Heat Pumps have been used widely for decades but recent developments have made them “cooler” than ever. Here’s what you should know: a heat pump is, simply put, a device that transfers heat from a colder area to hotter one using mechanical energy, typically using a vapor compression refrigeration cycle. Refrigerators, air conditioners and freezers are good examples, although the term heat pump (HP) is usually used for systems that heat spaces and possibly air condition.
There are many sources or, “heat sinks,” used to extract and deposit thermal energy. They can be air, the earth or water. For example, a geothermal heat pump transfers energy from the earth or water to air, which is then used to distribute the heat inside the conditioned area. There are numerous types of heat sinks, and associated terminology such as air-to-air, air-to- water, water-to-water, etc. The simplest and most common is air-to-air which has also been getting the most buzz lately.
When used for heating, HPs can improve comfort and indoor air quality while significantly reducing heating energy costs. Here is a recent chart of comparative heating energy costs.
As you can see, air-to-air HPs can be one of the least expensive ways to heat a space. Costs rival geothermal, natural gas and even firewood, which begs the question as to why haven’t they been used more in the past. There are characteristics that challenge their usage. The performance and efficiency of air-to-air HPs drop as the temperature difference between heat sinks increase. For example, the capacity and energy use of an air conditioner gets considerably worse on really hot days. Similarly, performance and efficiency drop when using HPs to heat a space on extremely cold days. The performance of a HP is rated by its coefficient of performance, or COP. This is a ratio of the energy moved divided by the energy required to run the device. Simple electric heat, for example, has a COP of one. For every kilowatt purchased it yields one kilowatt of heat energy. Modern and efficient air-to-air HPs have COPs ranging from approximately 3-4 when used for heating. Not bad, compared to electric heat, huh?
The challenge is that the heating output or capacity drops during the coldest outdoor temperatures precisely coincident with the times increased capacity is needed. Historically, air-to-air heat pumps were sized less than the maximum heating capacity needed since they would be grossly oversized for cooling (air conditioning) use in the summer months. This especially the case in northern climates where the winter has colder outdoor temperatures. The use of HPs in more southerly States has been more widespread because of the milder winter design temperatures. The “capacity droop” challenge historically required the use of supplemental or backup heat sources such as fossil fuels, or most commonly electric heat. These sources made the economics of HP heating much less attractive.
So what changed? The reliability, availability and use of variable frequency drives (or VFDs). Nearly all new and modern HPs are variable speed. This solves a major technological challenge; we can now size the system for the heating load without causing the equipment to short cycle, or rapidly start and stop while cooling, which tears up equipment and prevents good performance for cooling and dehumidification. During both heating and cooling modes the systems will ramp the speed and performance from anywhere between 10-100 percent. This enhances performance, equipment longevity and reduces noise. Dehumidifying performance is greatly improved as the unit runs longer at lower speed, enhancing moisture removal.
HP systems can easily be installed during new construction, but until recently they were more challenging as retrofits. Air-to-air systems distribute thermal energy via airflow. Many, if not most homes in the northeast have hydronic heating systems, thus no existing ductwork. Achieving a good outcome retrofitting ductwork can be expensive, invasive and challenging. In the heating mode, the typical warm supply air temperature is 80-110 degrees F, thus poorly designed airflow can create draftiness and discomfort. Fortunately, the “ductless” option alleviates this challenge.
In the early 1980’s, the first imports of ductless split system heat pumps began to arrive from Asian manufacturers. These were popular in countries like Japan, where typical living spaces are quite small by American standards. These ductless systems distribute air indoors directly from wall or ceiling mounted air handlers. They are compact, and very quiet to operate. One manufacturer even sells a unit disguised as a wall mounted fine art painting, where air silently passes in all directions from behind the frame. American manufacturers fought off the market penetration of this new disruptive technology for almost thirty years mainly by tightening codes, standards and implementing arcane testing criteria. Asian manufactures found ready markets in less developed countries of the Middle East, Caribbean and South America, where their use became commonplace in smaller commercial and residential spaces. Finally, the American market embraced the technology about five years ago, and today utilization has increased substantially. Several major US producers have purchased manufacturers in Asia and began production in the US as well.
Although these high efficiency air-to-air units come in a variety of systems configurations, the wall mounted indoor unit is most popular. Due to the size of the unit and low powered fans, air filtration is very limited. The ducted version can be supplied with much higher efficacy filtration and can even have central humidification added for the dry winter months.
In commercial applications, both ducted and ductless split system heat pumps are an effective solution for difficult to heat and cool areas due to their installation flexibility. Centralized HVAC systems serving the building might not offer such flexibility.
It is important to note that most split system heat pumps do not satisfy criteria for fresh air required by those versed in “best practice” and building codes. Outside air can be introduced in the ducted versions, but typically require an uncoupled parallel ventilation system such as an air-to-air heat exchanger to adequately introduce fresh air. Thus, unless additional provisions are made, the ductless split system heat pumps do not satisfy building ventilation needs alone.
At Thayer, we first witnessed testing of the innovative ductless split systems in 1982. They performed well and could have been very useful to our customers. The bad news is that we had to wait nearly twenty years for them to be commercially attainable and UL certified in the US. The good news is that now they are available, competitively priced, very reliable and extremely efficient.
In the last year or so, we designed several geothermal heat pump systems but found the high efficiency air-to-air systems have similar performance at a lower installed cost. An ironic twist to the relative ease of installation is the recent proliferation of installing contractors who do not have HVAC experience, qualifications or expertise. Maine was lobbied by special interest groups to make exceptions to normal technician licensing and this has resulted in a sharp increase in faulty installations. Although they are relatively easy to install compared to other system types, they still require proper selection, sizing, location and commissioning for a well working system.
In our state, Efficiency Maine has been promoting heat pump use for select high efficiency models with a $500 incentive which helps defray new system cost.
There are no “silver bullets” in the HVAC and energy efficiency marketplace. No one source of energy is “best.” We have what we like to call “silver birdshot;” We evaluate each building, and explore all possible options.
If interested, we can have one of our engineers evaluate your needs and develop options. “Call in the Experts” at Thayer.
Dan Thayer, P. E.
President, Thayer Corporation