9.6 Heat Pumps & Variable Refrigerant Flow (VRF): Balance Point, Defrost & Heat Recovery
Key Takeaways
- The balance point is the outdoor temperature at which declining air-source heat pump capacity equals the rising building heating load; below it, supplemental heat must make up the difference.
- Air-source heat pump heating capacity falls roughly linearly with outdoor temperature while building load rises linearly, so both lines can be written from two rating points and solved simultaneously.
- Water-source heat pumps operate on a common loop held between roughly 60 F and 90 F, so their capacity is nearly independent of outdoor temperature and units in heating mode reject load onto units in cooling mode.
- Two-pipe VRF operates all indoor units in the same mode, while three-pipe VRF heat recovery adds branch selector boxes that let some zones cool while others heat, moving energy between them instead of rejecting it.
- EPA places chillers and certain VRF systems in their own Technology Transitions subsectors rather than the residential and light commercial air conditioning subsector, so their compliance dates and their leak-repair applicability differ.
9.6 Heat Pumps & Variable Refrigerant Flow (VRF): Balance Point, Defrost & Heat Recovery
NCEES sub-topic 3D names "chillers, variable refrigerant flow, heat pumps, thermal storage" in a single line. Chillers were covered in Section 9.4. This section handles the two reversible-cycle families, and the calculation that defines them: where does the machine stop being able to heat the building by itself?
1. The Air-Source Heat Pump Capacity Problem
A heat pump moves heat rather than creating it, so its heating output tracks the temperature of the source it is pulling from. As outdoor temperature falls:
- The evaporating temperature falls, so suction density falls and the compressor moves less refrigerant mass per revolution.
- The compression ratio rises, so isentropic efficiency and volumetric efficiency both fall.
- Meanwhile the building load rises in direct proportion to the indoor-to-outdoor temperature difference.
The result is a scissors diagram. The two lines cross at the balance point.
Btu/hr
| * building heating load
| * (rises as OAT falls)
| *
| * BALANCE POINT
| * / \
| / \
| / \ heat pump capacity
|/ \ (falls as OAT falls)
+----------------------------------------- outdoor air temperature
0 F 20 F 40 F 60 F
Worked Example - Balance Point and Supplemental Heat
A house has a design heating load of 60,000 Btu/hr at 5 F outdoor with a 70 F indoor setpoint. A heat pump is rated at 36,000 Btu/hr at 47 F and 22,000 Btu/hr at 17 F.
Step 1 - Write the building load line. Heating load is zero when outdoor temperature equals indoor temperature, so the slope is 60,000 / (70 - 5) = 923 Btu/hr per F:
Step 2 - Write the heat pump capacity line from the two rating points. The slope is (36,000 - 22,000) / (47 - 17) = 467 Btu/hr per F:
Step 3 - Set them equal.
- 64,615 - 923 T = 14,067 + 467 T
- 50,548 = 1,390 T
- T_balance = 36.4 F
Step 4 - Size the supplemental heat at design. At 5 F the load is 923 x 65 = 60,000 Btu/hr, while the heat pump delivers 22,000 + 467 x (5 - 17) = 16,400 Btu/hr.
- Supplemental requirement: 60,000 - 16,400 = 43,600 Btu/hr
- As resistance heat: 43,600 / 3,412 = 12.8 kW
Two lessons fall straight out of this arithmetic. First, the heat pump covers only 27% of the design-day load, but because design conditions occur for very few hours, it still supplies the large majority of the annual heating energy. Second, supplemental heat is not optional sizing slack - at 5 F the house is 43,600 Btu/hr short, and if the strip heat is undersized the space simply does not reach setpoint.
2. Defrost, Its Penalty, and Its Control
When the outdoor coil surface falls below both freezing and the ambient dew point, frost accumulates, blocking airflow and insulating the fins. The worst frosting band is roughly 28 F to 42 F with high humidity - not the coldest weather, because very cold air holds almost no moisture.
Standard defrost reverses the four-way valve, running the machine in cooling mode so hot discharge gas melts the frost off the outdoor coil. During that interval:
- The indoor coil is now absorbing heat from the space, so supply air goes cold and supplemental heat must be energized to temper it.
- The outdoor fan is stopped so the discharge gas is not blown away.
- Seasonal heating capacity and efficiency are both reduced.
| Defrost Initiation Method | How It Decides | Consequence |
|---|---|---|
| Time-temperature | Fixed interval, e.g. every 30, 60, or 90 minutes, if coil temperature is below a setpoint | Simple and cheap; runs unnecessary defrosts in dry cold weather |
| Demand (adaptive) | Compares coil temperature to ambient, or measures air-side pressure drop, and defrosts only when frost is actually present | Fewer cycles, better seasonal performance, higher control cost |
Termination is normally on coil temperature (about 55 F to 70 F) with a time backstop.
Dual-Fuel Changeover
A dual-fuel or hybrid system pairs an air-source heat pump with a gas furnace and switches at an economic changeover temperature, not at the thermal balance point. The changeover is where the delivered cost of heat is equal:
Because heat pump COP falls with outdoor temperature and furnace efficiency does not, there is a single crossover temperature. Setting changeover at the balance point instead of the economic point is a common design error in both directions.
3. Water-Source and Ground-Source Configurations
| Configuration | Heat Source / Sink | Distinguishing Behavior |
|---|---|---|
| Air-source | Outdoor air | Capacity and COP fall steeply with outdoor temperature; requires defrost |
| Water-source (WSHP) | Common building loop held roughly 60 F to 90 F | Capacity is nearly constant year-round; a boiler adds heat and a cooling tower or fluid cooler rejects it when the loop drifts out of band |
| Ground-source (GSHP) | Vertical bores or horizontal field at deep-earth temperature | Highest COP; requires ground loop sizing against multi-year thermal balance, not just peak load |
| Water-to-water | Loop on both sides | Produces hot water for hydronic distribution rather than conditioning air directly |
The defining advantage of a WSHP loop is internal heat recovery: a perimeter unit in heating mode extracts heat from the loop while a core unit in cooling mode rejects heat into it. During shoulder seasons a large building can be nearly self-balancing, with the boiler and tower both idle.
4. Variable Refrigerant Flow
VRF systems modulate an inverter-driven compressor and electronic expansion valves at each indoor unit, distributing refrigerant rather than air or water to the zones.
Two-Pipe vs. Three-Pipe
| Feature | Two-Pipe Heat Pump VRF | Three-Pipe Heat Recovery VRF |
|---|---|---|
| Refrigerant lines | Liquid and suction/discharge | Liquid, suction, and discharge gas |
| Zone modes | All indoor units heat, or all cool | Individual zones heat and cool simultaneously |
| Mode routing | None required | Branch selector boxes or branch controllers |
| Best fit | Uniform-load buildings, residential, hotels | Buildings with simultaneous core cooling and perimeter heating |
| First cost | Lower | Higher, offset by recovered energy |
The energy argument for three-pipe VRF is the same as the WSHP loop argument: when one zone is rejecting heat and another needs it, transferring it directly is far cheaper than rejecting it outdoors and generating it again.
Design Constraints That Show Up on Exam Items
- Refrigerant line length and lift limits are manufacturer-specific and are the first constraint that kills a VRF layout in a tall building. Verify total piping length, length to the farthest indoor unit, and the maximum vertical separation between outdoor and indoor units against the specific product data - there is no universal value.
- Refrigerant concentration limits apply to every indoor unit. A large VRF charge combined with a small enclosed office can exceed the ASHRAE Standard 15 charge limit for the smallest room served. Section 8.3 works this calculation; with A2L refrigerants the limit tightens sharply, because R-32 carries an RCL of only 4.8 lbm per 1,000 ft3 against R-410A's 26.
- Ventilation is not included. VRF conditions recirculated room air. Outdoor air must come from a dedicated outdoor air system, and forgetting that is a code violation under Standard 62.1, not a comfort preference.
- Regulatory subsector matters. EPA classifies chillers and certain VRF systems in their own Technology Transitions subsectors rather than in residential and light commercial air conditioning, so do not carry the residential compliance dates or the residential leak-repair exemption across to a VRF question by analogy.
A building has a heating load of 48,000 Btu/hr at a 0 F design outdoor temperature with a 68 F indoor setpoint. An air-source heat pump delivers 30,000 Btu/hr at 47 F and 18,000 Btu/hr at 17 F. What is the balance-point temperature?
Using the same heat pump and building as the previous problem (48,000 Btu/hr load at 0 F, capacity 18,000 Btu/hr at 17 F falling at 400 Btu/hr-F), what electric resistance supplemental heat capacity is required at the 0 F design condition?
An air-source heat pump in a humid climate is observed to defrost most frequently at outdoor temperatures near 35 F and rarely at 10 F. What explains this?
An office building has a large interior zone requiring cooling year-round and a glazed perimeter requiring heating on winter mornings. Which system best exploits that load pattern, and why?