Most permit offices, LEED, Energy Star and International Residential Code (IRC) defines the requirement for sizing HVAC equipment: “Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies” (emphasis added).
ACCA (Air Conditioning Contractors of America) Manual S, Residential Heating and Cooling Equipment Selection, provides clear instructions for interpreting and applying original equipment manufacturer (OEM) expanded performance data to select equipment that meets application requirements (heating, sensible cooling or latent cooling) for the applied design conditions used to calculate loads with ACCA Manual J, Residential Load Calculation.
Manual S sets equipment sizing limits. These sizing parameters ensure that equipment capacities meet the minimum needs of occupants based on Manual J loads while preventing the problems associated with oversizing:
- Furnaces: 100-140% of total heating load
- Boilers: 100-140% of total heating load
- Air Conditioners: 115% of total cooling load
- When selecting cooling equipment, it is necessary to know the design conditions used to calculate the cooling load. Unlike heating equipment, cooling equipment OEM data offers a range of performance at different outdoor and indoor conditions.
- Heat Pumps: 115% or 125% of total cooling load
- Heat pumps in a cooling dominant climate are allowed to be 115% of the cooling level.
- Heat pumps in a heating dominant climate are allowed to be 125% of the cooling level. The size of the cooling equipment must be based on the same temperature and humidity conditions that were used to calculate the Manual J
AHRI Versus OEM Expanded Performance Data
Manufacturers of heating and cooling equipment certify the performance of their products with AHRI (Air Conditioning, Heating and Refrigeration Institute) who produces standards for rating such equipment. The problem is the data published in AHRI product directories simulate a very small geographic area in the U.S and should not be used. AHRI directories should only be used to compare equipment efficiency ratings.
OEM expanded performance data should be used to select properly sized equipment. For example, furnaces should be designed with a temperature rise range of 35°F–65°F. The furnace manufacturer will provide blower performance data indicating the air flow that the unit can deliver at different levels of resistance. If the air flow is outside of the temperature rise range:
- The equipment may cycle off at safety limits, suffer damage or possibly even create an unsafe condition.
- Incorrect air flow can cause too much temperature rise (slow-moving air may allow the heat exchanger to become too hot, which can result in warping or cracking of the metal heat exchanger)
- Too little temperature rise (fast-moving air may cause condensation in the metal heat exchanger, which can result in the production of an acid that can harm or penetrate the heat exchanger).
An air flow rate is acceptable if it yields a temperature rise within the range prescribed by the equipment manufacturer. Oversizing can lead to health issues associated with excessive humidity; higher costs for equipment and installation labor and materials; greater energy consumption; and more wear and tear on equipment.
A slightly different approach is used when using Manual S to verify cooling equipment selection than to select heating equipment. Verification begins by:
- Determining the outdoor air temperature and indoor wet-bulb temperature.
- Manufacturers all present their data in a different formats, but all should include airflow, entering air wet-bulb temperature,outdoor temperature, and cooling capacities (typically, total and sensible capacities).
- Another important design parameter is the volume of air that must flow over the indoor air conditioner coil to achieve the required cooling capacities.
- There are two aspects to the consideration of cooling load, sensible and latent loads.
- The sensible load is the heat that is measured by a thermometer or a thermostat. This is the “dry” heat one consciously feels.
- The latent load is the heat associated with airborne moisture as measured by a hygrometer or humidistat.
When you enter a home whose thermostat shows a cool temperature but which has high relative humidity (latent heat) initially you feel comfortable. But as your body adjusts to the temperature you begin to feel sticky, clammy and uncomfortable. You may even feel warm again. This is why two homes with the same thermostat setting can feel so different. Chances are the clammy, uncomfortable house was not properly sized using ACCA Manual S HVAC equipment sizing procedures…
ACCA Manual S reference:
Be sure to check out other Savoy Engineering Group posts on ACCA Manual S. They will help you to understand how the Manual S calculation is a lot more than just reading an AHRI certificate