Manual S Equipment Selection Explained

Manual J says how much heating and cooling a house needs. Manual S, ACCA's Residential Equipment Selection, is the next step: picking a furnace, air conditioner or heat pump whose real capacity, at the house's own conditions, fits those loads.

It sounds simple, match the box to the number. The catch is that equipment doesn't deliver its nameplate capacity at your conditions.

Why the nameplate isn't the answer

A "3-ton" air conditioner is rated at standard test conditions. At an actual design day the output depends on:

• Outdoor temperature. Cooling capacity drops as it gets hotter outside; heat pump heating capacity drops as it gets colder.
• Indoor conditions. The air entering the coil, its temperature and humidity, changes both total capacity and how much of it goes to moisture removal.
• Airflow. More air across the coil raises sensible capacity and lowers latent capacity; less air does the opposite.
• The matched system. The same outdoor unit performs differently with different indoor coils or air handlers.

Manufacturers publish expanded (extended) performance data: tables of capacity at a range of outdoor temperatures, entering conditions and airflows. Manual S reads the capacity at the design conditions from those tables, not from the model number.

The selection, step by step

1. Start from a Manual J. Room-by-room loads, with the cooling load split into sensible and latent parts, at local design conditions. A square-foot estimate isn't a valid starting point.
2. Set the design conditions. Outdoor summer and winter design temperatures, indoor setpoints, and the expected airflow.
3. Look up the candidate's capacity at those conditions in the expanded performance data: total, sensible and latent for cooling; output at the heating design temperature for heat pumps.
4. Compare with the loads. The equipment has to cover the sensible load and the latent load, and the total shouldn't exceed the load by more than Manual S allows. Cooling is held to a tighter oversizing limit than heating, because oversized cooling hurts comfort and humidity.
5. For heat pumps, size the backup. Find the balance point and size supplemental heat (electric or a furnace) to cover the gap at the heating design temperature.
6. Check the airflow the selection needs against what the duct system will deliver. That's where Manual D comes in.

Sensible and latent: the part rules of thumb miss

The sensible load is about temperature; the latent load is about moisture from people, cooking and humid outdoor air. In a dry climate the latent load is small and almost all capacity can go to sensible cooling. In a humid climate it can be a large share of the total.

An oversized unit cools the air quickly, satisfies the thermostat and shuts off, often before the coil has been cold long enough to pull much water out. That is the classic "cold and clammy" house, and it's why Manual S checks the latent capacity and limits oversizing rather than treating bigger as safer.

Furnaces and heat pumps in cold climates

For a furnace, the output (input × efficiency) is checked against the heating load, with an upper limit so it isn't grossly oversized. Very large furnaces short-cycle and can need more airflow than the ducts carry.

Heat pumps add a twist: heating capacity falls as it gets colder, just as the load rises. Selection looks at both seasons. In a cooling-dominated climate the cooling load usually sets the size and backup heat covers the coldest hours. In a heating-dominated climate, a variable-speed or cold-climate heat pump can often be sized closer to the heating load, as long as its minimum output keeps cooling from being oversized. Our guide to heat pump sizing walks through it, and the dual fuel guide covers when a furnace should take over.

What to keep for the permit and the customer

A clean Manual S selection records the design conditions, the loads, the equipment's capacity at those conditions (with the data source), the comparison, and the backup heat for heat pumps. It shows the inspector the equipment fits the house and shows the homeowner why this size, not a bigger one.

Where calc.blue fits

calc.blue carries the Manual J loads straight into equipment selection on the iPad: candidates are compared with the house at its own design temperatures on one chart, including backup heat and dual fuel, and the result flows into the permit packet and the homeowner report. It's built on ACCA Manual J 8th edition procedures. Join the waitlist.

ACCA, Manual J, Manual S and Manual D are trademarks of the Air Conditioning Contractors of America. calc.blue is not affiliated with or approved by ACCA.