Use
The load (amount of cooling and heating required) will vary based on how it is built. Insulation, air tightness, and internal loads all contribute to the equation.
Additionally, how a home is used has an impact. A family home with kids and a stay at home or work from home parent will be different than a 1br condo designed for a traveling professional. A lake house that is used primarily throughout the Summer and just kept warm enough through the Winter to avoid freezing will be a lot different than year-round residence.
All of these things must be understood to consider both the building load and usage.
Bringing it all together
Reading the above likely has you confused as to next steps. This list isn’t designed to allow you to complete the detailed calculation yourself as the modeling required to understand all components is beyond the scope of most homeowners. What it is intended to do is help you ask the right questions of a qualified professional.
You will likely need to use a licensed HVAC contractor to complete the work and have to pull a permit as well. While these steps alone don’t ensure you have the most efficient system (licenses and codes are primarily designed to meet minimum energy and design criteria), they give you a good resource to start with. Ask your licensed HVAC contractor to run the calculations for the various systems designs while considering the criteria above. They may not have the capability in-house; in which case you should ask them if they have a 3rd party resource that can help with the evaluation.
Extra money spent up front for a detailed energy analysis and proper design will go a long way to minimizing both the initial and long term cost. Don’t skip this step!
Conclusion
Heat pumps are often misunderstood. The technology has advanced greatly in recent years, making the systems viable in applications that they previously were not. However, there is a “pop culture” science misconception around them as well (i.e., “geothermal heat pumps are always a good investment and the most sustainable option! I saw it in a LEED building once so it must be true!”).
Having said that, they should definitely be considered when building new or completing a major renovation, especially if you don’t have natural gas available and/or live in a predominately cooling based climate (Southern states). We hope that this article has shed some light on the subject!