Document:

<PAGE>
                                                                   Exhibit 10.27

                 WAIVER AND AMENDMENT NO. 3 TO CREDIT AGREEMENT

            This Waiver and Amendment No. 3 to Credit Agreement, dated as of
March __, 2003 (this "Amendment"), is entered into by and between Comfort
Systems USA, Inc., a Delaware corporation ("Borrower"), the other Credit
Parties, General Electric Capital Corporation ("GE Capital"), as Agent (in such
capacity, "Agent") for the Lenders (as defined in the Credit Agreement referred
to below) and as a Lender, and the other Lenders party to the Credit Agreement.

                                    RECITALS

            WHEREAS, Borrower, the other Credit Parties, Agent and Lenders are
parties to that certain Credit Agreement, dated as of October 11, 2002 (as
amended by Amendment No. 1 to Credit Agreement, dated as of December 10, 2002,
and Amendment No. 2 to Credit Agreement, dated as of December 20, 2002, and as
the same may be further amended, restated, supplemented or otherwise amended
from time to time hereafter, the "Credit Agreement");

            WHEREAS, Borrower has notified Agent and Lenders that an Event of
Default is currently in existence under the Credit Agreement based upon
Borrower's breach of the "Minimum EBITDA" financial covenant set forth in
Section 4.2 of the Credit Agreement for the period ended December 31, 2002 (the
"Existing Event of Default");

            WHEREAS, Borrower has requested that Agent and Lenders waive the
Existing Event of Default and reset certain financial covenants set forth in
Section 4 of the Credit Agreement, and Agent and Lenders are willing to do so
pursuant to the terms and conditions set forth in this Amendment;

            WHEREAS, Borrower, Credit Parties, Agent and Lenders are each
desirous of entering into an amendment to certain other provisions the Credit
Agreement, as and to the limited extent set forth in this Amendment and subject
to the terms and conditions set forth herein; and

            WHEREAS, this document shall constitute a Loan Document and these
Recitals shall be construed as part of this Amendment;

            NOW THEREFORE, in consideration of the premises and the mutual
covenants herein contained, and for good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged by the parties, Borrower,
Credit Parties, Agent and Lenders each hereby agree as follows:

            1. Definitions. Except to the extent otherwise specified herein,
capitalized terms used in this Amendment shall have the same meanings ascribed
to them in the Credit Agreement and Annex A thereto.

            2. Waiver.
<PAGE>

            2.1. Existing Event of Default. Agent and Lenders hereby waive the
Existing Event of Default; provided, that, Borrower shall be in compliance with
all of the terms and conditions of the Credit Agreement as hereby amended. The
foregoing waiver is only applicable and shall only be effective in the specific
instance and for the specific purpose for which made. Such waiver is expressly
limited to the facts and circumstances referred to herein and shall not operate
(a) as a waiver of or consent to non-compliance with any other provision of the
Credit Agreement or any other Loan Document, (b) as a waiver of any other right,
power or remedy of Agent or Lenders under the Credit Agreement or any other Loan
Document or (c) as a waiver of or consent to any other Default or Event of
Default under the Credit Agreement or any other Loan Document.

            3. Amendments. The Credit Agreement is amended as follows:

            3.1. Annex A to the Credit Agreement is amended by deleting the
existing definitions of "Commitments" and "Revolving Loan Commitment" in their
entirety and replacing them with the following:

                        "Commitments" means (a) as to any Lender, the aggregate
            of such Lender's Revolving Loan Commitment and Term Loan Commitments
            set forth on Annex B to the Agreement or in the most recent
            Assignment Agreement executed by such Lender and (b) as to all
            Lenders, the aggregate of all Lenders' Revolving Loan Commitments
            and Term Loan Commitments, which aggregate commitment shall be
            Fifty-Five Million Dollars ($55,000,000) (comprised of aggregate
            Revolving Loan Commitments of $40,000,000 and Term Loan Commitments
            of $15,000,000), as such Commitments may be reduced, amortized or
            adjusted from time to time in accordance with the Agreement.

                        "Revolving Loan Commitment" means (a) as to any Lender,
            the commitment of such Lender to make its Pro Rata Share of
            Revolving Credit Advances or incur its Pro Rata Share of Letter of
            Credit Obligations (including, in the case of the Swing Line Lender,
            its commitment to make Swing Line Advances as a portion of its
            Revolving Loan Commitment) as set forth on Annex B or in the most
            recent Assignment Agreement, if any, executed by such Lender and (b)
            as to all Lenders, the aggregate commitment of all Lenders to make
            the Revolving Credit Advances (including, in the case of the Swing
            Line Lender, Swing Line Advances) or incur Letter of Credit
            Obligations, which aggregate commitment shall be Forty Million
            Dollars ($40,000,000), as such amount may be adjusted, if at all,
            from time to time in accordance with the Agreement."

            3.2. Annex A to the Credit Agreement is amended by inserting the
following definitions in alphabetical order therein:

                        "Emcor Sale" means that sale of certain assets of the
            Borrower to Emcor prior to the Closing Date during Fiscal Year 2002.

            3.3. Section 1.1(b) (Revolving Loans) is amended by inserting the
following proviso at the end of the second sentence thereof:

                                       2
<PAGE>

                        "and provided, further, that, notwithstanding anything
            contained in this Agreement or any other Loan Document to the
            contrary, a portion of the Revolving Loan Commitment in the
            principal amount of (i) at all times from the Closing Date through
            July 31, 2003, $15,000,000 and (ii) at all times from August 1, 2003
            through the Commitment Termination Date, $20,000,000 shall be
            available only to be utilized, subject to the satisfaction of the
            conditions set forth in this Agreement, upon the request of Borrower
            for the issuance of Letters of Credit pursuant to Section 1.1(d)
            hereof."

            3.4. Section 3.6 (Restrictions on Fundamental Changes) to the Credit
Agreement is hereby amended by deleting the text immediately following the first
sentence thereof in its entirety, beginning with the words "Notwithstanding the
foregoing" through and including clause (xii) thereof (such that such Section
3.6 shall consist only of the first sentence thereof).

            3.5. Section 4.2 (Minimum EBITDA) to the Credit Agreement is hereby
amended and restated in its entirety as follows:

                        "4.2 Minimum EBITDA. Borrower and its Subsidiaries on a
            consolidated basis shall have, at the end of each month set forth
            below, EBITDA for the 12-month period then ended of not less than
            the following:

<Table>
<Caption>
Period                                             EBITDA
------                                             ------

<S>                                                <C>
December 31, 2002                                  $ 23,639,000
January 31, 2003                                   $ 21,983,000
February 28, 2003                                  $ 22,745,000
March 31, 2003                                     $ 19,190,000
April 30, 2003                                     $ 18,010,000
May 31, 2003                                       $ 16,985,000
June 30, 2003                                      $ 16,420,000
July 31, 2003                                      $ 16,420,000
August 30, 2003                                    $ 16,420,000
September 30, 2003                                 $ 17,840,000
October 31, 2003                                   $ 17,840,000
November 30, 2003                                  $ 17,840,000
December 31, 2003                                  $ 23,565,000
January 31, 2004                                   $ 25,640,000
February 29, 2004                                  $ 26,820,000
March 31, 2004                                     $ 29,000,000
April 30, 2004                                     $ 29,000,000
</Table>

                                       3
<PAGE>
<Table>
<S>                                                <C>
May 31, 2004                                       $ 29,000,000
June 30, 2004                                      $ 29,000,000
July 31, 2004                                      $ 29,000,000
August 30, 2004                                    $ 29,000,000
September 30, 2004                                 $ 29,000,000
October 31, 2004                                   $ 29,000,000
November 30, 2004                                  $ 29,000,000
December 31, 2004                                  $ 29,000,000
January 31, 2005                                   $ 29,000,000
February 28, 2005                                  $ 29,000,000
March 31, 2005 and each month thereafter           $ 31,000,000"
</Table>

            3.6. Section 4.3 (Minimum Fixed Charge Coverage Ratio) to the Credit
Agreement is hereby amended and restated in its entirety as follows:

                        "4.3 Minimum Fixed Charge Coverage Ratio. Borrower and
            its Subsidiaries shall have on a consolidated basis at the end of
            each month set forth below, a Fixed Charge Coverage Ratio for the
            12-month period then ended of not less than the following:

                        2.3 to 1.0 for the month ending December 31, 2002;
                        2.3 to 1.0 for the month ending January 31, 2003;
                        2.3 to 1.0 for the month ending February 28, 2003;
                        2.5 to 1.0 for the month ending March 31, 2003;
                        2.5 to 1.0 for the month ending April 30, 2003;
                        2.5 to 1.0 for the month ending May 31, 2003;
                        2.5 to 1.0 for the month ending June 30, 2003;
                        2.5 to 1.0 for the month ending July 31, 2003;
                        2.5 to 1.0 for the month ending August 31, 2003;
                        2.6 to 1.0 for the month ending September 30, 2003;
                        2.6 to 1.0 for the month ending October 31, 2003;
                        2.6 to 1.0 for the month ending November 30, 2003;
                        2.7 to 1.0 for the month ending December 31, 2003; and
                        3.0 to 1.0 for each month ending thereafter."

            3.7. Section 4.4 (Minimum Interest Coverage Ratio) to the Credit
Agreement is herby amended and restated in its entirety as follows:

                        "4.4 Minimum Interest Coverage Ratio. Borrower and its
            Subsidiaries on a consolidated basis shall have at the end of each
            and every month, an Interest Coverage Ratio for the 12-month period
            then ended of not less than 3.00 to 1.0."

                                       4
<PAGE>

            3.8. Section 4.5 (Maximum Leverage Ratio) to the Credit Agreement is
hereby amended and restated in its entirety as follows:

                        "4.5 Maximum Leverage Ratio. Borrower and its
            Subsidiaries on a consolidated basis shall have, at the end of each
            month set forth below, a Leverage Ratio as of the last day of such
            month and for the 12-month period then ended of not more than the
            following:

                        1.75 to 1.0 for the month ending December 31, 2002;
                        1.75 to 1.0 for the month ending January 31, 2003;
                        1.75 to 1.0 for the month ending February 28, 2003;
                        1.75 to 1.0 for the month ending March 31, 2003;
                        2.20 to 1.0 for the month ending April 30, 2003;
                        2.20 to 1.0 for the month ending May 31, 2003;
                        2.20 to 1.0 for the month ending June 30, 2003;
                        2.20 to 1.0 for the month ending July 31, 2003;
                        2.20 to 1.0 for the month ending August 31, 2003;
                        2.10 to 1.0 for the month ending September 30, 2003;
                        2.10 to 1.0 for the month ending October 31, 2003;
                        2.10 to 1.0 for the month ending November 30, 2003;
                        1.50 to 1.0 for the month ending December 31, 2003;
                        1.50 to 1.0 for the month ending January 31, 2004;
                        1.50 to 1.0 for the month ending February 28, 2004;
                        1.50 to 1.0 for the month ending March 31, 2004;
                        1.50 to 1.0 for the month ending April 30, 2004;
                        1.50 to 1.0 for the month ending May 31, 2004;
                        1.50 to 1.0 for the month ending June 30, 2004;
                        1.50 to 1.0 for the month ending July 31, 2004;
                        1.50 to 1.0 for the month ending August 31, 2004;
                        1.25 to 1.0 for the month ending September 30, 2004; and
                        1.25 to 1.0 for each month ending thereafter."

            3.9. Section 4.6(1) (Compliance and Excess Cash Flow Certificate) to
the Credit Agreement is hereby amended by deleting the parenthetical "(but only
to the extent delivered at the end of a month that is also the end of a Fiscal
Quarter or Fiscal Year)" in its entirety.

            3.10. Schedule 2 to Exhibit 4.6(l) (Excess Cash Flow) to the Credit
Agreement is hereby amended by: (a) adding the following to "decreases in
Working Capital during the Fiscal Year*" (an item under the heading "Plus:"):

            ", excluding decreases in Working Capital related solely to the
            treatment of the Emcor Sale and discontinued operations";

            and

            (b) adding the following parenthetical following the words "Funded
Debt" in the third item under the heading "Less:"

                                       5
<PAGE>

            "(excluding payments paid in 2002 prior to the Closing Date in
            respect of Funded Debt in connection with the Emcor Sale)"

            3.11. Annex B to the Credit Agreement (Pro Rata Shares and
Commitment Amounts) is hereby amended by deleting existing Annex B in its
entirety and replacing it with new Annex B which is attached hereto as Exhibit
A.

            4. Representations and Warranties of Borrower and Credit Parties.
Borrower and each Credit Party hereby represents and warrants that:

            4.1 The execution, delivery and performance by it of this Amendment
has been duly authorized by all necessary corporate action, including, without
limitation, all necessary action by its Board of Directors and stockholders, and
that this Amendment is a legal, valid and binding obligation of it, enforceable
against it in accordance with its terms, except as the enforcement hereof may be
subject to the effect of any applicable bankruptcy, insolvency, reorganization,
moratorium or similar law affecting creditors' rights generally or to general
principles of equity.

            4.2 The execution, delivery and performance of this Amendment by
Borrower does not, and will not, contravene or conflict with any provision of
(i) law, (ii) any judgment, decree or order, or (iii) the certificate or
articles of incorporation or by-laws of such Credit Party, and does not, and
will not, contravene or conflict with, or cause any Lien to arise under, any
provision of any agreement, mortgage, lease, instrument or other document
binding upon or otherwise affecting Borrower, any other Credit Party or any of
their respective Subsidiaries or any property of Borrower, any other Credit
Party or any of their respective Subsidiaries.

            4.3 All of the representations and warranties contained in the
Credit Agreement and each other Loan Document are true and correct in all
material respects on and as of the date hereof as if made on the date hereof. No
Default or Event of Default exists under the Credit Agreement or any other Loan
Document or will exist after or be triggered by the execution and delivery of
this Amendment or the consummation of the transactions contemplated hereby. In
addition, each Credit Party represents and warrants that the Credit Agreement
and each of the other Loan Documents remains in full force and effect and each
is hereby ratified and confirmed in all respects.

            5. Conditions Precedent to Effectiveness. The effectiveness of each
of the waiver set forth in Section 2 and the amendments set forth in Section 3
hereof is subject to the satisfaction of each of the following conditions
precedent in a manner acceptable to Agent:

            5.1. Documentation. Borrower shall have delivered to Agent all of
the following, each duly executed and dated the date hereof, in form and
substance satisfactory to the Agent:

                                       6
<PAGE>

            (a)  Amendment. Counterparts of this Amendment, duly executed by the
                 Borrower, each of the other Credit Parties, Agent and all of
                 the Lenders;

            (b)  New Notes. (i) A replacement Revolving Note of Borrower in
                 favor of GE Capital (the "GE Capital Replacement Revolving
                 Note"), substantially in the form set forth as Exhibit B
                 hereto, delivered to the Agent for the Account of GE Capital,
                 as a Lender, in an amount equal to its Revolving Loan
                 Commitment of $29,333,333, (ii) a replacement Revolving Note of
                 Borrower in favor of Bank of Texas (the "Bank of Texas
                 Replacement Revolving Note"), substantially in the form set
                 forth as Exhibit B hereto, delivered to the Agent for the
                 account of Bank of Texas, as a Lender, in an amount equal to
                 its Revolving Loan Commitment of $5,333,333, (iii) a
                 replacement Revolving Note of Borrower in favor of Regions Bank
                 (the "Regions Bank Replacement Revolving Note" and together
                 with the GE Capital Replacement Revolving Note and the Bank of
                 Texas Replacement Revolving Note, the "New Notes"),
                 substantially in the form set forth as Exhibit B hereto,
                 delivered to the Agent for the account of Regions Bank, as a
                 Lender, in an amount equal to its Revolving Loan Commitment of
                 $5,333,333. Upon receipt of the GE Capital Replacement
                 Revolving Note, GE Capital shall promptly thereafter return to
                 Borrower for cancellation the original Revolving Note payable
                 to GE Capital in the maximum principal amount of $44,000,000.
                 Upon receipt of the Bank of Texas Replacement Revolving Note,
                 the Bank of Texas shall promptly thereafter return to Borrower
                 for cancellation the original Revolving Note payable to Bank of
                 Texas in the principal amount of $8,000,000. Upon receipt of
                 the Regions Bank Replacement Revolving Note, Regions Bank shall
                 promptly thereafter return to Borrower for cancellation the
                 original Revolving Note payable to Regions Bank in the
                 principal amount of $8,000,000; and

            (c)  Board Resolutions. Agent shall have received resolutions of the
                 board of directors of Borrower and each Credit Party
                 authorizing and approving the execution, delivery and
                 performance of this Amendment and the transactions contemplated
                 hereby and each other agreement, document or instrument
                 executed in connection herewith, certified as of the date of
                 this Amendment by the secretary or assistant secretary of such
                 Person as being in full force and effect without modification.

            5.2. No Default or Event of Default. No Default or Event of Default
shall have occurred and be continuing or would result from the effectiveness of
this Amendment or the consummation of any of the transactions contemplated
thereby.

            5.3. Amendment Fee. Borrower shall pay to Agent, for the account of
the Lenders, an amendment fee in an amount equal to $562,500, to be distributed
pro rata to all Lenders based on each Lender's Commitments immediately prior to
the effectiveness of this Amendment.

                                       7
<PAGE>

            6. Reference to and Effect Upon the Credit Agreement and other Loan
Agreements.

            6.1. Except as specifically amended in Section 3 above, the Credit
Agreement and each other Loan Document shall remain in full force and effect and
is hereby ratified and confirmed.

            6.2. The execution, delivery and effect of this Amendment shall be
limited precisely as written and shall not be deemed to (i) be a consent to any
waiver of any term or condition, or to any amendment or modification of any term
or condition (except as specifically waived in Section 2 above or amended in
Section 3 above), of the Credit Agreement or any other Loan Document or (ii)
prejudice any right, power or remedy which the Agent or any Lender now has or
may have in the future under or in connection with the Credit Agreement or any
other Loan Document. Each reference in the Credit Agreement to "this Agreement",
"hereunder", "hereof", "herein" or any other word or words of similar import
shall mean and be a reference to the Credit Agreement as amended hereby, and
each reference in any other Loan Document to the Credit Agreement or any word or
words of similar import shall be and mean a reference to the Credit Agreement as
amended hereby.

            7. Acknowledgment and Consent of Credit Parties. Each Credit Party
hereby consents to this Amendment and hereby confirms and agrees that (a) the
Guaranty and each other Loan Document to which it is a party is, and shall
continue to be, in full force and effect and is hereby ratified and confirmed in
all respects, and (b) the Collateral Documents to which such Credit Party is a
party and all of the Collateral described therein do, and shall continue to,
secure the payment of all of the Obligations.

            8. Counterparts. This Amendment may be executed in any number of
counterparts, each of which when so executed shall be deemed an original but all
such counterparts shall constitute one and the same instrument. Delivery of an
executed counterpart of a signature page to this Amendment by telecopier shall
be as effective as delivery of a manually executed counterpart signature page to
this Amendment.

            9. Costs and Expenses. As provided in Section 1.3 of the Credit
Agreement, Borrower shall pay the fees, costs and expenses incurred by Agent in
connection with the preparation, execution and delivery of this Amendment
(including, without limitation, reasonable attorneys' fees).

            10. GOVERNING LAW. THIS AMENDMENT SHALL BE GOVERNED BY AND CONSTRUED
AND ENFORCED IN ACCORDANCE WITH THE INTERNAL LAWS (AS OPPOSED TO CONFLICTS OF
LAW PROVISIONS) OF THE STATE OF NEW YORK.

            11. Headings. Section headings in this Amendment are included herein
for convenience of reference only and shall not constitute a part of this
Amendment for any other purpose.

                            [Signature Pages Follow]

                                       8
<PAGE>

            IN WITNESS WHEREOF, this Amendment has been duly executed as of the
date first written above.

                                       BORROWER:

                                       COMFORT SYSTEMS USA, INC.

                                       By:
                                           ------------------------------------
                                       Name:
                                             ----------------------------------
                                       Title:
                                              ---------------------------------

                                       9
<PAGE>

                   CREDIT PARTIES:

                         ACI MECHANICAL, INC.

                         ARC COMFORT SYSTEMS USA, INC.

                         ACCURATE AIR SYSTEMS, L.P., by
                           Atlas-Accurate Holdings, L.L.C.,
                           as General Partner

                         ACCU-TEMP GP, INC.

                         ACCU-TEMP LP, INC.

                         ACCU-TEMP LLC,
                           by Accu-Temp GP, Inc., as acting member

                         AIR SOLUTIONS USA, INC.

                         AIR TEMP, INC.

                         ATLAS-ACCURATE HOLDINGS, L.L.C.,
                           by CS53 Acquisition Corp.,
                           as acting member

                         ATLAS AIR CONDITIONING
                         COMPANY, L.P., by Atlas-Accurate
                           Holdings, L.L.C., as general partner

                         BATCHELOR'S MECHANICAL CONTRACTORS, INC.

                         BCM CONTROLS CORPORATION

                         CARSON BROTHERS, INC.

                         CEL, INC.

                         CENTRAL MECHANICAL, INC.

                         COMFORT SYSTEMS USA (ARKANSAS), INC.

                         COMFORT SYSTEMS USA (BALTIMORE), INC.

                                       10
<PAGE>

                         COMFORT SYSTEMS USA (BOWLING GREEN), INC.

                         COMFORT SYSTEMS USA (BRISTOL), INC.

                         COMFORT SYSTEMS USA (CLEVELAND), INC.

                         COMFORT SYSTEMS USA (FLORIDA), INC.

                         COMFORT SYSTEMS USA G.P., INC.

                         COMFORT SYSTEMS US (HARTFORD), INC.

                         COMFORT SYSTEMS USA (INTERMOUNTAIN), INC.

                         COMFORT SYSTEMS USA NATIONAL SERVICE ORGANIZATION, INC.

                         COMFORT SYSTEMS USA (OREGON), INC.

                         COMFORT SYSTEMS USA (SOUTH BOSTON), INC.

                         COMFORT SYSTEMS USA (SYRACUSE), INC.

                         COMFORT SYSTEMS USA (TEXAS), L.P.,
                           by Comfort Systems USA G.P., Inc.,
                           as general partner

                         COMFORT SYSTEMS USA (TWIN CITIES), INC.

                         COMFORT SYSTEMS USA (WESTERN MICHIGAN), INC.

                         CS44 ACQUISITION CORP.

                         CS53 ACQUISITION CORP.

                         DESIGN MECHANICAL INCORPORATED

                                       11
<PAGE>

                         EASTERN HEATING & COOLING, INC.

                         ESS ENGINEERING, INC.

                         GULFSIDE MECHANICAL, INC.

                         H & M MECHANICAL, INC.

                         HELM CORPORATION

                         HELM CORPORATION SAN DIEGO

                         HESS MECHANICAL CORPORATION

                         INDUSTRIAL COOLING INC.

                         J & J MECHANICAL, INC.

                         JAMES AIR CONDITIONING ENTERPRISE INC.

                         MARTIN HEATING, INC.

                         MECHANICAL SERVICE GROUP, INC.

                         MECHANICAL TECHNICAL
                         SERVICES, L.P., by Atlas-Accurate
                           Holdings, L.L.C., as general partner

                         MJ MECHANICAL SERVICES, INC.

                         NEEL MECHANICAL CONTRACTORS, INC.

                         NORTH AMERICAN MECHANICAL, INC.

                         OK SHEET METAL AND AIR CONDITIONING, INC.

                         QUALITY AIR HEATING & COOLING, INC.

                         S&K AIR CONDITIONING CO., INC.

                         S. I. GOLDMAN COMPANY, INC.

                                       12
<PAGE>

                         S.M. LAWRENCE COMPANY, INC.

                         SA ASSOCIATES, INC.

                         SALMON & ALDER, LLC,
                           by SA Associates, Inc., as acting member

                         SEASONAIR, INC.

                         SHEREN PLUMBING & HEATING, INC.

                         STANDARD HEATING & AIR CONDITIONING COMPANY

                         SUPERIOR MECHANICAL SYSTEMS, INC.

                         TARGET CONSTRUCTION, INC.

                         TEMP-RIGHT SERVICE, INC.

                         THE CAPITAL REFRIGERATION COMPANY

                         TRI-CITY MECHANICAL, INC.

                         UNITED ENVIRONMENTAL
                         SERVICES, L.P., by Atlas-Accurate Holdings,
                         L.L.C., as general partner

                         WEATHER ENGINEERING, INC.

                         WESTERN BUILDING SERVICES, INC.

                         By:
                                -----------------------------------------------
                         Name:
                                -----------------------------------------------
                         Title:
                                -----------------------------------------------

                                       13
<PAGE>

                                       GENERAL ELECTRIC CAPITAL CORPORATION,
                                       as Agent and a Lender

                                       By:
                                            --------------------------------
                                               Its Duly Authorized Signatory

                                       14
<PAGE>

                                       BANK OF TEXAS, NA, as a Lender

                                       By:
                                              ------------------------------
                                       Name:
                                              ------------------------------
                                       Title:
                                              ------------------------------

                                       15
<PAGE>

                                       REGIONS BANK, as a Lender

                                       By:
                                              ------------------------------
                                       Name:
                                              ------------------------------
                                       Title:
                                              ------------------------------

                                       16
<PAGE>

                                                                       EXHIBIT A
                                                              TO AMENDMENT NO. 3

                 ANNEX B (FROM ANNEX A - COMMITMENTS DEFINITION)
                                       to
                                CREDIT AGREEMENT

                     PRO RATA SHARES AND COMMITMENT AMOUNTS

Commitment                                      Lender
----------                                      ------
Revolving Loan Commitment
(including a Swing Line Commitment              GENERAL ELECTRIC CAPITAL
of $5,000,000)                                  CORPORATION
$29,333,333

Revolving Loan Commitment
$5,333,333                                      BANK OF TEXAS, NA

Revolving Loan Commitment
$5,333,333                                      REGIONS BANK

Term Loan Commitment:                           GENERAL ELECTRIC CAPITAL
$11,000,000                                     CORPORATION

Term Loan Commitment:                           BANK OF TEXAS, NA
$2,000,000

Term Loan Commitment:                           REGIONS BANK
$2,000,000

                                       17
<PAGE>

                                                                       EXHIBIT B
                                                              TO AMENDMENT NO. 3

                                 REVOLVING NOTE

                                                              New York, New York
$   ,000,000                                                     March   , 2003
 ---                                                                   --

            FOR VALUE RECEIVED, the undersigned, COMFORT SYSTEMS USA, INC.
("Borrower"), HEREBY PROMISES TO PAY to the order of ______________ ("Lender"),
at the offices of GENERAL ELECTRIC CAPITAL CORPORATION, a Delaware corporation,
as Agent for Lenders ("Agent"), at its address at 335 Madison Avenue, New York,
New York 10017 or at such other place as Agent may designate from time to time
in writing, in lawful money of the United States of America and in immediately
available funds, the amount of ____________ Million Dollars ($___,000,000) or,
if less, the aggregate unpaid amount of all Revolving Credit Advances made to
the undersigned under the "Credit Agreement" (as hereinafter defined). All
capitalized terms used but not otherwise defined herein have the meanings given
to them in the Credit Agreement or in Annex A thereto.

            This Revolving Note is one of the Revolving Notes issued pursuant to
that certain Credit Agreement dated as of October 11, 2002 by and among the
Borrower, the other Persons named therein as Credit Parties, Agent, Lender and
the other Persons signatory thereto from time to time as Lenders (including all
annexes, exhibits and schedules thereto, and as amended by Amendment No. 1 to
Credit Agreement, dated as of December 10, 2002, Amendment No. 2 to Credit
Agreement, dated as of December 20, 2002, and Waiver and Amendment No. 3 to
Credit Agreement dated as of the date hereof, and as the same may be further
amended, restated, supplemented or otherwise amended from time to time, the
"Credit Agreement"), and is entitled to the benefit and security of the Credit
Agreement, the Security Agreement and all of the other Loan Documents referred
to therein. Reference is hereby made to the Credit Agreement for a statement of
all of the terms and conditions under which the Loans evidenced hereby are made
and are to be repaid. The date and amount of each Revolving Credit Advance made
by Lenders to Borrower, the rates of interest applicable thereto and each
payment made on account of the principal thereof, shall be recorded by Agent on
its books; provided that the failure of Agent to make any such recordation shall
not affect the obligations of Borrower to make a payment when due of any amount
owing under the Credit Agreement or this Note in respect of the Revolving Credit
Advances made by Lender to Borrower.

            The principal amount of the indebtedness evidenced hereby shall be
payable in the amounts and on the dates specified in the Credit Agreement, the
terms of which are hereby incorporated herein by reference. Interest thereon
shall be paid until such principal amount is paid in full at such interest rates
and at such times, and pursuant to such calculations, as are specified in the
Credit Agreement.

            If any payment on this Revolving Note becomes due and payable on a
day other than a Business Day, the maturity thereof shall be extended to the
next succeeding Business Day and, with respect to payments of principal,
interest thereon shall be payable at the then applicable rate during such
extension.

                                       18
<PAGE>

            Upon and after the occurrence of any Event of Default, this
Revolving Note may, as provided in the Credit Agreement, be declared, and
immediately shall become, due and payable.

            Time is of the essence of this Revolving Note. Demand, presentment,
protest and notice of nonpayment and protest are hereby waived by Borrower.

            Except as provided in the Credit Agreement, this Revolving Note may
not be assigned by Lender to any Person.

                            [signature page follows]

                                       19
<PAGE>

            THIS REVOLVING NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE
WITH THE LAWS OF THE STATE OF NEW YORK APPLICABLE TO CONTRACTS MADE AND
PERFORMED IN THAT STATE.

                                          COMFORT SYSTEMS USA, INC.

                                          By:
                                                -------------------------------
                                          Name:
                                                -------------------------------
                                          Title:
                                                -------------------------------<PAGE>
                                                                   EXHIBIT 10.16

                 AGREEMENT, THIRD AMENDMENT TO CREDIT AGREEMENT
                      AND AMENDMENT TO OTHER LOAN DOCUMENTS

         This Agreement, Third Amendment to Credit Agreement and Amendment to
Other Loan Documents (this "Amendment") dated as of March 28, 2003 among the
financial institutions (collectively, the "Lenders") party to the Credit
Agreement (as such term is hereinafter defined); AMERICAN PLUMBING & MECHANICAL,
INC. (the "Borrower"), CREDIT LYONNAIS NEW YORK BRANCH, as Documentation Agent
for the Lenders (in such capacity, the "Documentation Agent"), and BANK ONE, NA
(successor by merger to The First National Bank of Chicago), as agent (in such
capacity, the "Agent") for the Lenders. The Subsidiaries of the Borrower are
joining in this Amendment to evidence their consent to the terms and provisions
hereof, to evidence their ratification of the Loan Documents to which they are
parties and for the other purposes set forth herein;

                                   WITNESSETH:

         WHEREAS, the Borrower, the Lenders, the Agent and the Documentation
Agent executed and delivered that certain Credit Agreement (as heretofore
amended and supplemented, the "Credit Agreement") dated as of March 31, 2001;
and

         WHEREAS, the Borrower, the Lenders, the Documentation Agent and the
Agent now desire to amend the Credit Agreement further to (a) provide for
further reductions of the Commitment of each Lender; (b) require evidence of
compliance with Section 6.26 of the Credit Agreement before any payment of
principal or interest on the Permitted Senior Subordinated Debt; (c) change the
interest rate to accrue on the Loans; (d) change certain financial covenants;
and (e) otherwise amend the Credit Agreement;

         NOW, THEREFORE, in consideration of the premises and the mutual
agreements, representations and warranties herein set forth, and for other good
and valuable consideration, the receipt and sufficiency of which are
acknowledged, the parties hereto hereby agree as follows:

         Section 1. Section 1.1 of the Credit Agreement is hereby amended by (i)
deleting the existing definition of each of the following terms to the extent
such that terms are currently defined therein and by substituting, in lieu
thereof, the following definition(s) for such terms, and (ii) adding the
following defined terms and the respective definition set forth below for each
such term that is not currently included in such Section 1.1:

         "13-Week Cash Flow Forecast" means a rolling cash flow forecast (in the
form attached hereto as Appendix C, with such modifications to such form and
scope as the Agent may from time to time reasonably request), prepared by the
Borrower with respect to the discontinued operations and the continuing
operations of the Borrower and its Subsidiaries and covering the then-current
calendar week and the thirteen-week period immediately following the date of
delivery of such report by the Borrower.

                                   APPENDIX C

<PAGE>

         "Commitment" means, for each Lender, the obligation of such Lender to
make Loans to, and participate in Facility LCs issued upon the application of,
the Borrower in an aggregate amount not exceeding the amount set forth opposite
its name on Appendix A hereto (and such Commitment shall reduce as shown on
Appendix A as of the date shown for each such reduction) or as set forth in any
Assignment Agreement relating to any assignment that has become effective
pursuant to Section 12.03.02 (with such Commitment to reduce proportionately on
each date when the assignor's Commitment was to be reduced as shown on Appendix
A), as such amount may be reduced from time to time by such Lender's Pro Rata
Share of each principal payment made on the Loans pursuant to clause (i) of
Section 2.02, as such amount may be further modified from time to time pursuant
to the terms hereof.

          "Consolidated EBITDA" means, with reference to any period, on a
trailing twelve-month basis (using the historical financial results of any other
business acquired in an Acquisition, to the extent applicable, without
duplication, on a pro forma basis, consistent with SEC regulations), the sum of
Consolidated Net Income plus, to the extent deducted in determining Consolidated
Net Income, (i) Consolidated Interest Expense, (ii) provisions for taxes based
on income or revenues, (iii) depreciation, (iv) amortization, (v) other non-cash
expenses, (vi) losses from discontinued operations described on Appendix B to
the Second Amendment to the Credit Agreement (provided, however, that the
aggregate amount of such losses added pursuant to this clause (vi) shall not
exceed $27,000,000), determined on a pre-tax basis, and (vii) on a pre-tax
basis, any extraordinary or non-recurring losses (other than losses from
discontinued operations) minus, to the extent included in determining
Consolidated Net Income, any extraordinary or non-recurring gains, all
calculated on a consolidated basis for the Borrower and its Subsidiaries and as
determined in accordance with GAAP.

         "Cumulative Annual EBITDA" means, with reference to any applicable
period, on a cumulative monthly basis for the calendar months then-elapsed in
the applicable fiscal year of the Borrower and its Subsidiaries (using the
historical financial results of any other business acquired in an Acquisition,
to the extent applicable, without duplication, on a pro forma basis, consistent
with SEC regulations), the sum of Consolidated Net Income for such applicable
period plus, to the extent deducted in determining such Consolidated Net Income,
(i) Consolidated Interest Expense for such applicable period, (ii) provisions
for taxes based on income or revenues for such applicable period, (iii)
depreciation for such applicable period, (iv) amortization for such applicable
period, (v) other non-cash expenses for such applicable period, (vi) losses,
during calendar year 2003 only, from discontinued operations described on
Appendix B to the Second Amendment to the Credit Agreement (provided, however,
that the aggregate amount of such losses added pursuant to this clause (vi)
shall not exceed $4,000,000), determined on a pre-tax basis, and (vii) on a
pre-tax basis, any extraordinary or non-recurring losses (other than losses from
discontinued operations) for such applicable period minus, to the extent
included in determining such Consolidated Net Income, any extraordinary or
non-recurring gains for such applicable period, all calculated on a consolidated
basis for the Borrower and its Subsidiaries and as determined in accordance with
GAAP.

                                   APPENDIX C

<PAGE>

         "Payment Date" means the last day of each calendar month.

         Section 2. Section 2.16 of the Credit Agreement is hereby amended to
read in its entirety as follows:

         2.16 Interest Payment Dates; Interest and Fee Basis. Interest accrued
on each Floating Rate Advance shall be payable on each Payment Date, commencing
with the first such date to occur after the date hereof and at maturity.
Interest accrued on each Eurodollar Advance shall be payable on each Payment
Date, on the last day of its applicable Interest Period, on any date on which
the Eurodollar Advance is prepaid, whether by acceleration or otherwise, and at
maturity. Interest, commitment fees and LC Fees shall be calculated for actual
days elapsed on the basis of a 360-day year; provided that interest on Floating
Rate Advances shall be calculated for actual days elapsed on the basis of a 365
days year. Interest shall be payable for the day an Advance is made but not for
the day of any payment on the amount paid if payment is received prior to 12:00
p.m. noon (Chicago time) at the place of payment. If any payment of principal of
or interest on an Advance or Swing Loan, or any fee or other amount payable
hereunder, shall become due on a day which is not a Business Day, such payment
shall be made on the next succeeding Business Day and, in the case of a
principal payment, such extension of time shall be included in computing
interest in connection with such payment.

          Section 3. Section 6.01(ii) of the Credit Agreement is hereby amended
to read in its entirety as follows:

         (ii) Within thirty-five (35) days after the close of each of the first
and third quarterly periods of each of its fiscal years and within forty-five
(45) days after the close of the second quarterly period of each of its fiscal
years, for itself and its Subsidiaries, consolidated and consolidating unaudited
balance sheets as at the close of each such period and consolidated and
consolidating profit and loss and reconciliation of surplus statements and a
statement of cash flows for the period from the beginning of such fiscal year to
the end of such quarter, together with a reconciliation of the calculation of
Consolidated EBITDA with respect to losses from discontinued operations, all
certified by its chief financial officer (consolidating statements need not
cover cash flow reporting).

         Section 4. Section 6.01(ix) of the Credit Agreement is hereby amended
to read in its entirety as follows:

         (ix) Within thirty (30) days after the end of each calendar month, for
itself and its Subsidiaries, (a) consolidated and consolidating unaudited
balance sheets as at the close of each such period and (b) consolidated and
consolidating profit and loss and reconciliation of surplus statements and a
statement of cash flows for the period from the beginning of such fiscal year to
the end of such month, all certified by its chief financial officer
(consolidating statements need not cover cash flow reporting), together with a
compliance certificate, in form and content acceptable to the Agent, signed by
an Authorized Officer of the Borrower showing the calculations necessary to
determine

                                   APPENDIX C

<PAGE>

compliance with this Agreement as of the end of such calendar month and stating
that no Default or Unmatured Default exists, or if any Default or Unmatured
Default exists, stating the nature and status thereof.

         Section 5. Section 6.20 of the Credit Agreement is hereby amended to
read in its entirety as follows:

         6.20. Permitted Senior Subordinated Debt. The Borrower will not, and
will not permit any Subsidiary to, (i) make any amendment or modification to the
indenture, note or other agreement evidencing or governing any Permitted Senior
Subordinated Debt, or directly or indirectly voluntarily prepay, defease or in
substance defease, purchase, redeem, retire or otherwise acquire, any Permitted
Senior Subordinated Debt, except any amendment or modification allowed under
Section 6.19, or (ii) pay any interest accruing on any Permitted Senior
Subordinated Debt unless (a) at least five (5) Business Days before the date
such payment is made, the Borrower provides the Agent with (i) evidence that the
Borrower was in compliance with Section 6.26 as of the last day of the most
recent fiscal quarter to end before the proposed date of such payment, and (ii)
a copy of the unqualified audit report described in Section 6.01(i) with respect
to the most recently-ended fiscal year of the Borrower and its Subsidiaries, and
(b) no Default or Unmatured Default then exists or would result from the payment
of such interest.

         Section 6. Section 6.26.01 of the Credit Agreement is hereby amended to
read in its entirety as follows:

         Total Debt to EBITDA Ratio. The Borrower will not permit the ratio,
determined as of the end of each of the following calendar months and as of the
end of each of the following calendar quarters for the then most-recently ended
twelve-month period, of (i) Consolidated Funded Total Debt to (ii) Consolidated
EBITDA to be greater than the respective ratios set forth below:

<Table>
<Caption>

                                                                          MAXIMUM TOTAL DEBT
APPLICABLE CALENDAR MONTH/ CALENDAR QUARTER                                TO EBITDA RATIO
-------------------------------------------                               ------------------

<S>                                                                       <C>
January, 2003                                                               5.25 to 1.00

February, 2003                                                              5.25 to 1.00

March, 2003 and First Quarter, 2003                                         5.25 to 1.00

April, 2003                                                                 5.25 to 1.00

May, 2003                                                                   5.25 to 1.00

June, 2003 and Second Quarter, 2003                                         5.25 to 1.00
</Table>

                                   APPENDIX C
<PAGE>
<Table>

<S>                                                                         <C>
July, 2003                                                                  5.00 to 1.00

August, 2003                                                                5.00 to 1.00

September, 2003 and Third Quarter, 2003                                     4.25 to 1.00

October, 2003                                                               4.25 to 1.00

November, 2003                                                              4.25 to 1.00

December, 2003 and Fourth Quarter, 2003                                     4.25 to 1.00

January, 2004                                                               4.25 to 1.00

February, 2004                                                              4.25 to 1.00

March, 2004 and First Quarter, 2004                                         4.25 to 1.00
</Table>

         Section 7. Section 6.26.02 of the Credit Agreement is hereby amended to
read in its entirety as follows:

         6.26.02. Senior Debt to EBITDA Ratio. The Borrower will not permit the
ratio, determined as of the end of each of the following calendar months and as
of the end of each of the following calendar quarters for the then most-recently
ended twelve-month period, of (i) Consolidated Senior Total Debt to (ii)
Consolidated EBITDA to be greater than the respective ratios set forth below:

<Table>
<Caption>

                                                                            MAXIMUM SENIOR
                                                                            TOTAL DEBT TO
APPLICABLE CALENDAR MONTH/CALENDAR QUARTER                                   EBITDA RATIO
------------------------------------------                                  ---------------

<S>                                                                        <C>
January, 2003                                                                2.50 to 1.00

February, 2003                                                               2.50 to 1.00

March, 2003 and First Quarter, 2003                                          2.55 to 1.00

April, 2003                                                                  2.55 to 1.00

May, 2003                                                                    2.50 to 1.00

June, 2003 and Second Quarter, 2003                                          2.50 to 1.00
</Table>

                                   APPENDIX C
<PAGE>

<Table>

<S>                                                                          <C>
July, 2003                                                                   2.40 to 1.00

August, 2003                                                                 2.40 to 1.00

September, 2003 and Third Quarter, 2003                                      2.00 to 1.00

October, 2003                                                                2.00 to 1.00

November, 2003                                                               2.00 to 1.00

December, 2003 and Fourth Quarter, 2003                                      1.80 to 1.00

January, 2004                                                                1.80 to 1.00

February, 2004                                                               1.80 to 1.00

March, 2004 and First Quarter, 2004                                          1.80 to 1.00

</Table>

         Section 8. Section 6.26.03 of the Credit Agreement is hereby amended to
read in its entirety as follows:

         6.26.03. Fixed Charge Coverage Ratio. The Borrower will not permit the
ratio, determined as of the end of each of the following calendar months and as
of the end of each of the following calendar quarters, of (i) Consolidated
EBITDA minus the sum of Consolidated Cash Taxes and Consolidated Capital
Expenditures, in each case for the 12 months then ending, to (ii) Consolidated
Cash Interest Expense for the 12 months then ending, plus current maturities of
principal Indebtedness as of the end of such month or quarter, as applicable,
plus one seventh (1/7th) of the Aggregate Outstanding Credit Exposure as of the
end of such month or quarter, as applicable, all calculated for the Borrower and
its Subsidiaries on a consolidated basis, to be less than the respective ratios
set forth below:

<Table>
<Caption>

                                                                            MINIMUM FIXED
                                                                           CHARGE COVERAGE
APPLICABLE CALENDAR MONTH/CALENDAR QUARTER                                      RATIO
------------------------------------------                                 ----------------

<S>                                                                        <C>
January, 2003                                                                0.85 to 1.00

February, 2003                                                               0.85 to 1.00

March, 2003 and First Quarter, 2003                                          0.90 to 1.00
</Table>

                                   APPENDIX C
<PAGE>

<Table>

<S>                                                                        <C>
April, 2003                                                                  0.95 to 1.00

May, 2003                                                                    0.95 to 1.00

June, 2003 and Second Quarter, 2003                                          0.95 to 1.00

July, 2003                                                                   1.00 to 1.00

August, 2003                                                                 1.00 to 1.00

September, 2003 and Third Quarter, 2003                                      1.10 to 1.00

October, 2003                                                                1.10 to 1.00

November, 2003                                                               1.10 to 1.00

December, 2003 and Fourth Quarter, 2003                                      1.10 to 1.00

January, 2004                                                                1.10 to 1.00

February, 2004                                                               1.10 to 1.00

March, 2004 and First Quarter, 2004                                          1.10 to 1.00
</Table>

         Section 9. Section 6.26.05 of the Credit Agreement is hereby amended to
read in its entirety as follows:

         Cumulative Annual EBITDA. The Borrower will have Cumulative Annual
EBITDA as of the end of each of the following calendar months and as of the end
of each of the following calendar quarters in at least the following respective
minimum amounts :

<Table>
<Caption>

                                                                         MINIMUM CUMULATIVE
APPLICABLE CALENDAR MONTH/CALENDAR QUARTER                                  ANNUAL EBITDA
------------------------------------------                               ------------------

<S>                                                                      <C>
January, 2003                                                                $1,265,000

February, 2003                                                               $3,081,000

March, 2003 and First Quarter, 2003                                          $5,264,000

April, 2003                                                                  $7,873,000

May, 2003                                                                    $10,611,000
</Table>

                                   APPENDIX C
<PAGE>

<Table>

<S>                                                                          <C>
June, 2003 and Second Quarter, 2003                                          $13,578,000

July, 2003                                                                   $16,801,000

August, 2003                                                                 $19,949,000

September, 2003 and Third Quarter, 2003                                      $25,664,000

October, 2003                                                                $28,871,000

November, 2003                                                               $31,138,000

December, 2003 and Fourth Quarter, 2003                                      $33,225,000

January, 2004                                                                $1,350,000

February, 2004                                                               $3,286,000

March, 2004 and First Quarter, 2004                                          $5,614,000
</Table>

         Section 10. Attachments. Schedules I and III to the Credit Agreement
are hereby deleted and there are hereby substituted therefor new Schedules I and
III, which shall be identical to Schedules I and III, respectively, attached
hereto. The changes reflected in the revised Pricing Schedule shall be effective
as of the date hereof with respect to all then-outstanding Advances and Facility
LCs. Appendix A to the Credit Agreement is hereby deleted and there is hereby
substituted therefor a new Appendix A, which shall be identical to Appendix A
attached hereto.

         Section 11. Additional Representations, Warranties and Covenants of the
Borrower. The Borrower hereby represents and warrants to the Agent, the
Documentation Agent and each Lender that (i) Schedule III attached to this
Amendment is a complete, accurate and updated description, as of the effective
date of this Amendment, of all of the Subsidiaries of the Borrower, and (ii) to
the best knowledge of the Borrower as of the effective date of this Amendment,
Schedule VI attached hereto is a complete, accurate and updated description of
all deposit accounts and other accounts owned, held, or maintained by, or for
the benefit of, the Borrower and/or all of its Subsidiaries as of the effective
date of this Amendment. Within ten (10) Business Days after the effective date
of this Amendment, the Borrower shall furnish to the Agent an updated Schedule
VI , being an updated description of all deposit accounts and other accounts
owned, held, or maintained by, or for the benefit of, the Borrower and/or all of
its Subsidiaries as of the date of delivery of such updated Schedule VI to the
Agent, which updated Schedule VI shall be accompanied by a written certification
(in form and substance reasonably acceptable to the Agent) from the Borrower
that such updated Schedule VI is a complete, accurate and updated description of
all deposit accounts and other accounts owned, held, or maintained by, or for
the benefit of, the Borrower and/or all of its Subsidiaries as of the date of
delivery of such updated Schedule VI to the Agent.

                                   APPENDIX C
<PAGE>

         Section 12. 13-Week Cash Flow Forecast. In addition to any and all
other or additional reports and/or other information required to be delivered by
the Borrower to the Agent and/or the Lenders under the Credit Agreement and/or
the other Loan Documents, the Borrower hereby covenants and agrees to deliver to
the Agent and to each Lender an updated 13-Week Cash Flow Forecast on the first
and fifteenth days of each calendar month hereafter.

         Section 13. Waiver. The Borrower has informed the Agent that, as of
December 31, 2002, the Borrower will not be in compliance with Sections 6.26.01,
6.26.03 and 6.26..05 of the Credit Agreement as such provisions were in effect
before giving effect to this Amendment. The Required Lenders hereby waive
compliance, as of December 31, 2002, with Sections 6.26.01, 6.26.03 and 6.26.05
of the Credit Agreement; provided, however, that such waiver is conditioned upon
compliance with Sections 6.26.01, 6.26.03 and 6.26.05 of the Credit Agreement as
of December 31, 2002 as such Sections would have been in effect if this
Amendment had been effective as of December 31, 2002.

         Section 14. Additional Fee. The Borrower hereby acknowledges and agrees
that it has been attempting to obtain, and shall hereafter use its reasonable
best efforts to obtain, refinancing and/or replacement financing in an amount at
least sufficient to repay, satisfy and terminate in full all of the Obligations
prior to July 31, 2003 (with the proceeds of any such refinancing and/or
replacement financing to be utilized for such purpose) and the Borrower further
agrees to furnish the Agent and the Lenders with periodic updates with respect
to the status/progress of the Borrower's efforts to obtain such refinancing
and/or replacement financing. In addition to any other fees, charges,
reimbursements and other sums payable by the Borrower under the Credit Agreement
and/or the other Loan Documents, the Borrower hereby agrees that, unless all of
the Obligations have been paid, satisfied and terminated in full prior to July
31, 2003, on July 31, 2003 the Borrower shall pay to the Agent (for the ratable
benefit of each Lender in accordance with its respective Commitment as of the
date of the effectiveness of this Amendment) a fee equal to 1.00% times the
Commitment (as of the date of the effectiveness of this Amendment) of each
Lender.

         Section 15. Suspension of Swing Loans. Unless and until the Agent, or
any successor Agent, shall notify the Borrower and the Lenders in writing to the
contrary, the Agent shall have no duty or obligation whatsoever to make any
further or additional Swing Loans and the Borrower shall have no right to
request or receive any further or additional Swing Loans.

         Section 16. Modification to Certain Collateral Documents.
Notwithstanding anything to the contrary contained in any of the Loan Documents,
the Agent, the Documentation Agent and the Borrower hereby acknowledge and agree
that, unless and until the Agent shall notify the Borrower in writing that the
Agent desires to exercise any of such rights under the applicable Loan
Document(s), none of the Agent, the Documentation Agent and/or any of the
Lenders shall have any right, duty and/or obligation to exercise any voting
rights or other corporate governance rights with respect to any of the Property
of the Borrower or any of its Subsidiaries that may constitute stock of, or
other indicia of ownership of, any Person.

         Section 17. Further Assurances. The Borrower hereby agrees that, on or
before June 1, 2003, the Borrower and each of the applicable Subsidiaries of the
Borrower and any and all

                                   APPENDIX C
<PAGE>

necessary or appropriate other Persons shall have, at Borrower's expense,
executed and delivered any and all further instruments, documents, agreements,
filings and registrations, and shall have taken any and all further or
additional actions, as shall be necessary or advisable, in the opinion of the
Agent, in order to create, evidence, further effectuate, maintain, protect,
preserve, register, perfect, or continue the perfection of an Acceptable
Security Interest in all of the deposit and other accounts of the Borrower
and/or each of its Subsidiaries (including, without limitation, the deposit and
other accounts described or referred to in Schedule VI attached hereto). Without
limiting the foregoing or being limited by the foregoing, the Borrower agrees
that it shall, and shall cause all of its Subsidiaries to, at Borrower's expense
and as from time to time requested by the Agent, promptly execute and deliver
all further instruments, documents, agreements, filings and registrations, and
take all further actions, as shall, from time to time, be necessary or
advisable, in the opinion of the Agent, in order to: (i) further evidence and/or
effectuate the guarantee by each such Subsidiary of all of the Obligations, (ii)
create, evidence, further effectuate, maintain, protect, preserve, register,
perfect, or continue the perfection of an Acceptable Security Interest in all of
the Property of the Borrower and its Subsidiaries, as security for the
Obligations, (iii) confirm and validate the Credit Agreement, the other Loan
Documents and the Agent's rights and remedies thereunder, (iv) enable the Agent
to exercise and enforce its rights and remedies under any of the Loan Documents
in respect of the Property of the Borrower and/or its Subsidiaries covered by
any of the Collateral Documents, or (v) otherwise give the Agent the full
benefits of the rights and remedies described in or granted under the Credit
Agreement and/or any of the other Loan Documents.

         Section 18. Release. THE BORROWER, FOR ITSELF AND ON BEHALF OF EACH
SUBSIDIARY, AND EACH SUBSIDIARY OF THE BORROWER, ACKNOWLEDGES THAT ON THE DATE
HEREOF ALL OBLIGATIONS OF THE BORROWER TO THE AGENT, THE DOCUMENTATION AGENT,
THE LC ISSUER AND THE LENDERS ARE PAYABLE WITHOUT DEFENSE, OFFSET, COUNTERCLAIM
OR RECOUPMENT. IN ADDITION, EACH OF THE BORROWER, FOR ITSELF AND ON BEHALF OF
EACH SUBSIDIARY AND EACH SUBSIDIARY OF THE BORROWER, HEREBY RELEASES ANY AND ALL
CLAIMS, CAUSES OF ACTION OR OTHER DISPUTES IT MAY HAVE AGAINST THE AGENT, THE
DOCUMENTATION AGENT, THE LC ISSUER, EACH LENDER, THEIR RESPECTIVE COUNSEL OR ANY
OF THEIR RESPECTIVE AGENTS, DIRECTORS, OFFICERS, EMPLOYEES, REPRESENTATIVES,
SUCCESSORS OR ASSIGNS OF ANY KIND OR NATURE ARISING OUT OF, RELATED TO, OR IN
ANY WAY CONNECTED WITH, THE LOAN DOCUMENTS, IN EACH CASE WHICH MAY HAVE ARISEN
ON OR BEFORE THE DATE OF THIS AMENDMENT AND REGARDLESS OF WHETHER THE SAME AROSE
OUT OF THE NEGLIGENCE OF THE PERSON BEING RELEASED OR OTHERWISE, EXCEPT AND TO
THE EXTENT THAT THE SAME AROSE OUT OF THE GROSS NEGLIGENCE OR WILLFUL MISCONDUCT
OF THE PERSON OTHERWISE BEING RELEASED. THE BORROWER AND EACH SUBSIDIARY OF THE
BORROWER HEREBY ACKNOWLEDGES THAT IT HAS READ THIS AMENDMENT AND HAS CONFERRED
WITH ITS COUNSEL AND ADVISORS REGARDING ITS CONTENT, INCLUDING THIS SECTION, AND
HEREBY AGREES TO WAIVE ANY CLAIM THAT THE TERMS OF THIS AMENDMENT (INCLUDING,
WITHOUT LIMITATION, THE RELEASES CONTAINED HEREIN) AND THE LOAN

                                   APPENDIX C
<PAGE>

DOCUMENTS (AS AMENDED HEREBY) ARE INVALID OR OTHERWISE UNENFORCEABLE.

         Section 19. Conditions. This Amendment shall not become effective until
(i) it has been executed and delivered by the Borrower, each Guarantor and the
Required Lenders, (ii) the Borrower shall have delivered to the Agent a
certificate of the Secretary or any Assistant Secretary of the Borrower
authorizing the execution, delivery and performance of this Amendment, and (iii)
the Borrower shall have paid to the Agent (for payment to the applicable
Lenders) a fee equal to 1.00% times the Commitment (as of the date of the
effectiveness of this Amendment, after giving effect to this Amendment) of each
Lender which shall have joined in the execution of this Amendment by the later
of (a) March 28, 2003, and (b) the date it has been executed by the Required
Lenders.

         Section 20. Representations True; No Default. The Borrower represents
and warrants that the representations and warranties contained in the Loan
Documents are true and correct in all material respects on and as of the date
hereof as though made on and as of such date. The Borrower hereby certifies that
no event has occurred and is continuing which constitutes an Unmatured Default
or a Default.

         Section 21. Ratification. Except as expressly amended hereby, the Loan
Documents shall remain in full force and effect. The Credit Agreement, as hereby
amended, and all rights and powers created thereby or thereunder and under the
other Loan Documents are in all respects ratified and confirmed and remain in
full force and effect.

         Section 22. Definitions and References. Any term used herein which is
defined in the Credit Agreement shall have the meaning therein ascribed to it.
The terms "Agreement" and "Credit Agreement" as used in the Loan Documents or
any other instrument, document or writing furnished to the Agent or any Lender
by any Obligor and referring to the Credit Agreement, shall mean the Credit
Agreement as hereby amended.

         Section 23. Miscellaneous. This Amendment (a) shall be binding upon and
inure to the benefit of the Borrower, the Subsidiaries of the Borrower, the
Lenders, the Agent, the Documentation Agent and their respective successors,
assigns, receivers and trustees (but the Borrower shall not assign its rights
hereunder without the express prior written consent of the Required Lenders);
(b) may be modified or amended only by a writing signed by the party against
whom the same is to be enforced; (c) may be executed in several counterparts,
and by the parties hereto on separate counterparts, and each counterpart, when
so executed and delivered, shall constitute an original agreement, and all such
separate counterparts shall constitute but one and the same agreement, and (d)
together with the other Loan Documents, embodies the entire agreement and
understanding between the parties with respect to the subject matter hereof and
supersedes all prior agreements, consents and understandings relating to such
subject matter.

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
signed by their respective duly authorized officers, effective as of the date
first above written.

                                   APPENDIX C
<PAGE>

         THE LOAN DOCUMENTS (INCLUDING THIS AMENDMENT) REPRESENT THE FINAL
AGREEMENT AMONG THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.

         THERE ARE NO UNWRITTEN ORAL AGREEMENTS AMONG THE PARTIES.

                                 AMERICAN PLUMBING & MECHANICAL, INC.

                                 By:
                                    -------------------------------------------
                                    Steve Smith,
                                    Treasurer

                                   APPENDIX C
<PAGE>

                                 BANK ONE, NA (successor by merger to The First
                                 National Bank of Chicago), individually and as
                                 Agent

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------

                                   APPENDIX C
<PAGE>

                                 CREDIT LYONNAIS NEW YORK BRANCH, individually
                                 and as Documentation Agent

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------

                                   APPENDIX C
<PAGE>

                                 UNION BANK OF CALIFORNIA, N.A.

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------

                                   APPENDIX C
<PAGE>

                                 FLEET NATIONAL BANK

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------

                                   APPENDIX C
<PAGE>

                                 COMERICA BANK

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------

                                   APPENDIX C
<PAGE>

                                 BAY VIEW FINANCIAL CORPORATION

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------

                                   APPENDIX C
<PAGE>

         Each of the Subsidiaries of the Borrower (including, without
limitation, the Subsidiaries of the Borrower that are specifically listed below)
hereby (a) consents to the execution and delivery of this Amendment by the
Borrower and each of the other Subsidiaries of the Borrower, and (b)
acknowledges, confirms, and agrees that each Loan Document heretofore,
contemporaneously herewith, or hereafter executed by, or on behalf of, such
Subsidiary under, or in connection with, the Credit Agreement applies to,
relates to, and/or secures, as applicable, and shall continue to apply to,
relate to and/or secure, as applicable, the Credit Agreement (as such Credit
Agreement may heretofore, contemporaneously herewith and/or hereafter be, or
have been, amended, modified, supplemented and/or restated, in whole or in part)
and shall continue to constitute a Loan Document under such Credit Agreement.

                                 AMERICAN PLUMBING & MECHANICAL, INC. and EACH
                                 OF THE SUBSIDIARIES OF AMERICAN PLUMBING &
                                 MECHANICAL, INC. (including, without
                                 limitation, the specific Subsidiaries of
                                 American Plumbing & Mechanical, Inc.
                                 specifically listed below)

                                 By:
                                    --------------------------------------------
                                 Name:
                                      ------------------------------------------
                                 Title:
                                       -----------------------------------------
                                 For and on behalf of each of such Subsidiaries,
                                 in all capacities

                                 AMPAM SACRAMENTO, INC.

                                 AMPAM COLORADO, INC.

                                 AMPAM COMMERCIAL NORTHWEST, INC.

                                 AMPAM CORPORATE SERVICES, L.P.
                                   By AMPAM Holdings, LLC, its general partner
                                     By American Plumbing & Mechanical, Inc. its
                                        sole Member
                                 AMPAM DALLAS L.P.
                                   By AMPAM Holdings, LLC, its general partner
                                     By American Plumbing & Mechanical, Inc. its
                                        sole Member

                                 AMPAM HOLDINGS, LLC
                                   By American Plumbing & Mechanical, Inc.
                                        its sole Member

                                 AMPAM MANAGEMENT COMPANY, INC.

                                   APPENDIX C
<PAGE>

                                 AMPAM NORTH CAROLINA, INC.

                                 AMPAM PARTNERS, LLC
                                   By American Plumbing & Mechanical, Inc.
                                      its sole Member

                                 AMPAM SERVICES, INC.

                                 AMPAM ATLAS PLUMBING, L.L.C.
                                   By American Plumbing & Mechanical, Inc.
                                      its sole Member

                                 AMPAM CHRISTIANSON, L.P.
                                   By AMPAM Holdings, LLC, its general partner
                                      By American Plumbing & Mechanical, Inc.
                                         its sole Member

                                 FRANKLIN FIRE SPRINKLER COMPANY

                                 AMPAM J.A. CROSON COMPANY

                                 J.A. CROSON COMPANY

                                 AMPAM RIGGS PLUMBING, INC.

                                 AMPAM LDI MECHANICAL, INC.

                                 AMPAM MILLER MECHANICAL, INC.

                                 AMPAM COMMERCIAL SOUTHEAST, INC.

                                 AMPAM PARKS MECHANICAL INC.

                                 AMPAM POWER PLUMBING, L.P.
                                   By AMPAM Holdings, LLC, its general partner
                                      By American Plumbing & Mechanical, Inc.
                                         its sole Member

                                 AMPAM RCR COMPANIES

                                 AMPAM COMMERCIAL SHERWOOD MECHANICAL, INC.

                                 TEEPE'S RIVER CITY MECHANICAL, INC.

                                   APPENDIX C

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