Pro Tip: Most residential systems aim for a Friction Rate between 0.06 and 0.10 in. w.c. per 100 ft.
Available Static Pressure (ASP)
0.00
Total Effective Length (TEL)
0
Friction Rate (per 100ft)
0.00
Understanding Manual D Calculations
ACCA Manual D is the industry standard for sizing residential duct systems. A correct calculation ensures that the HVAC equipment can deliver the required airflow (CFM) to every room while overcoming the resistance of the ductwork and components.
Step 1: Calculate Available Static Pressure (ASP)
ASP is the amount of pressure remaining to move air through the ductwork after accounting for "fixed" losses. You find this by taking the equipment's rated External Static Pressure (ESP) and subtracting the pressure drops of internal components:
Filter Drop: The resistance of your air filter.
Coil Drop: The resistance of the indoor evaporator coil (often found in manufacturer's data).
Other: Resistance from dampers, grilles, and diffusers.
Step 2: Determine Total Effective Length (TEL)
TEL is not just the length of the pipe. Every elbow, tee, and boot adds "equivalent length" because air creates more friction when changing direction.
TEL = Measured Duct Length + Equivalent Length of all Fittings.
Step 3: Calculate the Friction Rate (FR)
The Friction Rate is the design value used on a duct slide rule (ductulator). The formula is:
FR = (ASP × 100) / TEL
Manual D Design Example
Variable
Typical Value
Blower ESP
0.50 in. w.c.
Component Losses
0.32 in. w.c.
ASP (0.50 – 0.32)
0.18 in. w.c.
TEL (Length + Fittings)
240 ft
Friction Rate Result
0.075 in. w.c. / 100ft
function calculateManualD() {
var esp = parseFloat(document.getElementById('manual_esp').value) || 0;
var filter = parseFloat(document.getElementById('manual_filter').value) || 0;
var coil = parseFloat(document.getElementById('manual_coil').value) || 0;
var other = parseFloat(document.getElementById('manual_other').value) || 0;
var length = parseFloat(document.getElementById('manual_length').value) || 0;
var fittings = parseFloat(document.getElementById('manual_fittings').value) || 0;
var asp = esp – (filter + coil + other);
var tel = length + fittings;
var resultsDiv = document.getElementById('manual_d_results');
var msgDiv = document.getElementById('manual_msg');
resultsDiv.style.display = 'block';
if (tel <= 0) {
msgDiv.innerHTML = "Error: Total Effective Length must be greater than zero.";
msgDiv.style.backgroundColor = "#f8d7da";
msgDiv.style.color = "#721c24";
return;
}
if (asp <= 0) {
msgDiv.innerHTML = "Warning: Available Static Pressure is 0 or negative. Your equipment cannot handle these component drops.";
msgDiv.style.backgroundColor = "#fff3cd";
msgDiv.style.color = "#856404";
document.getElementById('res_asp').innerHTML = asp.toFixed(3);
document.getElementById('res_tel').innerHTML = tel.toFixed(0);
document.getElementById('res_fr').innerHTML = "N/A";
return;
}
var fr = (asp * 100) / tel;
document.getElementById('res_asp').innerHTML = asp.toFixed(3);
document.getElementById('res_tel').innerHTML = tel.toFixed(0);
document.getElementById('res_fr').innerHTML = fr.toFixed(3);
if (fr 0.18) {
msgDiv.innerHTML = "Result: High Friction Rate. Ductwork may be noisy or restrictive.";
msgDiv.style.backgroundColor = "#fff3cd";
msgDiv.style.color = "#856404";
} else {
msgDiv.innerHTML = "Result: Good Friction Rate. Standard residential design range.";
msgDiv.style.backgroundColor = "#d4edda";
msgDiv.style.color = "#155724";
}
}