If you close that return, you need to make up the return air somewhere else. Instead, take off the grill and see if there's air leaking from around it (instead of the duct itself). Then insulate and/ or seal the gaps. Might be a good place for some spray foam. BTW, if the duct is used for only AC, you can cover it during the off season. They sell magnetic covers for floor registers, so maybe there's something for larger sizes.

I don't see where a 15 amp/ 120 volt circuit would have anything to do with it. There are some conditions where there is a 20 amp limit, but that can usually be handled with an SSU. 430.24 Several Motors or a Motor(s) and Other Load(s). Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads. FPN: See Annex D, Example No. D8. Exception No. 1: Where one or more of the motors of the group are used for short-time, intermittent, periodic, or varying duty, the ampere rating of such motors to be used in the summation shall be determined in accordance with 430.22(E). For the highest rated motor, the greater of either the ampere rating from 430.22(E) or the largest continuous duty motor full-load current multiplied by 1.25 shall be used in the summation. Exception No. 2: The ampacity of conductors supplying motor-operated fixed electric space-heating equipment shall comply with 424.3(B). Exception No. 3: Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time, the conductor ampacity shall be permitted to be based on the summation of the currents of the motors and other loads to be operated at the same time that results in the highest total current. 430.32 (B) One Horsepower or Less, Automatically Started. Any motor of 1 hp or less that is started automatically shall be protected against overload by one of the following means. (1) Separate Overload Device. By a separate overload device following the requirements of 430.32(A)(1). 430.36 Fuses — In Which Conductor. Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded conductor and also in the grounded conductor if the supply system is 3-wire, 3-phase ac with one conductor grounded. 430.42 Motors on General-Purpose Branch Circuits. Overload protection for motors used on general-purpose branch circuits as permitted in Article 210 shall be provided as specified in 430.42(A), (B), (C), or (D). (A) Not over 1 Horsepower. One or more motors without individual overload protection shall be permitted to be connected to a general-purpose branch circuit only where the installation complies with the limiting conditions specified in 430.32(B) and 430.32(D) and 430.53(A)(1) and (A)(2). 430.53 Several Motors or Loads on One Branch Circuit. Two or more motors or one or more motors and other loads shall be permitted to be connected to the same branch circuit under conditions specified in 430.53(D) and in 430.53(A), (B), or (C). (A) Not Over 1 Horsepower. Several motors, each not exceeding 1 hp in rating, shall be permitted on a nominal 120-volt branch circuit protected at not over 20 amperes or a branch circuit of 600 volts, nominal, or less, protected at not over 15 amperes, if all of the following conditions are met: (1) The full-load rating of each motor does not exceed 6 amperes. (2) The rating of the branch-circuit short-circuit and ground-fault protective device marked on any of the controllers is not exceeded. (3) Individual overload protection conforms to 430.32. (B) If Smallest Rated Motor Protected. If the branchcircuit short-circuit and ground-fault protective device is selected not to exceed that allowed by 430.52 for the smallest rated motor, two or more motors or one or more motors and other load(s), with each motor having individual overload protection, shall be permitted to be connected to a branch circuit where it can be determined that the branchcircuit short-circuit and ground-fault protective device will not open under the most severe normal conditions of service that might be encountered. (D) Single Motor Taps. For group installations described above, the conductors of any tap supplying a single motor shall not be required to have an individual branch-circuit short-circuit and ground-fault protective device, provided they comply with one of the following: (1) No conductor to the motor shall have an ampacity less than that of the branch-circuit conductors. (2) No conductor to the motor shall have an ampacity less than one-third that of the branch-circuit conductors, with a minimum in accordance with 430.22, the conductors to the motor overload device being not more than 7.5 m (25 ft) long and being protected from physical damage. (3) Conductors from the branch-circuit short-circuit and ground-fault protective device to a listed manual motor controller additionally marked "Suitable for Tap Conductor Protection in Group Installations" shall be permitted to have an ampacity not less than 1⁄10 the rating or setting of the branch-circuit short-circuit and ground-fault protective device. The conductors from the controller to the motor shall have an ampacity in accordance with 430.22. The conductors from the branch-circuit short-circuit and ground-fault protective device to the controller shall (1) be suitably protected from physical damage and enclosed either by an enclosed controller or by a raceway and be not more than 3 m (10 ft) long or (2) have an ampacity not less than that of the branch-circuit conductors.

I'm having a hard time visualizing a mobile home affixed to a walkout basement. Please explain. Better yet, post some pictures!

Thanks, Mike, Yes, it's kind of a separate room, drywall on all 4 sides with 2 doors and a vent for inverse pressure(that's what one HVAC guy said)(it's just an opening on the furnace room and living room. Yes, the furnace is high efficiency type. (Lennox EL196E) Yes, there is a hot water tank next to it. Yes, there is a Rubatex insul Tube 180 3/4. I think this is where it's getting air for combustion. Not sure how to measure the size of the furnace. I have closed the front opening in the duct. I have left the top opening, now I think the air will be sucked in from there. the area between the duct and wood you see in the image(i think its called joist) is leading to the furnace room, My concern is, this big of an opening is sucking all the air and the vents on the 2nd floor is not able to suck or are low on return air. if there is low suction on the 2nd floor then no return vent on the 1st floor can cause issues???