<PAGE>
 
                                                                   EXHIBIT 10.20

                               SERVICE AGREEMENT
                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      -----------------------------------

This Agreement ("Agreement") is made and entered into this 1st day of January,
1998, by and between Algonquin Gas Transmission Company, a Delaware Corporation
(herein called "Algonquin") and Boston Gas Company (herein called "Customer"
whether one or more persons).

WHEREAS, Algonquin and Customer entered into two Service Agreements (No. 93302
and No. 93202) both dated June 1, 1993, for service under Rate Schedule AFT-1;
and

WHEREAS, Algonquin and its customers entered into an agreement in which
provided, among other things, for revised daily and annual contract entitlements
under Rate Schedule AFT-1; and

WHEREAS, to enhance both parties' ease in administering nominations and capacity
releases, among other things, Algonquin and Customer desire to combine the two
aforementioned service agreements into a single service agreement for service
under Rate Schedule AFT-1;

NOW, THEREFORE, in consideration of the premises and of the mutual covenants
herein contained, the parties do agree as follows:


                                   ARTICLE I
                              SCOPE OF AGREEMENT

1.1  Subject to the terms, conditions and limitations hereof and of Algonquin's
     Rate Schedule AFT-1, Algonquin agrees to receive from or for the account of
     Customer for transportation on a firm basis quantities of natural gas
     tendered by Customer on any day at the Point(s) of Receipt; provided,
     however, Customer shall not tender without the prior consent of Algonquin,
     at any Point of Receipt on any day a quantity of natural gas in excess of
     the applicable Maximum Daily Receipt Obligation for such Point of Receipt
     plus the applicable Fuel Reimbursement Quantity; and provided further that
     Customer shall not tender at all Point(s) of Receipt on any day or in any
     year a cumulative quantity of natural gas, without the prior consent of
     Algonquin, in excess of the following quantities of natural gas plus the
     applicable Fuel Reimbursement Quantities:

          Maximum Daily Transportation Quantity      27,729 MMBtu
                                                 ----------

          Maximum Annual Transportation Quantity 10,121,085 MMBtu
                                                 ----------      
<PAGE>
 
                               SERVICE AGREEMENT
                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      ----------------------------------

1.2  Algonquin agrees to transport and deliver to or for the account of Customer
     at the Point(s) of Delivery and Customer agrees to accept or cause
     acceptance of delivery of the quantity received by Algonquin on any day,
     less the Fuel Reimbursement Quantities; provided, however, Algonquin shall
     not be obligated to deliver at any Point of Delivery on any day a quantity
     of natural gas in excess of the applicable Maximum Daily Delivery
     Obligation.


                                  ARTICLE II
                               TERM OF AGREEMENT

2.1  This Agreement shall become effective as of the date set forth hereinabove
     and shall continue in effect for a term ending on and including October 31,
     2009 ("Primary Term") and shall remain in force from year to year
     thereafter unless terminated by either party by written notice one year or
     more prior to the end of the Primary Term or any successive term
     thereafter. Algonquin's right to cancel this Agreement upon the expiration
     of the Primary Term hereof or any succeeding term shall be subject to
     Customer's rights pursuant to Sections 8 and 9 of the General Terms and
     Conditions.

2.2  This Agreement may be terminated at any time by Algonquin in the event
     Customer fails to pay part or all of the amount of any bill for service
     hereunder and such failure continues for thirty days after payment is due;
     provided Algonquin gives ten days prior written notice to Customer of such
     termination and provided further such termination shall not be effective
     if, prior to the date of termination, Customer either pays such outstanding
     bill or furnishes a good and sufficient surety bond guaranteeing payment to
     Algonquin of such outstanding bill; provided that Algonquin shall not be
     entitled to terminate service pending the resolution of a disputed bill if
     Customer complies with the billing dispute procedure currently on file in
     Algonquin's tariff.


                                  ARTICLE III
                                 RATE SCHEDULE

3.1  Customer shall pay Algonquin for all services rendered hereunder and for
     the availability of such service under Algonquin's Rate Schedule AFT-1 as
     filed with the Federal Energy Regulatory Commission and as the same may be
     hereafter revised or changed. The rate to be charged Customer for
     transportation hereunder shall not be more than the maximum rate under Rate
     Schedule AFT-1, nor less than the minimum rate under Rate Schedule AFT-1.

3.2  This Agreement and all terms and provisions contained or incorporated
     herein are subject to the provisions of Algonquin's applicable rate
     schedules and of Algonquin's General Terms and Conditions on file with the
     Federal Energy Regulatory Commission, or other duly constituted authorities
     having jurisdiction,
<PAGE>
 
                               SERVICE AGREEMENT
                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      -----------------------------------

     and as the same may be legally amended or superseded, which rate schedules
     and General Terms and Conditions are by this reference made a part hereof.

3.3  Customer agrees that Algonquin shall have the unilateral right to file with
     the appropriate regulatory authority and make changes effective in (a) the
     rates and charges applicable to service pursuant to Algonquin's Rate
     Schedule AFT-1, (b) Algonquin's Rate Schedule AFT-1, pursuant to which
     service hereunder is rendered or (c) any provision of the General Terms and
     Conditions applicable to Rate Schedule AFT-1. Algonquin agrees that
     Customer may protest or contest the aforementioned filings, or may seek
     authorization from duly constituted regulatory authorities for such
     adjustment of Algonquin's existing FERC Gas Tariff as may be found
     necessary to assure that the provisions in (a), (b), or (c) above are just
     and reasonable.


                                  ARTICLE IV
                              POINT(S) OF RECEIPT

Natural gas to be received by Algonquin for the account of Customer hereunder
shall be received at the outlet side of the measuring station(s) at or near the
Primary Point(s) of Receipt set forth in Exhibit A of the service agreement,
with the Maximum Daily Receipt Obligation and the receipt pressure obligation
indicated for each such Primary Point of Receipt. Natural gas to be received by
Algonquin for the account of Customer hereunder may also be received at the
outlet side of any other measuring station on the Algonquin system, subject to
reduction pursuant to Section 6.2 of Rate Schedule AFT-1.


                                   ARTICLE V
                             POINT(S) OF DELIVERY

Natural gas to be delivered by Algonquin for the account of Customer hereunder
shall be delivered on the outlet side of the measuring station(s) at or near the
Primary Point(s) of Delivery set forth in Exhibit B of the service agreement,
with the Maximum Daily Delivery Obligation and the delivery pressure obligation
indicated for each such Primary Point of Delivery. Natural gas to be delivered
by Algonquin for the account of Customer hereunder may also be delivered at the
outlet side of any other measuring station on the Algonquin system, subject to
reduction pursuant to Section 6.4 of Rate Schedule AFT-1.
<PAGE>
 
                               SERVICE AGREEMENT
                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      -----------------------------------

                                  ARTICLE VI
                                   ADDRESSES

Except as herein otherwise provided or as provided in the General Terms and
Conditions of Algonquin's FERC Gas Tariff, any notice, request, demand,
statement, bill or payment provided for in this Agreement, or any notice which
any party may desire to give to the other, shall be in writing and shall be
considered as duly delivered when mailed by registered, certified, or first
class mail to the post office address of the parties hereto, as the case may be,
as follows:

     (a)  Algonquin:  Algonquin Gas Transmission Company
                      5400 Westheimer Court
                      Houston, TX 77056

     (b)  Customer:   Boston Gas Company
                      One Beacon Street
                      Boston, MA 02108

or such other address as either party shall designate by formal written notice.


                                  ARTICLE VII
                                INTERPRETATION

The interpretation and performance of the Agreement shall be in accordance with
the laws of the Commonwealth of Massachusetts, excluding conflicts of law
principles that would require the application of the laws of a different
jurisdiction.
<PAGE>
 
                               SERVICE AGREEMENT
                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      -----------------------------------

                                 ARTICLE VIII
                          AGREEMENTS BEING SUPERSEDED

When this Agreement becomes effective, it shall supersede the following
agreements between the parties hereto, except that in the case of conversions
from former Rate Schedules F-2 and F-3, the parties' obligations under Article
II of the service agreements pertaining to such rate schedules shall continue in
effect.

Service Agreements (Nos. 93202 and 93302) executed by Customer and Algonquin
under Rate Schedule AFT-1 dated June 1, 1993.

IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be signed
by their respective agents thereunto duly authorized, the day and year first
above written.

                                   ALGONQUIN GAS TRANSMISSION COMPANY

                                   By:    /s/ Robert B. Evans
                                          --------------------------------
                                   Title: Sr. Vice President
                                          --------------------------------

                                   BOSTON GAS COMPANY

                                   By:    /s/ William R. Luthern
                                          --------------------------------
                                   Title:  VICE PRESIDENT
                                          --------------------------------
<PAGE>
 
                               SERVICE AGREEMENT
                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      -----------------------------------

                                   EXHIBIT A
                              Point(s) of Receipt
                              -------------------
                            Dated: January 1, 1998

              To the service agreement under Rate Schedule AFT-1
            between Algonquin Gas Transmission Company (Algonquin)
                       and Boston Gas Company (Customer)
                        concerning Point(s) of Receipt
 
<TABLE>
<CAPTION>
Primary                     Maximum Daily              Maximum              
Point of                 Receipt Obligation       Receipt Pressure          
Receipt                       (MMBtu)                  (Psig)               
-------                  ------------------       ----------------          
<S>                      <C>                      <C>                       
Centerville, NJ                 6,335             Any pressure requested by 
                                                  Algonquin not in excess of
                                                  750 Psig. 

Lambertville, NJ               21,394             Any pressure requested by     
                                                  Algonquin not in excess of
                                                  750 Psig.
</TABLE>                                                                    

Signed for Identification

Algonquin:  /s/ Robert B. Evans 
            --------------------
                                
Customer:   /s/ W. R. Luthern   
            -------------------- 
<PAGE>
 
                                                                          93302C
 
                               SERVICE AGREEMENT

                      (APPLICABLE TO RATE SCHEDULE AFT-1)
                      ----------------------------------- 

                                   EXHIBIT B
                             Point(s) of Delivery
                             --------------------

                            Dated: January 1, 1998

              To the service agreement under Rate Schedule AFT-1
            between Algonquin Gas Transmission Company (Algonquin)
                       and Boston Gas Company (Customer)
                        concerning Point(s) of Delivery
                         
<TABLE>
<CAPTION>
Primary                          Maximum Daily              Minimum     
Point of                      Delivery Obligation      Delivery Pressure
Delivery                           (MMBtu)                   (Psig)     
--------                      -------------------      -----------------
<S>                           <C>                      <C>               
Except as noted, 
on the outlet side 
of meter station 
located at:

Everett, MA
(at Customer's
reduction valves.)                   20,229                    75        
Waltham, MA                           3,000                   125      
East Braintree, MA                    3,000                   125      
Weston, MA                                0                     -       
Wellesley, MA                             0                     -      
Ponkapoag, MA                             0                     -       
Norwood, MA                           1,500                    75       
Mansfield Street,                                     
 Somerville, MA                                       
 (Alternate Delivery Point)               0                     -
</TABLE>


Signed for Identification:


Algonquin:  /s/ Robert B. Evans 
            --------------------
                                
Customer:   /s/ W. R. Luthern   
            --------------------