Sec Form 4 Filing - Harris-Peterson Candace @ CECO ENVIRONMENTAL CORP - 2026-06-01

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Harris-Peterson Candace 2. Issuer Name and Ticker or Trading Symbol CECO ENVIRONMENTAL CORP [ CECO] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Human Resources Officer (Last) (First) (Middle) 5080 SPECTRUM DRIVE, SUITE 800E 3. Date of Earliest Transaction (MM/DD/YY) 06/01/2026 (Street) ADDISON, TX75001 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 06/01/2026 A 3,133 ( 3 ) A 3,133 D Common Stock 06/01/2026 A 2,735 ( 4 ) A 5,868 D Common Stock 06/01/2026 A 6,313 ( 5 ) A 12,181 D Common Stock 06/01/2026 A 1,401 ( 6 ) A 13,582 D Common Stock 06/01/2026 A 4,679 ( 7 ) A 18,261 D Common Stock 06/01/2026 A 625 ( 8 ) A 18,886 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Harris-Peterson Candace 5080 SPECTRUM DRIVE SUITE 800E ADDISON, TX75001 Chief Human Resources Officer Signatures /s/ Kiril Kovachev as Attorney-in-Fact for Candace Harris-Peterson 06/03/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On June 1, 2026 (the "Closing Date"), pursuant to an Agreement and Plan of Merger, dated as of February 23, 2026 (the "Merger Agreement"), by and among CECO Environmental Corp. (the "Issuer"), Thermon Group Holdings, Inc. ("Thermon"), Longhorn Merger Sub, Inc. ("Merger Sub, Inc.") and Longhorn Merger Sub LLC ("Merger Sub LLC"), (i) Merger Sub, Inc. merged with and into Thermon, with Thermon continuing as a wholly-owned subsidiary of the Issuer and the surviving corporation of the merger (the "First Merger") and (ii) Thermon, as the surviving corporation of the First Merger, merged with and into Merger Sub LLC, with Merger Sub LLC being the surviving entity of the merger. ( 2 )(Continued from Footnote 1) Pursuant to the Merger Agreement, at the effective time of the First Merger (the "Effective Time"), each outstanding restricted stock unit of Thermon (each a "Thermon RSU") and outstanding award of performance units (each a "Thermon PU"), was automatically assumed by the Issuer and converted into a restricted stock unit with respect to a number of shares of the Issuer's common stock equal to the product of (x) the number of shares of Thermon's common stock subject to such Thermon RSU or Thermon PU and (y) 0.8110 (each, a "Converted RSU Award"). ( 3 )On May 12, 2026, the Reporting Person was granted an award of Thermon RSUs, which was assumed and converted into a Converted RSU Award that vests in equal installments on each of June 1, 2027, June 1, 2028 and June 1, 2029. ( 4 )On June 1, 2025, the Reporting Person was granted an award of Thermon RSUs, which was assumed and converted into a Converted RSU Award that vested as to one-third on June 1, 2026 and vests as to one-third on each of June 1, 2027 and June 1, 2028. ( 5 )On June 1, 2025, the Reporting Person was granted an award of Thermon PUs, which was assumed and converted into a Converted RSU Award that vests in full on March 31, 2028. ( 6 )On June 1, 2024, the Reporting Person was granted an award of Thermon RSUs, which was assumed and converted into a Converted RSU Award that vested as to one-half on June 1, 2026 and vests as to one-half on June 1, 2027. ( 7 )On June 1, 2024, the Reporting Person was granted an award of Thermon PUs, which was assumed and converted into a Converted RSU Award that vests in full on March 31, 2027. ( 8 )On June 1, 2023, the Reporting Person was granted an award of Thermon RSUs, which was assumed and converted into a fully vested Converted RSU Award.

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