MI HVAC cheat sheet

Sensible heat output of electric resistance heat = 3.412 Btu per watt-hour.

Gas furnace AFUE is the ratio of annual heat output to annual fuel energy input; 90%+ AFUE units are condensing and require a secondary heat exchanger plus condensate drain.

Forced-air furnace temperature rise = supply air temp minus return air temp; it must fall within the nameplate rise range at the rated airflow.

Hydronic baseboard output is typically rated near 170°F average water temperature; output drops roughly with the difference between water and room temperature.

Heat pump balance point is the outdoor temperature where heat pump capacity equals building load; below it, supplemental heat is needed.

A heat pump's coefficient of performance (COP) = heat delivered in Btu divided by energy input in Btu; typical air-source COP is 2 to 4.

Btuh = 500 × gpm × water temperature drop (°F) for hydronic systems using water.

Steam systems use traps to pass condensate while stopping steam; a failed trap can cause water hammer or lost heat.

Total heat (enthalpy) difference across the evaporator coil: capacity (Btu/h) = 4.5 × cfm × Δh.

Sensible capacity = 1.08 × cfm × temperature difference; latent capacity = 0.68 × cfm × humidity ratio difference (grains).

Superheat = suction line temperature minus evaporating (saturation) temperature; target is typically 8–12°F at the evaporator for fixed-orifice systems.

Subcooling = condensing (saturation) temperature minus liquid line temperature; measured at the condenser, typically 8–12°F for TXV systems.

Refrigerant boils at a temperature tied to pressure; a low suction pressure with high superheat suggests undercharge or a restricted metering device.

Standard air conditioning airflow is about 350–450 cfm per ton of cooling capacity (12,000 Btu/h per ton).

A dirty condenser raises condensing temperature and pressure, increasing compressor amp draw and reducing capacity.

Recovery of refrigerant is required before opening a circuit; venting refrigerant to atmosphere is prohibited under federal law.

Equal friction method sizes ducts by selecting a constant friction rate (commonly 0.1 in. w.c. per 100 ft) and reading sizes from a duct calculator.

Duct velocity pressure = (velocity in fpm / 4005)² expressed in inches of water column.

Friction loss in ductwork rises with the square of airflow; doubling cfm in the same duct roughly quadruples friction.

Equivalent length converts fittings to straight-duct length; elbows, boots, and takeoffs add to total effective length.

Return air pathways must be sized so that total external static pressure stays within the blower's rated range.

ASHRAE 62.2 residential ventilation: whole-house mechanical ventilation rate depends on floor area and bedroom count.

Duct leakage testing pressurizes the duct system to 25 Pa and reports leakage as cfm per 100 sq ft of conditioned floor area.

Flexible duct must be installed fully extended; compressing it reduces airflow and increases static pressure.

Manual J computes room-by-room heating and cooling loads from envelope, orientation, internal gains, and infiltration.

Manual S performs equipment selection using the Manual J loads and manufacturer performance data.

Manual D sizes the duct system; Manual T selects registers and diffusers.

Design conditions use winter outdoor design temperature for heating and summer design dry-bulb and wet-bulb for cooling.

Infiltration load can be estimated with the air-change method or a blower-door-derived leakage value.

Oversizing a cooling system shortens runtime, raises indoor humidity, and reduces part-load efficiency.

1 ton of cooling = 12,000 Btu/h; sensible heat ratio = sensible load divided by total load.

Duct and infiltration losses must be accounted for so delivered capacity matches the room load.

Ohm's law: voltage = current × resistance; power in watts = volts × amps for resistive loads.

A low-voltage thermostat circuit is typically 24 VAC supplied by a step-down transformer in the air handler.

A pressure switch proves inducer draft before ignition; a rollout switch and flame sensor are safety interlocks in gas furnaces.

A high-limit switch opens on excess supply air temperature and interrupts the burner circuit.

A capacitor's microfarad rating should be tested against the nameplate; a failed run capacitor causes hard starting or high amp draw.

Locked rotor amps (LRA) exceed running load amps (RLA); a compressor drawing near LRA is failing to start.

Grounding, bonding, and GFCI protection are required by the National Electrical Code as adopted in Michigan.

A float switch in the condensate pan interrupts the cooling circuit to prevent overflow damage.

Natural gas heating value is roughly 1,000 Btu per cubic foot; LP gas is about 2,500 Btu per cubic foot.

Gas piping is sized by longest length and total connected load using NFPA 54 / IFGC capacity tables.

Combustion air: approximately 50 cubic feet of space per 1,000 Btu/h of input is the classic rule for unconfined spaces.

Complete combustion of natural gas produces carbon dioxide and water vapor; incomplete combustion produces carbon monoxide.

A Category IV condensing appliance uses positive-pressure venting and must be vented with approved plastic pipe.

Draft hood or induced-draft fan maintains proper venting; blocked flue or inadequate draft causes spillage of flue gases.

A manometer measures gas manifold pressure; natural gas manifold pressure is commonly 3.5 in. w.c.

Excess air and stack temperature are combustion-analysis indicators of efficiency and burner condition.

HVAC work in Michigan is regulated under the Michigan Mechanical Code and the Stille-DeRossett-Hale Single State Construction Code Act.

Mechanical permits are issued by the local enforcing agency; work must be inspected before concealment.

Mechanical contractors must be licensed by the state; a license requires passing an examination and paying fees.

Apprentices must work under the supervision of a licensed mechanical contractor or journeyman.

Code adoption in Michigan happens at the state level; local units generally cannot adopt conflicting mechanical codes.

Permit fees and inspection schedules are set by the local enforcing agency within state guidelines.

Contractors must carry the insurance and registration required for the license to remain valid.

Final inspection sign-off is required before a system is placed in service or concealed.

A takeoff lists quantities of material (duct, pipe, fittings, equipment) from the drawings before pricing.

Labor hours are estimated from productivity rates per unit (per foot of duct, per fixture, per ton) multiplied by quantity.

Overhead and profit are added to direct job costs; markup is applied to cost, whereas margin is a percentage of the selling price.

A change order documents scope changes and their cost impact and must be approved before work proceeds.

Contract documents should define scope, exclusions, payment schedule, and warranty terms.

Cash flow planning accounts for the lag between paying for materials and collecting from the customer.

Retainage is a percentage withheld until substantial completion or final acceptance.

Job cost tracking compares actual labor and material costs with the estimate to measure performance.