ACCA Residential HVAC Design cheat sheet

Manual J computes room-by-room heating and cooling loads from envelope, infiltration, internal, and duct gains; equipment selection follows, not precedes, the load calculation.

Cooling load uses design outdoor dry-bulb and coincident wet-bulb; sensible and latent loads are computed separately and summed into total capacity.

Infiltration is estimated from envelope tightness (e.g., blower-door-derived ACH50 converted to natural ACH) plus a wind/stack factor, not from a fixed rule of thumb.

Manual S matches equipment to load: total capacity must meet or exceed total load, and sensible capacity must meet sensible load at the design entering wet-bulb.

Oversizing shortens runtime, raises cycling losses, degrades latent removal, and causes temperature swings; Manual S explicitly limits permissible oversizing.

Manual D sizes ducts to available static pressure: total external static minus device and coil drops is apportioned across supply and return runs using friction rate and equivalent length.

Equivalent length sums straight-run length plus fitting allowances; a higher equivalent length at fixed available static yields a lower friction rate and larger ducts.

Manual D requires a room-by-room airflow allocation consistent with the Manual J room loads so each space receives its calculated cfm.

ANSI/ACCA 5 QI-2015 (HVAC Quality Installation Specification) defines verifiable criteria for design, installation, commissioning, and documentation.

Acceptable field practices include charging by superheat (fixed orifice) or subcooling (TXV) per manufacturer tables, never by gauge feel alone.

Airflow verification targets roughly 350-450 cfm per ton of cooling in residential systems; low airflow risks coil icing and compressor overheating.

Duct leakage testing quantifies distribution losses; sealed and tested ducts reduce effective load and improve delivered capacity.

Commissioning records should capture static pressure, temperature split, refrigerant charge method, airflow, and combustion analysis where applicable.

Combustion safety testing includes draft measurement and CO testing on gas appliances; spillage or elevated CO requires corrective action before sign-off.

Refrigerant line evacuation to a deep vacuum (commonly 500 microns or below, held) confirms removal of moisture and non-condensables before charging.

Section 608 of the Clean Air Act governs refrigerant handling; technicians must be certified for the equipment class they service.

Ozone-depleting refrigerants are phased out; R-410A is an HFC blend with zero ODP but high GWP, and HFO blends such as R-454B are lower-GWP alternatives.

R-410A operates at roughly 50-70% higher pressures than R-22, requiring dedicated gauges, hoses, and recovery equipment rated for the higher pressure.

Recovery cylinders must be DOT-approved, not overfilled (typically 80% fill limit), and labeled with refrigerant type and amount.

A2L refrigerants are mildly flammable; handling requires ignition-source awareness, ventilation, and adherence to listed charge limits and labeling.

Leak detection precedes recovery; EPA rules require repair of leaks above threshold rates in appliances of specified charge sizes, with verification testing.

Never vent refrigerant to atmosphere; recovery, recycling, and reclamation are the only permitted disposition paths.

Permits and inspections are typically required for new installations, changeouts, and duct alterations; scope varies by jurisdiction and must be confirmed locally.

Load calculations and equipment selection documentation are commonly required submittals for permit review and for utility or rebate program qualification.

Manual J/S/D compliance is referenced by many energy codes (e.g., IECC) as the basis for sizing and duct design.

A written contract should define scope, equipment model numbers, warranties, payment schedule, and change-order procedures.

Refrigerant purchase and handling records support EPA compliance and may be requested during audits.

Warranty validity often depends on documented commissioning data (charge verification, airflow, static pressure); missing records can void manufacturer coverage.

Change orders should be documented in writing before work proceeds to preserve both margin and legal standing.