NATE Heat Pump Install cheat sheet

Heat pump COP = heating capacity delivered (Btu/h) ÷ electrical input (Btu/h); HSPF2 = total heating Btu ÷ total electrical watt-hours over the region IV test, so HSPF2 ≈ COP × 3.412 adjusted for defrost and cycling.

Cooling capacity balance point: compressor capacity falls roughly 1.5–3% per °F of outdoor rise above 95°F, while building load rises; the intersection of the two curves sets the design balance point.

Heating balance point is where heat pump capacity equals structure heat loss; below it, supplemental (auxiliary) heat carries the deficit, and the thermal balance point should be at or below design outdoor temperature for all-electric designs.

Standard cycle order: compressor discharge → reversing valve → outdoor coil (heating mode condenser) → expansion device → indoor coil (evaporator) → accumulator/suction → compressor.

Superheat = suction line temp − evaporating saturation temp; subcooling = condensing saturation temp − liquid line temp. Target superheat on fixed-orifice systems is load-dependent (typically 5–15°F); TXV systems target 8–12°F subcooling.

Reversing valve is a pilot-operated 4-way slide: energized solenoid = cooling on most residential designs; a stuck valve shows equal pressures or wrong-mode temperatures at both coils.

Defrost termination is normally on-demand (demand defrost) using coil temperature and pressure differential, with a 10-minute maximum timer and 30/60/90-minute initiation intervals; strip heat energizes during defrost to temper supply air.

Key components: scroll or rotary compressor, accumulator (heat pumps always require one), bi-flow or two check-type expansion valves, liquid-line filter-drier, suction-line accumulator, crankcase heater, and service valves.

Manual J block load uses design conditions: heating design temperature = 99% winter design dry bulb; cooling design temperature = 1% summer design dry bulb, both from ASHRAE/NOAA tables for the specific location.

Sensible heat equation: Btu/h = 1.08 × CFM × ΔT (sea level); latent: Btu/h = 0.68 × CFM × Δgrains; altitude correction multiplies airflow factors by the density ratio.

Equipment sizing rule: total capacity within 100–115% of calculated total load and sensible capacity ≥ sensible load at design; oversizing raises cycling, humidity, and defrost losses.

Line-set length and vertical separation reduce nominal capacity - apply manufacturer's correction table (typically 1–3% per 10 ft of equivalent length beyond 25 ft) before comparing to load.

Site survey items: available electrical service and panel capacity, clearances to property lines and combustibles, snow/prevailing wind exposure, condensate routing, structural support, and existing duct static pressure.

Ducted systems: verify external static pressure budget against blower table; a 0.5 in. w.c. design ESP is common for residential air handlers, 0.8–1.0 in. w.c. for high-static units.

Cold-climate selection: verify rated capacity at 5°F and 17°F (HSPF2 Region V data), compressor lockout setpoint, and whether the unit is rated for operation below 0°F.

Minimum circuit ampacity (MCA) = 1.25 × largest motor RLA + sum of other loads; maximum fuse/breaker (MOP) is set by the unit nameplate - never upsize beyond it to stop nuisance trips.

Wire sizing: use 60°C ampacity column for NM-B and 75°C for THHN; ampacity must be ≥ MCA, and the breaker protects the conductor per NEC 440.

Disconnect must be within sight of and within 50 ft of the outdoor unit; a non-fused pull-out is acceptable when the branch breaker provides protection.

Ground the outdoor unit with an equipment grounding conductor sized per NEC Table 250.122; do not rely on conduit as the ground path.

Low-voltage control colors (conventional): R = 24V hot, C = common, Y = compressor contactor, G = indoor blower, W = auxiliary/emergency heat, O = reversing valve (cooling active) and B = reversing valve (heating active) - never both O and B on one system.

Thermostat R-to-C must read 24VAC ±10% under load; a reading below 20VAC indicates transformer overload, undersized wire, or a shorted control.

Communicating systems use two-wire polarity-insensitive data buses; keep data wire off line-voltage conduit, use manufacturer-specified cable, and terminate shields at one end only.

Defrost control board, low-ambient kit, crankcase heater, and outdoor thermistor are separate loads - verify transformer VA capacity covers all accessories plus the thermostat.

Nitrogen purge is mandatory during brazing at ~2–3 SCFH; flow must be visible at the outlet, and the system must be open to atmosphere at one point so pressure cannot build.

Use silver-phosphorus brazing alloy (BCuP) for copper-to-copper; add flux or use a silver-bearing alloy with flux for copper-to-brass or copper-to-steel joints.

Evacuation target: 500 microns or below with a standing vacuum test - isolate the pump, hold 500 microns for 10–15 minutes with no rise above 500; a rise to 1,000–2,000 microns indicates moisture, above 2,000 indicates a leak.

Triple evacuation: evacuate to 1,500 microns, break with dry nitrogen to 0 psig, repeat, then final evacuate to 500 microns; this removes moisture faster than a single long pull.

Weigh-in charging is required for units with a fixed orifice or when line-set length exceeds the factory charge; add refrigerant per the manufacturer's oz-per-foot table for liquid-line size.

Subcooling method (TXV/EEV): measure liquid-line temperature and convert suction or liquid pressure to saturation temperature; target the nameplate subcooling (±3°F), adjusting charge in small increments.

Superheat method (fixed orifice): target superheat comes from the manufacturer's chart using indoor wet-bulb and outdoor dry-bulb; add charge if superheat is above target, recover if below.

Line-set limits: typical residential maximum 50–100 ft total equivalent length and 50 ft vertical; oil return requires an inverted trap at the top of a riser and a P-trap every 20–25 ft of vertical rise on larger systems.

Nominal airflow: 400 CFM per ton for cooling; 350 CFM/ton improves latent removal in humid climates, 450 CFM/ton maximizes sensible capacity in dry climates.

Static pressure budget: supply trunk 0.1 in. w.c., branch runs 0.1 in. w.c., return 0.05–0.1 in. w.c., filter 0.1–0.2 in. w.c., coil 0.2–0.3 in. w.c.; total should not exceed the blower's rated ESP.

Return air must be ducted or fully panned on heat pumps; a starved return drops airflow, raises superheat, and can freeze the indoor coil in cooling.

Flex duct: keep runs taut, limit sag, use long-radius elbows, and support at ≤ 5 ft intervals; each 90° flex elbow can cost the equivalent of 25–35 ft of straight duct.

Duct leakage in unconditioned space should be sealed with mastic (not cloth duct tape); target ≤ 10% total leakage and ≤ 5% to outside for new construction in most energy codes.

Indoor unit must be level within 1/4 in. per foot for condensate drainage; secondary drain pan with a float switch is required where overflow could cause damage.

Filter selection: MERV 8–13 is typical; higher MERV raises static pressure - verify blower table at the filter's rated pressure drop, not a nominal value.

Minimum clearances: typically 12–18 in. on the service side, 12 in. on the fan discharge side, 6–12 in. on other sides, and 24–36 in. above grade; always use the manufacturer's installation manual values.

Do not place the outdoor unit under a roof drip line, eave, or deck where ice and snow shed onto the coil; avoid recirculation zones where discharge air re-enters the coil.

Wall-mounted brackets must be lagged into framing or blocking; ground pads must be level and above the local frost line, with a 2–3 in. riser to keep the coil above snow.

Defrost drain: route the condensate away from the foundation with a freeze-resistant path; a heated drain pan or drain-line heat tape is used where freezing is expected.

Defrost cycle sequence: reversing valve shifts to cooling, outdoor fan stops (on most designs), strip heat energizes, coil warms to terminate temperature (~50–70°F coil temp), then normal heating resumes.

Excessive defrost (short intervals, long cycles) is caused by a faulty coil sensor, low charge, restricted airflow, or a bad defrost board; verify sensor resistance against the temperature chart before replacing the board.

Ice buildup at the bottom of the coil often indicates a blocked drain pan or a unit set too low; raise the unit and clear the pan before condemning the defrost control.

Commissioning sequence: verify electrical and refrigerant tightness, evacuate, weigh in charge, then run in cooling for 15–20 minutes to stabilize before recording superheat/subcooling.

Record and compare: indoor entering/leaving dry-bulb and wet-bulb, outdoor dry-bulb, suction and liquid pressures, superheat, subcooling, amp draw vs. nameplate RLA, and external static pressure.

Temperature split in cooling: 18–22°F between return and supply at 400 CFM/ton with 50% indoor RH; a split below 15°F suggests low airflow or low charge.

Heating mode: verify supply air temperature rise is within the manufacturer's range (typically 25–45°F for a heat pump without strip heat), and check that auxiliary heat stages in only when needed.

Delta-T method for airflow: CFM = sensible Btu/h ÷ (1.08 × ΔT); use the manufacturer's blower table and measured ESP as the primary airflow check.

Verify defrost initiation and termination, low-ambient lockout, crankcase heater operation, and thermostat auxiliary lockout/balance point settings before signing off.

Document static pressure, charge method used, line-set length and added refrigerant, and all setpoints; compare measured capacity against the load calculation to confirm the balance point.

Low suction pressure + high superheat + normal subcooling = restricted or underfed evaporator (low airflow, restricted piston/TXV, or low charge); check airflow before adding refrigerant.

High suction pressure + low superheat + low subcooling = overfeeding TXV or overcharge; high head pressure with normal subcooling points to condenser airflow or non-condensables.

Compressor won't start: check 24V at contactor coil, then capacitor (microfarad reading vs. rated ±6%), then start components, then winding resistance and insulation to ground.

Heat pump heats but won't cool (or vice versa): suspect the reversing valve solenoid, the O/B thermostat configuration, or a stuck valve slide - verify coil temperatures in both modes.

Defrost complaints: check outdoor coil sensor resistance, board timing, and whether the unit is actually iced; a dirty coil or low charge mimics defrost failure.

Preventive maintenance: clean coils, check refrigerant charge by superheat/subcooling, test capacitors, inspect contactor points, flush condensate drain, and verify thermostat and safety lockouts.

Recovery and recycling: use EPA-certified recovery equipment, never vent refrigerant, and record the amount recovered; cylinders must not be overfilled (80% fill rule for liquid).

Section 608 certification is required to purchase or handle regulated refrigerants; EPA Type II covers high-pressure appliances, Type I small appliances, Type III low-pressure, and Universal covers all.

Know the refrigerant safety group: R-410A and R-32 are A1 (lower toxicity, no flame propagation); R-454B and R-32 are A2L (lower flammability) and require A2L-rated components, leak detection, and charge limits per ASHRAE 15 and the model codes.

R-410A operating pressures are roughly 50–70% higher than R-22; use only rated hoses, gauges, and recovery equipment, and never mix refrigerant types in a cylinder.

Leak repair requirements under 608: repairs must be completed within 30 days for commercial refrigeration above the leak-rate threshold; evacuation to 0 psig before opening the system is required.

Electrical safety: lock out/tag out the disconnect, verify with a meter, discharge capacitors before touching terminals, and treat 240V control circuits as live until proven dead.

Use PPE: safety glasses, gloves, and a respirator when brazing; ventilate the work area and keep a fire extinguisher within reach.

Local codes govern: permit requirements, seismic or wind anchoring, condensate disposal, and clearance to property lines; verify with the AHJ before installation.

Refrigerant cylinders must be upright, secured, and below 125°F; never heat a cylinder with an open flame - use a warm-water bath or a cylinder heater.

Heat pump COP = heating capacity delivered (Btu/h) ÷ electrical input (Btu/h); HSPF2 = total heating Btu ÷ total electrical watt-hours over the region IV test, so HSPF2 ≈ COP × 3.412 adjusted for defrost and cycling.

Cooling capacity balance point: compressor capacity falls roughly 1.5–3% per °F of outdoor rise above 95°F, while building load rises; the intersection of the two curves sets the design balance point.

Heating balance point is where heat pump capacity equals structure heat loss; below it, supplemental (auxiliary) heat carries the deficit, and the thermal balance point should be at or below design outdoor temperature for all-electric designs.

Standard cycle order: compressor discharge → reversing valve → outdoor coil (heating mode condenser) → expansion device → indoor coil (evaporator) → accumulator/suction → compressor.

Superheat = suction line temp − evaporating saturation temp; subcooling = condensing saturation temp − liquid line temp. Target superheat on fixed-orifice systems is load-dependent (typically 5–15°F); TXV systems target 8–12°F subcooling.

Reversing valve is a pilot-operated 4-way slide: energized solenoid = cooling on most residential designs; a stuck valve shows equal pressures or wrong-mode temperatures at both coils.

Defrost termination is normally on-demand (demand defrost) using coil temperature and pressure differential, with a 10-minute maximum timer and 30/60/90-minute initiation intervals; strip heat energizes during defrost to temper supply air.

Key components: scroll or rotary compressor, accumulator (heat pumps always require one), bi-flow or two check-type expansion valves, liquid-line filter-drier, suction-line accumulator, crankcase heater, and service valves.

Manual J block load uses design conditions: heating design temperature = 99% winter design dry bulb; cooling design temperature = 1% summer design dry bulb, both from ASHRAE/NOAA tables for the specific location.

Sensible heat equation: Btu/h = 1.08 × CFM × ΔT (sea level); latent: Btu/h = 0.68 × CFM × Δgrains; altitude correction multiplies airflow factors by the density ratio.

Equipment sizing rule: total capacity within 100–115% of calculated total load and sensible capacity ≥ sensible load at design; oversizing raises cycling, humidity, and defrost losses.

Line-set length and vertical separation reduce nominal capacity - apply manufacturer's correction table (typically 1–3% per 10 ft of equivalent length beyond 25 ft) before comparing to load.

Site survey items: available electrical service and panel capacity, clearances to property lines and combustibles, snow/prevailing wind exposure, condensate routing, structural support, and existing duct static pressure.

Ducted systems: verify external static pressure budget against blower table; a 0.5 in. w.c. design ESP is common for residential air handlers, 0.8–1.0 in. w.c. for high-static units.

Cold-climate selection: verify rated capacity at 5°F and 17°F (HSPF2 Region V data), compressor lockout setpoint, and whether the unit is rated for operation below 0°F.

Minimum circuit ampacity (MCA) = 1.25 × largest motor RLA + sum of other loads; maximum fuse/breaker (MOP) is set by the unit nameplate - never upsize beyond it to stop nuisance trips.

Wire sizing: use 60°C ampacity column for NM-B and 75°C for THHN; ampacity must be ≥ MCA, and the breaker protects the conductor per NEC 440.

Disconnect must be within sight of and within 50 ft of the outdoor unit; a non-fused pull-out is acceptable when the branch breaker provides protection.

Ground the outdoor unit with an equipment grounding conductor sized per NEC Table 250.122; do not rely on conduit as the ground path.

Low-voltage control colors (conventional): R = 24V hot, C = common, Y = compressor contactor, G = indoor blower, W = auxiliary/emergency heat, O = reversing valve (cooling active) and B = reversing valve (heating active) - never both O and B on one system.

Thermostat R-to-C must read 24VAC ±10% under load; a reading below 20VAC indicates transformer overload, undersized wire, or a shorted control.

Communicating systems use two-wire polarity-insensitive data buses; keep data wire off line-voltage conduit, use manufacturer-specified cable, and terminate shields at one end only.

Defrost control board, low-ambient kit, crankcase heater, and outdoor thermistor are separate loads - verify transformer VA capacity covers all accessories plus the thermostat.

Nitrogen purge is mandatory during brazing at ~2–3 SCFH; flow must be visible at the outlet, and the system must be open to atmosphere at one point so pressure cannot build.

Use silver-phosphorus brazing alloy (BCuP) for copper-to-copper; add flux or use a silver-bearing alloy with flux for copper-to-brass or copper-to-steel joints.

Evacuation target: 500 microns or below with a standing vacuum test - isolate the pump, hold 500 microns for 10–15 minutes with no rise above 500; a rise to 1,000–2,000 microns indicates moisture, above 2,000 indicates a leak.

Triple evacuation: evacuate to 1,500 microns, break with dry nitrogen to 0 psig, repeat, then final evacuate to 500 microns; this removes moisture faster than a single long pull.

Weigh-in charging is required for units with a fixed orifice or when line-set length exceeds the factory charge; add refrigerant per the manufacturer's oz-per-foot table for liquid-line size.

Subcooling method (TXV/EEV): measure liquid-line temperature and convert suction or liquid pressure to saturation temperature; target the nameplate subcooling (±3°F), adjusting charge in small increments.

Superheat method (fixed orifice): target superheat comes from the manufacturer's chart using indoor wet-bulb and outdoor dry-bulb; add charge if superheat is above target, recover if below.

Line-set limits: typical residential maximum 50–100 ft total equivalent length and 50 ft vertical; oil return requires an inverted trap at the top of a riser and a P-trap every 20–25 ft of vertical rise on larger systems.

Nominal airflow: 400 CFM per ton for cooling; 350 CFM/ton improves latent removal in humid climates, 450 CFM/ton maximizes sensible capacity in dry climates.

Static pressure budget: supply trunk 0.1 in. w.c., branch runs 0.1 in. w.c., return 0.05–0.1 in. w.c., filter 0.1–0.2 in. w.c., coil 0.2–0.3 in. w.c.; total should not exceed the blower's rated ESP.

Return air must be ducted or fully panned on heat pumps; a starved return drops airflow, raises superheat, and can freeze the indoor coil in cooling.

Flex duct: keep runs taut, limit sag, use long-radius elbows, and support at ≤ 5 ft intervals; each 90° flex elbow can cost the equivalent of 25–35 ft of straight duct.

Duct leakage in unconditioned space should be sealed with mastic (not cloth duct tape); target ≤ 10% total leakage and ≤ 5% to outside for new construction in most energy codes.

Indoor unit must be level within 1/4 in. per foot for condensate drainage; secondary drain pan with a float switch is required where overflow could cause damage.

Filter selection: MERV 8–13 is typical; higher MERV raises static pressure - verify blower table at the filter's rated pressure drop, not a nominal value.

Minimum clearances: typically 12–18 in. on the service side, 12 in. on the fan discharge side, 6–12 in. on other sides, and 24–36 in. above grade; always use the manufacturer's installation manual values.

Do not place the outdoor unit under a roof drip line, eave, or deck where ice and snow shed onto the coil; avoid recirculation zones where discharge air re-enters the coil.

Wall-mounted brackets must be lagged into framing or blocking; ground pads must be level and above the local frost line, with a 2–3 in. riser to keep the coil above snow.

Defrost drain: route the condensate away from the foundation with a freeze-resistant path; a heated drain pan or drain-line heat tape is used where freezing is expected.

Defrost cycle sequence: reversing valve shifts to cooling, outdoor fan stops (on most designs), strip heat energizes, coil warms to terminate temperature (~50–70°F coil temp), then normal heating resumes.

Excessive defrost (short intervals, long cycles) is caused by a faulty coil sensor, low charge, restricted airflow, or a bad defrost board; verify sensor resistance against the temperature chart before replacing the board.

Ice buildup at the bottom of the coil often indicates a blocked drain pan or a unit set too low; raise the unit and clear the pan before condemning the defrost control.

Commissioning sequence: verify electrical and refrigerant tightness, evacuate, weigh in charge, then run in cooling for 15–20 minutes to stabilize before recording superheat/subcooling.

Record and compare: indoor entering/leaving dry-bulb and wet-bulb, outdoor dry-bulb, suction and liquid pressures, superheat, subcooling, amp draw vs. nameplate RLA, and external static pressure.

Temperature split in cooling: 18–22°F between return and supply at 400 CFM/ton with 50% indoor RH; a split below 15°F suggests low airflow or low charge.

Heating mode: verify supply air temperature rise is within the manufacturer's range (typically 25–45°F for a heat pump without strip heat), and check that auxiliary heat stages in only when needed.

Delta-T method for airflow: CFM = sensible Btu/h ÷ (1.08 × ΔT); use the manufacturer's blower table and measured ESP as the primary airflow check.

Verify defrost initiation and termination, low-ambient lockout, crankcase heater operation, and thermostat auxiliary lockout/balance point settings before signing off.

Document static pressure, charge method used, line-set length and added refrigerant, and all setpoints; compare measured capacity against the load calculation to confirm the balance point.

Low suction pressure + high superheat + normal subcooling = restricted or underfed evaporator (low airflow, restricted piston/TXV, or low charge); check airflow before adding refrigerant.

High suction pressure + low superheat + low subcooling = overfeeding TXV or overcharge; high head pressure with normal subcooling points to condenser airflow or non-condensables.

Compressor won't start: check 24V at contactor coil, then capacitor (microfarad reading vs. rated ±6%), then start components, then winding resistance and insulation to ground.

Heat pump heats but won't cool (or vice versa): suspect the reversing valve solenoid, the O/B thermostat configuration, or a stuck valve slide - verify coil temperatures in both modes.

Defrost complaints: check outdoor coil sensor resistance, board timing, and whether the unit is actually iced; a dirty coil or low charge mimics defrost failure.

Preventive maintenance: clean coils, check refrigerant charge by superheat/subcooling, test capacitors, inspect contactor points, flush condensate drain, and verify thermostat and safety lockouts.

Recovery and recycling: use EPA-certified recovery equipment, never vent refrigerant, and record the amount recovered; cylinders must not be overfilled (80% fill rule for liquid).

Section 608 certification is required to purchase or handle regulated refrigerants; EPA Type II covers high-pressure appliances, Type I small appliances, Type III low-pressure, and Universal covers all.

Know the refrigerant safety group: R-410A and R-32 are A1 (lower toxicity, no flame propagation); R-454B and R-32 are A2L (lower flammability) and require A2L-rated components, leak detection, and charge limits per ASHRAE 15 and the model codes.

R-410A operating pressures are roughly 50–70% higher than R-22; use only rated hoses, gauges, and recovery equipment, and never mix refrigerant types in a cylinder.

Leak repair requirements under 608: repairs must be completed within 30 days for commercial refrigeration above the leak-rate threshold; evacuation to 0 psig before opening the system is required.

Electrical safety: lock out/tag out the disconnect, verify with a meter, discharge capacitors before touching terminals, and treat 240V control circuits as live until proven dead.

Use PPE: safety glasses, gloves, and a respirator when brazing; ventilate the work area and keep a fire extinguisher within reach.

Local codes govern: permit requirements, seismic or wind anchoring, condensate disposal, and clearance to property lines; verify with the AHJ before installation.

Refrigerant cylinders must be upright, secured, and below 125°F; never heat a cylinder with an open flame - use a warm-water bath or a cylinder heater.