Question 1

What is a heat load calculation?

Accepted Answer

A heating load is the rate of heat loss from a conditioned space to the outdoors at the winter design conditions. A cooling load is the rate of heat gain at the summer design conditions, divided into sensible and latent components. Heating equipment must supply heat at the heating-load rate to hold the indoor design temperature; cooling equipment must remove heat at the cooling-load rate.

Question 2

What are ACCA and Manual J?

Accepted Answer

Who ACCA is. The Air Conditioning Contractors of America is a nonprofit trade association for the contractors who install and maintain heating, cooling, refrigeration, and building-performance systems. It reports about 60,000 professionals across more than 3,000 member companies. ACCA develops and publishes the technical standards for residential HVAC system design under an ANSI-accredited standards process.

Question 3

How is the Manual J calculation performed?

Accepted Answer

The calculation is performed in Amply, an iPad application listed by ACCA as Powered by Manual J.

Question 4

How does the load become an equipment selection at altitude?

Accepted Answer

The Manual J load is the demand. Equipment is selected against it under ACCA Manual S, Residential Equipment Selection (ANSI/ACCA 3 Manual S-2023), which applies manufacturer performance data at the local design conditions and sets size limits so that installed equipment neither short-cycles nor runs short. Between the load and the selection sits altitude.

Question 5

Why does airflow matter?

Accepted Answer

In a forced-air system the air is the carrier. Air conditioners and heat pumps are rated at a specified airflow, commonly 350 to 450 cubic feet per minute for each 12,000 BTU per hour of cooling capacity. A furnace's heating airflow is set by the temperature-rise range on its rating plate for its rated input.

Question 6

What is the Digital TrueFlow Grid?

Accepted Answer

We use the Digital TrueFlow Grid, an air handler flow meter made by The Energy Conservatory.

Question 7

How is the airflow test performed?

Accepted Answer

The system is run at design conditions before any reading is taken. With the existing filter in place, a static-pressure probe is set through a 3/8-inch port in the supply plenum and the normal operating pressure is recorded. The filter is then removed and the Digital TrueFlow Grid is installed in its place, in an adapter plate sized to the slot, sealed so no air bypasses it and facing upstream into the airflow.

Question 8

What does the airflow measurement decide?

Accepted Answer

The measured airflow is compared with the airflow the selected equipment requires from the manufacturer's data. If the ducts deliver it at acceptable static pressure, the system is acceptable as it stands. If not, the outcomes are to modify the ducts, replace them, or select equipment the ducts can support.

Question 9

What does a blower-door test measure?

Accepted Answer

A blower door is a calibrated fan mounted in an exterior doorway. It depressurizes or pressurizes the building to a standard test pressure, usually 50 pascals, and the airflow required to hold that pressure is the measurement. The result is reported as CFM50, or as ACH50 once divided by the building's volume: air changes per hour at 50 pascals.

Question 10

Can the ductwork be modified to carry more air?

Accepted Answer

When measured airflow falls short of what the selected equipment requires, the duct system can be modified. Field guides sort an existing system into three outcomes: acceptable as is, modify, or replace. The least invasive gains are usually on the return side: a larger return drop, an added return, a lower-resistance filter, sealed leaks, and replacement of crushed or kinked flex duct. Supply-side gains mean larger trunks or branches.

Design & Analytics

TrueGrid Airflow Analytics

Manual J Heat Load Calculation