MD HVAC cheat sheet

One ton of refrigeration equals 12,000 Btu/h, 200 Btu/min, or 3.517 kW.

Sensible airflow equation: Qs = 1.08 × CFM × ΔT, where Qs is Btu/h and ΔT is dry-bulb difference in °F.

Total airflow equation: Qt = 4.5 × CFM × Δh, where Δh is enthalpy difference in Btu/lb.

Compression ratio = discharge pressure (psia) ÷ suction pressure (psia); use absolute pressure, not gauge pressure.

Superheat = suction line temperature − evaporator saturation temperature at suction pressure; subcooling = condensing saturation temperature − liquid line temperature.

The metering device creates the pressure drop; the evaporator absorbs heat; the condenser rejects heat; the compressor raises pressure and temperature.

Saturation temperature increases as pressure increases for a pure refrigerant; zeotropic blends have bubble and dew points and measurable glide.

A deep vacuum is measured in microns; 25,400 microns equals 1 in Hg, and 500 microns is a common dehydration target.

Natural gas heating value is about 1,000 Btu/ft³; propane vapor is about 2,500 Btu/ft³ and No. 2 fuel oil about 140,000 Btu/gal.

For water, hydronic heat output Q = 500 × GPM × ΔT; 1 GPM at 20°F ΔT delivers about 10,000 Btu/h.

Electric resistance heat: 1 watt = 3.413 Btu/h; a 5 kW heater delivers about 17,065 Btu/h.

Furnace temperature rise is found from Q = 1.08 × CFM × ΔT; compare measured rise with the nameplate range.

Typical natural gas manifold pressure is about 3.5 in. w.c.; typical LP manifold pressure is about 10–11 in. w.c.

Combustion requires roughly 10 ft³ of air per ft³ of natural gas at stoichiometric conditions; excess air is supplied for safe combustion.

A residential hot-water boiler commonly uses a 30 psig pressure relief valve; the relief valve must not be capped or plugged.

Oil burner nozzles are rated in gallons per hour; No. 2 oil at 140,000 Btu/gal and 1 GPH yields about 140,000 Btu/h input.

Hydronic expansion tanks accommodate water volume change; precharge and system fill pressure must match design.

CFM = duct velocity in ft/min × duct cross-sectional area in ft².

Velocity pressure: VP = (V ÷ 4005)² in. w.c. for standard air at about 0.075 lb/ft³.

External static pressure equals supply static plus return static, measured with a manometer.

A common duct design friction rate is 0.1 in. w.c. per 100 ft of equivalent length.

MERV ratings: MERV 8 captures coarse particles; MERV 13 captures finer particles and may increase pressure drop.

ASHRAE Standard 62.1 addresses commercial ventilation; Standard 62.2 addresses residential ventilation.

Continuous residential bathroom exhaust is commonly 20 cfm; intermittent operation is commonly 50 cfm.

CO2 concentration is a ventilation indicator; outdoor air is about 400 ppm and indoor targets often range from 600 to 1,000 ppm.

Ohm's law: E = I × R; power: P = E × I = I² × R.

In a series circuit current is common; in a parallel circuit voltage is common.

Measure voltage in parallel with the load, current in series with the circuit, and resistance with power removed.

Locked-rotor current for induction motors is typically about 5–7 times running full-load current.

A run capacitor is rated in microfarads and stays in the circuit; a start capacitor is switched out after startup.

Low-voltage thermostat terminals: R = 24 V power, W = heat, Y = cooling, G = fan, C = common.

A contactor coil is a control load; the coil voltage must match the control transformer output.

Discharge capacitors before testing; use a meter rated for the voltage and verify zero stored voltage.

Motor overload protection is sized to motor nameplate current and service factor, not to the branch-circuit breaker alone.

EPA Section 608 certification is required to service, repair, or dispose of appliances containing regulated refrigerants.

Knowingly venting regulated refrigerant to the atmosphere is prohibited; recover refrigerant before opening a system.

ASHRAE Standard 15 governs refrigerant safety classifications, occupancy, and machinery-room requirements.

A CO alarm should be installed outside sleeping areas and on each level of a home with combustion appliances.

Gas piping must include a shutoff valve and a sediment trap ahead of the appliance controls.

Electrical disconnects must be within sight of and readily accessible from the equipment served.

Lockout/tagout requires isolating all energy sources and verifying zero energy before service work.

Flue and vent terminations must maintain manufacturer and code clearances from windows, doors, and air intakes.

Recovery cylinders must be DOT-approved, labeled, and filled no more than 80% by weight.

Manual J calculates residential heating and cooling loads; Manual S selects equipment; Manual D designs ducts.

A permit and inspection are generally required for new HVAC installations, replacements, and gas piping changes.

A written contract should state scope, price, payment schedule, warranty, and change-order procedure.

Change orders should be documented in writing before additional work is performed.

Lien rights allow unpaid contractors to file a claim against property; deadlines vary by state.

OSHA requires fall protection, eye protection, and safe ladder use on construction sites.

Commissioning verifies airflow, refrigerant charge, temperature rise, and controls operation against design.

Warranty terms should identify labor coverage, parts coverage, and required maintenance.

A startup checklist records model and serial numbers, static pressure, voltage, amperage, and refrigerant pressures.

One ton of refrigeration equals 12,000 Btu/h, 200 Btu/min, or 3.517 kW.

Sensible airflow equation: Qs = 1.08 × CFM × ΔT, where Qs is Btu/h and ΔT is dry-bulb difference in °F.

Total airflow equation: Qt = 4.5 × CFM × Δh, where Δh is enthalpy difference in Btu/lb.

Compression ratio = discharge pressure (psia) ÷ suction pressure (psia); use absolute pressure, not gauge.

Superheat = suction line temperature − evaporator saturation temperature at suction pressure; subcooling = condensing saturation temperature − liquid line temperature.

The metering device creates the pressure drop; the evaporator absorbs heat; the condenser rejects heat; the compressor raises pressure and temperature.

Saturation temperature increases as pressure increases for a pure refrigerant; zeotropic blends have bubble and dew points and measurable glide.

A deep vacuum is measured in microns; 25,400 microns equals 1 in Hg, and 500 microns is a common dehydration target.

Natural gas heating value is about 1,000 Btu/ft³; propane vapor is about 2,500 Btu/ft³ and No. 2 fuel oil about 140,000 Btu/gal.

For water, hydronic heat output Q = 500 × GPM × ΔT; 1 GPM at 20°F ΔT delivers about 10,000 Btu/h.

Electric resistance heat: 1 watt = 3.413 Btu/h; a 5 kW heater delivers about 17,065 Btu/h.

Furnace temperature rise = 1.08 × CFM × ΔT; compare measured rise with the nameplate range.

Typical natural gas manifold pressure is about 3.5 in. w.c.; typical LP manifold pressure is about 10–11 in. w.c.

Combustion requires roughly 10 ft³ of air per ft³ of natural gas at stoichiometric conditions; excess air is supplied for safe combustion.

A residential hot-water boiler commonly uses a 30 psig pressure relief valve; the relief valve must not be capped or plugged.

Oil burner nozzles are rated in gallons per hour; No. 2 oil at 140,000 Btu/gal and 1 GPH yields about 140,000 Btu/h input.

Hydronic expansion tanks accommodate water volume change; precharge and system fill pressure must match design.

CFM = duct velocity in ft/min × duct cross-sectional area in ft².

Velocity pressure: VP = (V ÷ 4005)² in. w.c. for standard air at about 0.075 lb/ft³.

External static pressure equals supply static plus return static, measured with a manometer and corrected for probe.

A common duct design friction rate is 0.1 in. w.c. per 100 ft of equivalent length.

MERV ratings: MERV 8 captures coarse particles; MERV 13 captures finer particles and may increase pressure drop.

ASHRAE Standard 62.1 addresses commercial ventilation; 62.2 addresses residential ventilation.

Continuous residential bathroom exhaust is commonly 20 cfm; intermittent operation is commonly 50 cfm.

CO2 concentration is a ventilation indicator; outdoor air is about 400 ppm and indoor targets often range 600–1,000 ppm.

Return air, supply air, and outdoor air dampers must be balanced to maintain design static pressure and airflow.

Ohm's law: E = I × R; power: P = E × I = I² × R.

In a series circuit current is common; in a parallel circuit voltage is common.

Measure voltage in parallel with the load, current in series with the circuit, and resistance with power removed.

Locked-rotor current for induction motors is typically about 5–7 times running full-load current.

A run capacitor is rated in microfarads and stays in the circuit; a start capacitor is switched out after startup.

Low-voltage thermostat terminals: R = 24 V power, W = heat, Y = cooling, G = fan, C = common.

A contactor coil is a control load; the coil voltage must match the control transformer output.

Discharge capacitors before testing; use a meter rated for the voltage and verify zero stored voltage.

Motor overload protection is sized to motor nameplate current and service factor, not to the branch-circuit breaker alone.

EPA Section 608 certification is required to service, repair, or dispose of appliances containing regulated refrigerants.

Knowingly venting regulated refrigerant to the atmosphere is prohibited; recover refrigerant before opening a system.

ASHRAE Standard 15 governs refrigerant safety classifications, occupancy, and machinery-room requirements.

A CO alarm should be installed outside sleeping areas and on each level of a home with combustion appliances.

Gas piping must include a shutoff valve and a sediment trap ahead of the appliance controls.

Electrical disconnects must be within sight of and readily accessible from the equipment served.

Lockout/tagout requires isolating all energy sources and verifying zero energy before service work.

Flue and vent terminations must maintain manufacturer and code clearances from windows, doors, and air intakes.

Recovery cylinders must be DOT-approved, labeled, and filled no more than 80% by weight.

Manual J calculates residential heating and cooling loads; Manual S selects equipment; Manual D designs ducts.

A permit and inspection are generally required for new HVAC installations, replacements, and gas piping changes.

A written contract should state scope, price, payment schedule, warranty, and change-order procedure.

Change orders should be documented in writing before additional work is performed.

Lien rights allow unpaid contractors to file a claim against property; deadlines vary by state.

OSHA requires fall protection, eye protection, and safe ladder use on construction sites.

Commissioning verifies airflow, refrigerant charge, temperature rise, and controls operation against design.

Warranty terms should identify labor coverage, parts coverage, and required maintenance.

A startup checklist records model/serial numbers, static pressure, voltage, amperage, and refrigerant pressures.