Complexities of heat pumps
The local electric utility company often refers their customers to me for assistance in determining the reason (or reasons) for their high utility bills. The questions I ask during the initial contact often reveal some important clues which typically revolve around their climate control systems. Overwhelmingly, Dayton area homeowners who contact me disclose that their heating system includes a heat pump. More than once however, I’ve had to read between the lines of unknowing clients who simply state their home is “all electric” and didn’t even know that a heat pump was installed or what it was called. This common denominator has prompted me to write this article in hopes it helps others.
Heat pumps have garnered favoritism in recent years as energy efficient climate control systems. This is true in many regions of the country, but comes with some caveats that if ignored, can result in quite low energy efficiency and very high utility bills. Most utility providers offer rebate programs for heat pump upgrades, and many HVAC companies are now recommending replacement of aging split system air conditioners with newer heat pump technology. Unfortunately very few homeowners completely understand what a heat pump is, how it works, or the complexities of heat pump operation. This is especially true when it comes to the most common air to air (or air source) heat pump systems. Management, maintenance, and proper operation of these systems are paramount to reaping the most benefit of a heat pump.
To get to the heart of the matter let’s have a quick look at how heat pumps work, because understanding this will help to understand the best way to manage them.
Heat pumps are quite an amazing technology that provide both cooling and heating from essentially a single machine. In the simplest of terms, a heat pump is an air conditioner that can work in reverse. During the summer months, the heat pump works as a traditional air conditioner by removing heat from your living space, and transferring that heat to the outside air. During the winter months, an air source heat pump provides heat by extracting warmth from the outside air and transferring that heat to your living space. As amazing as it may seem, a heat pump can extract warmth from outside air to heat your home even if it’s freezing!
When properly installed in what we call a “high performance” home, an air-source heat pump can deliver up to three times more heat energy to a home than the electrical energy it consumes. This makes them more efficient than a traditional electric furnace or heater (i.e. electric radiant heat or fan heaters). So if these systems are so energy efficient, why do many of their owners complain of high electrical use and uncomfortable rooms? Let’s look at a few reasons…
Finding balance in an imperfect world
A central concept in heat pump operation is the “balance point”. In the realm of heating, ventilating, and air conditioning (HVAC) design, there are two terms that often get confused – heating/cooling load and heating/cooling capacity. The load is how much cooling or heating a house needs. The capacity is how much cooling or heating an HVAC system can supply.
With most traditional heat sources (furnaces, and boilers), the capacity of the heating system is not generally affected by the outdoor conditions. This is mostly true in an ideal world anyway, but there is some loss of capacity as the outside temperature drops and forces of nature play their role on the system, especially with poorly insulated ducts or ducts routed through ventilated attics and crawl spaces.
In air source heat pumps however, the source of heat is the outdoor air. But as temperatures drop outside, there’s less heat available to bring inside, so the heating capacity of a heat pump is highly dependent on the outdoor conditions. In fact, the capacity goes in the opposite direction from the load. The image here may help to explain.
Every house and heat pump will have the capacity and load lines in different places and under varying ambient conditions, so this graph simply serves as a visual reference to show that where the two lines cross becomes the balance point. Anything after the balance point significantly reduces the economic benefit of air-source heat pumps. The important points to note about the relationship between heat pump heating capacity and a home’s heating load are as follows:
- First, as the outside temperature falls, the load (demand for heat) will increase and the capacity (ability to provide heat) will decrease.
- Second, at a certain temperature, the capacity will equal the load creating the balance point, but this balance point does not remain constant.
- Last but not least, for temperatures below the balance point, the home will need supplemental heat to meet the load (demand).
The balance point for a moderately efficient home in our area will typically fall between 35°-40° Fahrenheit. This is not a given though, and the balance point will vary by region and overall home performance. Many homes with average to below average efficiency will have a higher balance point in the 40’s, while some homes considered “high performance” (although very rare) may be lower in the 20’s or even teens.
Remember when I said that a properly installed air-source heat pump can deliver up to three times more heat energy to a home than the electrical energy it consumes?
Well that is in a perfect world in a perfectly high performance home! Once outside temperatures fall below the balance point, energy consumption to output ratio in the average home is more like 1 to 1.5. Not much better than an all-electric forced air furnace.
This means that when average temperatures fall below 40 degrees Fahrenheit, the average home with an air-source heat pump will be consuming nearly as much energy as a home without! To maintain optimum efficiency during heat pump operation we must lower the balance point, but this isn’t always easily done and can actually come with negative impacts. One method to lower the balance point is to make the house more energy efficient (improve insulation, reduce air infiltration, etc.). The other would be to increase the equipment size.
Either of these options will benefit your heating efficiency, but without careful consideration, can result in an oversized cooling system which creates a whole other set of problems saved for another topic. We’ll just sum it up that the balance point can be a moving target which is extremely hard to hit without proper care. Improperly sized or carelessly installed heat pump, non-existent consideration for home performance, and neglected routine maintenance often results in a disastrous outcome for your electric bill as you’ll see by reading further.
As it gets colder outside, your heat pump’s ability to pull heat inside is reduced dramatically. Eventually it can’t meet the heating load of the house by itself. That’s where supplemental heat kicks in. At temperatures below the balance point, the supplemental heat will make up the difference for the increased load demand. For most heat pumps, the supplemental heat source is electric resistance (strip) heat. When the heat pump can no longer pull enough heat from outside to meet the heating load of the house, the electric resistance heat (electric furnace) comes on and supplements the heat pump. If your home has natural gas, propane, or fuel oil, the auxiliary heat (rather than supplemental heat) may be supplied by a combustion furnace. This is called a dual-fuel system. Most of these dual-fuel systems should be connected in a way that when it gets too cold outside for the heat pump to supply all of the heat, the heat pump shuts off completely and the combustion furnace takes over to supply the heat demand. All too often, this is not the case and the heat pump continues to wastefully operate. If you have a dual-fuel system and the heat pump continues to run while the combustion furnace is fired up, then there is a problem that needs corrected by a competent technician.
Your supplemental heat source is almost certainly going to be electric resistance heat if you have an all-electric home. The supplemental heat works in conjunction with the heat pump to meet the heating demand. This means that the heat pump and the resistance heat will be operating at the same time… significantly increasing your energy consumption. Your energy consumption to output ratio just became a 2 or even 1.5 as a result! So unless your home is a high-performance home (and very few are actually high performance), you’ll be spinning the electrical meter like a vintage record player any time the outside temperature falls below 38-40 degrees Fahrenheit… which happens to occur quite frequently between the months of December and March in our Dayton region. Now this heat pump system which was sold to you as a very efficient system, doesn’t seem quite so efficient in cold weather does it? Well, because it’s not! But the heat pump itself isn’t fully to blame. The salesman or technician only sold you part of the big picture! We’ll talk more about that later…
Defrosting isn’t just for freezers
So, while a heat pump can indeed be a very efficient device for heating your home in moderate temperatures, it will have to work much harder to extract heat from really cold outside air. In fact, when the outside temperature drops below around 38-40 degrees Fahrenheit, it will start to struggle and run much less efficiently. The compressor unit (outside) will tend to freeze up, and the unit has to then use even more energy to stop that from happening. This is done by the unit entering “defrost mode” much the same as your food freezer does. Most heat pump defrost modes are cycled by a set timer that operates every 30 minutes or so. Some heat pumps have sensors that cycle the defrost mode depending on ambient temperature conditions rather than timed cycles. These are called demand-defrost controls and are more energy efficient, but of course increases the initial upfront cost of the heat pump itself.
The brain behind the brawn
Unfortunately, some homeowners have a misunderstanding about how the heat pump and its thermostat work together, and sometimes that misunderstanding comes from the installers themselves! The thermostat that controls your heat pump is slightly different than most traditional system thermostats. A heat pump’s thermostat will have three modes (cool, heat, and emergency heat) instead of the traditional two modes of cool and heat. Homeowners overlook that the thermostat says ‘emergency‘ instead of ‘supplemental,’ and think that when it gets cold outside they must switch over to emergency heat mode. This is often because someone (unfortunately some installers included) told them it was necessary to do so when outside temperatures fall below a certain point. A properly functioning heat pump system is designed to automatically switch to ‘supplemental’ heat when needed. The emergency mode is really only for use when the heat pump unit itself suddenly or unexpectedly fails.
On a similar note (and not just with heat pumps) is bumping the thermostat higher or lower in an effort to heat or cool the home faster. The time it takes to change the room temperature has absolutely no direct correlation to how far you temporarily adjust the thermostat, no matter how high or low you adjust it! Bumping the system to a significantly higher temperature on a heat pump will often call for the supplemental heat to activate in order to accommodate the immediate demand, but still won’t heat the room any faster… just more expensively. Patience is a virtue when it comes to heat pumps! If your home is too cold, gradual bumps of one or two degrees at a time are recommended to reach your desired comfort level. Until then, temporarily donning a sweater is much more economical! If you switch to emergency heat when the system is functioning normally, or force the supplemental heat before home demand really requires it, you’re going to end up paying a lot more for electric to heat your house than was really necessary.
Size matters, but so does quality, diameter, and speed
Proper installation is more critical with a heat pump than with a gas furnace. A gas furnace producing 130 degree air can sometimes overcome energy loss caused by ductwork leaks or drafty, poorly insulated homes. A heat pump will have a more difficult time keeping up with a home like this however. Combine a drafty home or leaky ducts with a heat pump system that was not properly installed will only magnify the needed effort. Many times the heat pump itself is blamed, when in actuality the installation was not done properly, or the heat pump was in fact not the best choice for the application. If there are problems with the installation, they usually surface when the weather is coldest because humans are more sensitive to slight temperature drops as opposed to slight temperature increases.
Properly sizing the heat pump system for the home is extremely important, even more so when a heat pump is replacing a split-system air conditioner. Unfortunately contractors like to play the “rules of thumb” game rather than calculate actual heating and cooling demands of any particular home. When it comes to precision, being “all thumbs” is asking for failure. Because many existing homes were not designed for heat pump technology, much of the HVAC system remains grossly oversized (specifically the ducts) for their efficient operation. More often than not, the original ducts throughout the home are far too large in diameter and way too leaky for the heat pump to efficiently work. Heat pumps can have problems with low airflow and leaky ducts. There should be about 400 to 500 cubic feet per minute (cfm) airflow for each ton of the heat pump’s air-conditioning capacity. Efficiency and performance deteriorate if airflow is much less than 350 cfm per ton. Technicians can increase the airflow by cleaning the evaporator coil or increasing the fan speed, but more often than not, modification of the ductwork is needed in order to realize any significant improvement in performance.
Heat pumps are marketed by listing their optimum stated efficiencies. These are achieved under ideal “laboratory” conditions where the ducts are sized correctly, ambient temperature remains constant, filters are pristine, and airflow is at optimum rate. In the real world, these conditions are MUCH different. If a homeowner knew that the ducts throughout the home would need replaced or significantly altered (let alone properly sealed and insulated) to achieve optimum results, the additional cost alone would deter them from installing any upgrades to an otherwise “adequate” heating and cooling system. And that’s just part of the whole picture!
The house as a whole needs to be treated as one complete system. Many homes (even new construction rated as “Energy Star” compliant) are as leaky as a whiffle ball holding air. Comprehensive air sealing of the building envelope plays an important role in the heat load or demand of your home. We’re not just talking about caulking your leaky windows and replacing gaskets at doors either. There are numerous sources of hidden air leaks occurring from attics and foundations that can result in very poor heating and cooling system performance. But alas, that too is another topic for another day.
An ounce of prevention is worth every bit its weight in gold and more
Routine maintenance can’t be stressed enough for air source heat pumps. Dirty filters and clogged evaporator coils are leading culprits to poor performance and high electric bills. So checking/changing/cleaning them at least monthly are a required chore that should never be neglected. As well, seasonal servicing from a qualified provider is just as important. Refrigerant loss and worn out mechanical parts can attribute to low performance or even failure of the system when it’s needed most. Afterhours and weekend emergency calls to service technicians will cost far more than a seasonal checkup and tuning.
So, when contemplating why your utility bills remain high, an otherwise energy efficient heat pump may be partially to blame, or even the wrong choice altogether. No matter what the brochure or salesman says, when it comes to energy efficiency and lower utility bills, there’s no singular magic bullet especially with an air-source heat pump.
If the entire home isn’t addressed as a system, don’t expect a single piece of the puzzle to cure all your home energy ailments. But addressing the whole house as a system, and knowing how all the systems should work together in harmony, is an excellent starting point.