Help! The lowest level of house always colder than upper-level

The problem:

Customer states that their lower floor is always 5-10 degrees colder than the upper level. They have a single system for the entire house and have tried closing vents upstairs to force air to blow out of lower level.

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Possible causes:

Something in duct work can be reducing airflow; uninsulated walls or major air infiltration through damaged doors or windows; disconnected ducts.

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Diagnosis:

When we arrived at the house, we first opened all the vents, airflow should never be restricted or blocked, damage to the system may occur. After measuring the existing duct work and space we noticed that the system was possibly oversized for the structure and that the ductwork was not balanced properly, a 6” duct was run to every visible supply in the house.

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Solution:

In order to deliver proper airflow, we performed a load calculation and a manual “d” for the house. We presented the customer with a proposal to add a thermostat to the lower level and a system of dampers to “zone” the house where both thermostats would control the equipment.