Heat pumps are fantastic, all-electric options for heating applications. Advancements in VRF technology have increased the viability of heat pumps over traditional fuel-fired heating equipment. This article will explore the fundamentals of the heat pump cycle, emphasizing the effect of refrigerant type and operating pressure on the cooling and heating performance, the sizing procedure for air-source heat pumps, and the considerations necessary when applying VRF system heat pump technology.
Heat Pump Cycle
Heat pumps operate by moving heat from lower temperature areas to higher temperature areas. They typically take advantage of an electrically driven compressor and a closed vapor compression cycle. Commercial-grade heat pumps come in many forms and can provide useful heating and cooling or simultaneously provide useful heating and cooling1. Technically, most refrigeration equipment can be classified as heat pumps, although the term is typically reserved for equipment that provides useful heat.
Heat and Refrigerants
After realizing heat pumps move heat, it’s essential to understand that heat is still available below freezing. If all heat is removed from an object, the object's temperature will decrease to minus 459.6°F [minus 273.2°C]. This temperature is called absolute zero and is the temperature at which all molecular activity stops. Heat pumps take advantage of the heat available from a source, even air at minus 30°F, and elevate it to a higher temperature using the work of compression. They can also be thought of as energy pumps as they extract energy from one temperature level and elevate (pump) it to a higher temperature level.
Using a visual aid, such as the P-H diagram in Figure 2 for a vapor compression cycle, can provide insight into the proposed refrigeration equipment's operation and potential capabilities.
Varying the refrigerant type and associated evaporator pressure in a given heat pump allows the unit to provide helpful heat when operating at subzero temperatures. The P-H diagram for an ideal refrigeration cycle is shown in Figure 2. Driving the refrigerant pressure higher will result in a higher temperature gas at the compressor outlet. The higher the temperature, the more useful the available heat is from a heat pump. For subcritical operation, the peak of the refrigerant boundary or dome indicates the max temperature of the refrigerant while still allowing it to condense. For R-410A, that critical temperature is 160.4°F. Factoring in a reasonable approach on the condenser and the ability to reject enough heat (enthalpy per pound) while condensing the refrigerant is why subcritical R-410A heat pumps cannot produce water temperatures greater than roughly 140°F when used in a water heating application.
The evaporator in a refrigerant cycle allows the refrigerant to boil, which allows heat to be removed from a source. The temperature at which a potential refrigerant boils sets the theoretical lower limit of heat pump operation. For example, at atmospheric pressure, R-410A boils at minus 60°F, and ammonia boils at minus 28°F. At higher pressures, the refrigerant boils and condenses at higher temperatures. At lower pressures, refrigerants will boil at lower temperatures. The dashed lines in the PH diagram show the temperature at which common refrigerants will boil at atmospheric pressure. Notice that there is still room on the PH diagram to lower the evaporation pressure of the refrigerant. Some refrigerants like HFC-134a are can operate at lower than atmospheric pressures, giving them the ability to operate in colder climates. Varying the evaporator pressure in a heat pump cycle or choosing a refrigerant with a low boiling point can help with low ambient operation.
Table 1 shows the atmospheric boiling points and typical ambient temperature limits of some common refrigerants. As can be seen for R-717(ammonia), the evaporation temperature of a heat pump cycle is also determined by a system's refrigerant charge pressure.
A fundamental understanding of the heat pump cycle and the operational limits of certain refrigerants gives engineers a foundation for designing and selecting heat pump systems.
Sizing Heat Pumps
Step One: Pick a Heat Source — The first step in sizing a heat pump to meet a building's heating load is deciding what medium will act as the heat source for the unit. As discussed earlier, heat pumps move heat from lower temperature sources to a medium at higher temperatures. Numerous sources of useful heat can be used when applying heat pumps. Selecting a heat source depends on the climate, geographic location, and cost. The ASHRAE Handbook on HVAC Systems and Equipment provides valuable details on the heat sources listed below.
- Ambient air; and
- Exhaust or relief air streams from building.
- Well water;
- Surface water;
- Condenser water systems; and
- Ground coupled; and
- Direct expansion.
Ambient air is the most common medium used as a heat source in applied and unitary heat pumps. The two most critical variables in successfully applying air-source heat pumps are the local outdoor temperature and the potential for frost formation. For heat transfer to occur, the outdoor air temperature needs to be sufficiently warmer than the temperature of the evaporating refrigerant moving through the outdoor coil. During heating operation, the temperature of the evaporating refrigerant in the outdoor coil is typically 10°-20°F colder than the outdoor air temperature. The following heat pump sizing steps will use an air-source unit as the basis.
Step Two: Defining the Building's Heating and Cooling Load Lines — The second step involves determining the design heating and cooling loads a heat pump needs to provide. When performing heating load calculations, it’s recommended that the 99% values be used and the 99.6% and mean of extremes be reserved for exceptionally harsh cases.
After determining the design cooling and heating loads, they should be plotted to create a graph of cooling and heating capacity versus outdoor temperature. Start by plotting the heating design load at a coincident outdoor air temperature as the first point; the second point should be the outdoor air temperature at which the space does not require heating. A typical heating set point is 70°F, but with solar and internal heat gains, the outdoor temperature at which heating is required (known as the balance point temperature) is lower than the heating set point temperature. Balance point temperatures are typically 5°-15°F lower than the heating set point temperature depending on internal heat gains. Balance point temperatures will vary with the available solar radiation and internal heat generation. Like plotting the heating load line, plot the cooling load line for the space. Figure 3 shows the heating and cooling load lines for a building with a peak heating and cooling load of 83 MBH and 60 MBH, respectively.
Step Three: All Heat Pumps or Hybrid Systems — The next step in sizing and selecting a heat pump is to determine the primary operation goal of the equipment. Heat pumps can be sized to meet a building's design cooling load, design heating load, or a balance between the design cooling and heating load5. In a moderate climate, where the author practices, a balanced approach will typically result in a unit that can cover both the cooling and heating loads. In colder climates, the heating to cooling hours ratio can be higher than 4:1, and unit selection to cover all or a portion of the heat load will be desirable. To cover all of the heating hours for the building heating load in Figure 3, a unit will need to be selected that could provide 83 MBH at 15°F below zero. To cover a portion of the heating load, a unit to cover 60 MBH at 8°F above freezing should be selected, and a supplemental heat source shall be used to cover the remaining hours when the temperature is lower than 8°F.
Step Four: Plotting Heat Pump Performance — The next step is to plot the proposed heat pump's rated capacity against the ambient outdoor temperature along with the heating and cooling load lines. Figure 4 shows the building cooling and heating load lines from the previous step, along with the heating performance of a nominal 6- and 8-ton air-source VRF unit. As shown in Figure 4, the performance lines for 6- and 8-ton units cross the building heating load lines at a temperature of 8°F and minus 5°F, respectively. The temperature at which the heat pump capacity is equal to the building load is the thermal balance point. The area to the left of the balance point (hatched section), bound by the heating load profile, heat pump capacity profile, and the chosen design temperature, indicates where supplemental heating is required. Supplemental heating can come from electric resistance elements or combustion heat.
As discussed previously, the climate can significantly impact the performance of an air-source heat pump. The annual heating hours can be five times greater than the cooling hours in cold climates. In addition, heating hours can extend over a greater range of harsh outdoor temperatures. The fraction of total annual heating that occurs above a given outdoor temperature was graphed for four locations across the U.S., Figure 5. Starting from the right-hand side (i.e., warmer outdoor temperatures), these plots accumulate or integrate the heating degree-day loads for a typical year as one moves toward colder outdoor temperatures and display the percentage of total annual heating load accumulated on the vertical axis. For example, in San Diego, approximately 90% of the heating hours occur above an outdoor temperature of 45°F. In Minneapolis, only 28% of the heating hours occur above 45°F.
The design heating data for the four cities graphed in Figure 5 is listed in Table 1. What is important to note is the significant difference in temperature between the 99.6 % heating temperature and the 50-year extreme design temperature. For example, Boise, Idaho, has a 99.6% design day temperature of 11.4°F and a 50-year extreme temperature 20°F lower than the 99.6% temperature. A conservative design for a traditional fuel-fired heating system will typically use the 99.6% heating data. A heat pump system with no backup heating source would need to operate at the 50-year extreme temperature to be considered a resilient heating system. The loss of heat during extreme weather events can present a real hazard for building occupants.
Variable Refrigerant Flow
Modern VRF units are great all-electric options for heating, especially compared to traditional packaged heat pumps. Heat recovery VRF systems consist of an outdoor unit, indoor units, and heat recovery boxes. The benefit of using a heat recovery VRF system is the ability to trade energy between thermal zones. Packaged heat pumps are typically single zone or require a separate reheat system to subdivide the unit into several thermal zones. Whether split into many zones or a single zone, packaged heat pumps have limited and costly methods for trading energy between zones needing heat and zones with excess heat. Figure 6 shows a schematic diagram of a three-pipe heat recovery VRF system. ODU-1 at the top of the figure is connected to six equally sized fan coil units, four of which are in heating. The heating zones can take some of the excess heat rejected from the two zones in cooling, with the remainder of the heat provided by the ODU-1. ODU-2 at the bottom of the figure is connected to six equally sized fan coil units, five of which are in cooling. The one zone in heating can accept some of the heat rejected by the five other fan coils in cooling mode, and ODU-2 rejects the excess heat. With water-cooled VRF systems, multiple outdoor units can be connected to a two- or four-pipe condenser water system, allowing ODU-1 to accept the excess heat from ODU-2, resulting in secondary heat recovery. Two-pipe condenser water systems will give each ODU a similar temperature. In contrast, four-pipe condenser water systems can give a zone in heating warmer water and a zone in cooling colder water.
Aside from the ability for heat recovery between zones, VRF offers the following additional benefits over traditional packaged heat pumps.
- Excellent part-load performance due to inverter-duty, variable-speed compressors allowing for capacity modulation from 10% to 100%;
- Reduced ductwork and duct losses are confined to the ventilation air system;
- Increased heating performance over traditional packaged rooftop units at low ambient conditions, as shown in Figure 7. Low ambient temperature kits can allow heating operation down to ranges of 0° to minus 13°F, which are typically out of reach for traditional packaged rooftop units;
- Heat recovery outdoor units can be paired with traditional packaged equipment. This is done by using a coil connection kit provided by the VRF manufacturer; and
- Heat recovery units can be paired with refrigerant-to-water heat exchangers that can be used for applications such as domestic water preheating.
When sizing VRF systems, it is essential to factor in the piping length adjustment and defrosts factors. Systems operating with long piping lengths and at lower temperatures will perform lower than published nominal capacities.
While VRF is an attractive option for air-side heating, some considerations need to be appropriately assessed before selecting this system. The following should be considered on a project-by-project basis.
- In order to provide adequate indoor ventilation, VRF systems typically need to be coupled with a dedicated outdoor air unit (DOAS) to provide ventilation for each space;
- In scenarios where a heat pump system is equipped with auxiliary heat, such as electrical resistance heating, the system may operate at a coefficient of performance (COP) at or near 1.0, putting it on par with the efficiency of electric resistance heating;
- Compliance with ASHRAE Standard 15-2019, “Safety Standard for Refrigeration Systems,” can be challenging;
- VRF systems are proprietary, and specifying a manufacturer will lock the owner into that VRF manufacturer for the system's life;
- Maintenance for VRF systems is more specialized than a packaged heat pump, typically requiring owners to enter into or maintain service contracts with third-party vendors. This can be more troublesome when integrating a VRF system onto campus with dedicated maintenance and operations staff;
- It can be challenging to design VRF systems for spec buildings, where tenant improvements can drastically change the internal loads of a building, which could result in undersized equipment and refrigerant mains; and
The control systems that operate VRF systems are proprietary and different for each VRF manufacturer. Typically, control of the VRF unit components is left to the internal control system, and the central building management system can integrate the read-only points and limited command points.
Heat pumps are a proven all-electric heating technology that will play a significant role in the architectural and engineering industry's mission to decarbonize and electrify the built environment. Like any technology, they have pros and cons that need to be considered. Though, when utilized properly, they give engineers another suitable system option for their toolboxes.