<SEC-DOCUMENT>0001179110-13-006397.txt : 20130403
<SEC-HEADER>0001179110-13-006397.hdr.sgml : 20130403
<ACCEPTANCE-DATETIME>20130403171751
ACCESSION NUMBER:		0001179110-13-006397
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20130402
FILED AS OF DATE:		20130403
DATE AS OF CHANGE:		20130403

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			COMFORT SYSTEMS USA INC
		CENTRAL INDEX KEY:			0001035983
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRICAL WORK [1731]
		IRS NUMBER:				760526487
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		675 BERING DRIVE
		STREET 2:		SUITE 400
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77057
		BUSINESS PHONE:		7138309600

	MAIL ADDRESS:	
		STREET 1:		675 BERING DRIVE
		STREET 2:		SUITE 400
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77057

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Murdy William F
		CENTRAL INDEX KEY:			0001205063

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-13011
		FILM NUMBER:		13740630

	MAIL ADDRESS:	
		STREET 1:		COMFORT SYSTEMS USA, INC.
		STREET 2:		675 BERING DRIVE, SUITE 400
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77057

	FORMER NAME:	
		FORMER CONFORMED NAME:	MURDY WILLIAM F
		DATE OF NAME CHANGE:	20021109
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-04-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001035983</issuerCik>
        <issuerName>COMFORT SYSTEMS USA INC</issuerName>
        <issuerTradingSymbol>FIX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001205063</rptOwnerCik>
            <rptOwnerName>Murdy William F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O COMFORT SYSTEMS USA, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>675 BERING DRIVE, SUITE 400</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77057</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-04-02</value>
                <footnoteId id="F1"/>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5803</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>13.84</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>242066</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-04-02</value>
                <footnoteId id="F1"/>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5840</value>
                    <footnoteId id="F4"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>13.84</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>236226</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Forfeited shares are related to performance stock and restricted stock units that vested on April 1, 2013 and were priced based on the average of the high and low stock price on April 1, 2013. Forfeitures were process on April 2, 2013.</footnote>
        <footnote id="F2">Reflects the third and final vesting of performance stock granted on March 25, 2010.</footnote>
        <footnote id="F3">Price is based on the average of the high and low stock price on April 1, 2013.</footnote>
        <footnote id="F4">Reflects the second vesting of performance stock granted on March 24, 2011.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Trent T. McKenna, by power of attorney</signatureName>
        <signatureDate>2013-04-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
