Graduate Studies, UNL

 

Dissertations and Doctoral Documents, University of Nebraska-Lincoln, 2023–

First Advisor

David Yuill

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Mechanical Engineering

Date of this Version

12-2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Mechanical Engineering

Under the supervision of Professor David Yuill

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, Seyed Ali Rooholghodos. Used by permission

Abstract

Faults in residential comfort systems can have detrimental effects on the performance of the system, reducing the energy efficiency, affecting indoor comfort, and shortening equipment lifespan. Many of the faults occur at installation. However, there has not previously been a systematic and nationally representative study to indicate the prevalence of the faults, meaning the distribution of faults by fault type and fault intensity. Understanding true fault prevalence is essential to be able to identify derivers, and to assess the incremental electricity consumption of the new installation systems.

This study presents on-site data from 341 new central cooling systems—air conditioners and heat pumps—in eight U.S. cities across various climate zones. Using non-invasive diagnostics, I classified three installation faults—refrigerant charge deviation, indoor-coil airflow, and liquid-line restriction—by type and intensity, producing fault distributions at state and regional levels. I then analyzed potential drivers using house/system metadata (such as floor area, vintage, efficiency, thermostat behavior, contractor participation), along with climate and regional practice indicators, employing multivariate models to find fault correlates. To estimate energy impacts for 2023, I allocated national AHRI shipments to states through three streams: new construction, replacements, and first-time installs. A baseline of annual electricity use without faults was created using state-level operating intensity, shipment tonnage, and SEER standards. Finally, fault distributions were mapped to COP-based energy multipliers to estimate excess electricity use in 2023 for new installations.

Some key findings on fault intensities, prevalence, and energy impacts include:

  1. Evaporator airflow fault intensity: On average 16% below the 400 cfm/ton nominal; –20% (~360 cfm/ton) is the most common level; heat pumps lower than ACs.
  2. Refrigerant charge fault intensity: Average –6.5% (undercharge); 23% of systems have a serious deviation (≥ 20% under/over).
  3. Liquid-line restriction fault intensity: ~25% minor restriction; ~1% serious (rare).
  4. Socioeconomic status showed no meaningful association with any fault type.
  5. Energy: 2.49 TWh excess electricity use for 2023 new installs above the no-fault baseline (8.94% of 27.86 TWh), driven chiefly by South Atlantic (~856 GWh), West South Central (~504 GWh), and Pacific (~249 GWh).

Advisor: David Yuill

Download

Share

COinS