Exhibit 5.1
 
                                                      December 17, 1996
 
AMTROL Inc.
1400 Division Road
West Warwick, Rhode Island 02893
 
Ladies and Gentlemen:
 
    We have acted as special counsel for AMTROL Inc., a Rhode Island corporation
("AMTROL"), in connection with the Registration Statement on Form S-4 (the
"Registration Statement") filed by AMTROL with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended,
relating to the issuance by AMTROL of $115,000,000 aggregate principal amount of
its 10 5/8% Senior Subordinated Notes due 2006 (the "Exchange Notes"). The
Exchange Notes are to be offered by AMTROL in exchange for $115,000,000
aggregate principal amount of its outstanding 10 5/8% Senior Subordinated Notes
due 2006 (the "Notes").
 
    We have examined the Registration Statement, the Indenture dated as of
November 1, 1996 between AMTROL Acquisition, Inc. and The Bank of New York, as
trustee (the "Trustee"), as supplemented by the First Supplemental Indenture
dated as of November 13, 1996 between AMTROL and the Trustee (the "Indenture"),
which have been filed with the Commission as Exhibits to the Registration
Statement. In addition, we have examined, and have relied as to matters of fact
upon, the originals or copies, certified or otherwise identified to our
satisfaction, of such corporate records, agreements, documents and other
instruments and such certificates or comparable documents of public officials
and of officers and representatives of AMTROL, and have made such other and
further investigations, as we have deemed relevant and necessary as a basis for
the opinion hereinafter set forth.
 
    In such examination, we have assumed the genuineness of all signatures, the
legal capacity of natural persons, the authenticity of all documents submitted
to us as originals, the conformity to original documents of all documents
submitted to us as certified or photostatic copies, and the authenticity of the
originals of such latter documents.
 
    Based upon the foregoing, and subject to the qualifications and limitations
stated herein, we are of the opinion that, assuming due authorization of the
Exchange Notes in accordance with Rhode Island law, the Exchange Notes, when
executed and authenticated in accordance with the provisions of the Indenture
and delivered in exchange for the Notes as contemplated in the Registration
Statement and the Indenture, will constitute valid and legally binding
obligations of AMTROL.
 
    Our opinion set forth above is subject to the effects of bankruptcy,
insolvency, fraudulent conveyance, reorganization, moratorium and other similar
laws relating to or affecting creditors' rights generally, general equitable
principles (whether considered in a proceeding in equity or at law) and an
implied covenant of good faith and fair dealing.
 
    We are members of the Bar of the State of New York and we do not express any
opinion herein concerning any law other than the law of the State of New York
and the federal law of the United States.
 
    We hereby consent to the use of this opinion as an Exhibit to the
Registration Statement and to the reference to our firm under the caption "Legal
Matters" in the Prospectus included therein.
 
                                          Very truly yours,
 
                                          /s/ Simpson Thacher & Bartlett
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                                          SIMPSON THACHER & BARTLETT