Manual S - Equipment Selection

What Manual S Does

ACCA Manual S (Residential Equipment Selection) takes the load calculations from Manual J and matches them to specific equipment. This is where the rubber meets the road - you have a calculated heating load and a calculated cooling load, and you need to pick a furnace, air conditioner, or heat pump that satisfies both without being oversized or undersized.

Manual S is not about picking the cheapest or most expensive unit. It is about selecting equipment whose capacity at actual design conditions falls within a specific range relative to the Manual J loads. The goal is a system that runs long cycles, controls humidity, maintains even temperatures, and operates efficiently.

The 3rd Edition Stratified Sizing Rules

Manual S 3rd Edition (2023/2024) replaces the previous universal 115% cap with stratified limits based on equipment type. Variable-speed compressors can modulate output to match low-load conditions, which mitigates the short-cycling risks that justified the old single cap. The limits apply to the selected equipment's sensible cooling capacity at actual design conditions relative to the Manual J total building cooling load.

For example, if Manual J calculates a total building cooling load of 24,000 BTU/h:

• Single-stage AC maximum: 24,000 x 1.20 = 28,800 BTU/h
• Two-stage AC maximum: 24,000 x 1.25 = 30,000 BTU/h (evaluated at low stage)
• Variable-capacity AC range: 21,600 to 31,200 BTU/h (90% to 130%)

Why Sensible Capacity - Not Total Capacity

Manual S sizes on sensible capacity, not total capacity. This is a critical distinction. Total capacity = sensible capacity + latent capacity. A unit might have 36,000 BTU/h total capacity but only 26,000 BTU/h sensible and 10,000 BTU/h latent. If the Manual J sensible load is 24,000 BTU/h, the 26,000 sensible capacity falls within the acceptable window and the equipment is acceptable from a sensible standpoint.

Sizing on total capacity would lead to oversized equipment in humid climates where latent loads are significant. A unit with 36,000 BTU/h total might seem like the right match for a 34,000 BTU/h total load, but if the sensible load is only 24,000 BTU/h, the sensible capacity would be far above the allowed maximum.

Heating Capacity Requirements for Fossil Fuel Equipment

For gas furnaces, boilers, and other fossil fuel heating equipment, Manual S requires the output capacity to fall between 100% and 140% of the Manual J total heating load. The wider range exists because heating oversizing has fewer negative consequences than cooling oversizing - there is no humidity control concern in heating mode, and modern furnaces modulate or cycle effectively.

However, the heating equipment must also satisfy the cooling airflow requirement. If a gas furnace provides the blower for a split AC system, the furnace's blower must deliver the required cooling CFM (typically 400 CFM per ton) at the system's design static pressure.

Manual S 3rd Edition uses stratified cooling sizing limits: single-stage AC up to 120%, two-stage AC up to 125% (evaluated at low stage), and variable-capacity AC 90% to 130% of the total building cooling load. Variable-speed compressors justify the wider allowance because they modulate output to prevent short-cycling. Fossil fuel heating equipment must be sized 100% to 140% of the heating load. Always size on sensible cooling capacity, not total capacity.