Ninja DoubleStack XL Smart SL451, Deep Gray

The SL451 is the DoubleStack XL with a Smart Thermometer

The SL451 stacks two 5-quart baskets, one above the other, for 10 quarts total, and adds Ninja's Foodi Smart Thermometer. The listing describes a Smart Cook System built around the integrated thermometer, for doneness from rare to well-done, and SharkNinja's page names the integrated Foodi Smart Thermometer. The box, per the listing, holds the air fryer, two 5-quart baskets, two nonstick crisper plates, two stacked meal racks, the thermometer and 9 recipes.

SharkNinja lists 120 volts, 1,690 watts, a one-year warranty, 20.83 by 13.07 by 17.72 inches and 25.35 pounds. Amazon's details say 11.25 by 19.22 by 15.14 inches and 26.2 to 26.3 pounds. The stacked meal racks give two cooking levels per basket, which is how the listing arrives at four foods at once.

SharkNinja's support site files the SL451 under the SL401 Series owner's guide, which states it supports SL401, SL401C and SL451. The guide covers the air fryer itself in detail but has no section on the thermometer or Smart Cook System, so the doneness settings are not documented in anything we could read. Look for thermometer instructions in the box, or ask SharkNinja, before relying on it.

Temperature fields on the listing disagree with the guide

The Amazon details give a temperature range of up to 400°F in one field and 80 to 200°C in two others. The owner's guide sets Air Broil at a fixed 450°F with no adjustment, and gives defaults of 390°F for Air Fry and 325°F for Bake. The listing fields and the guide do not agree, so ask SharkNinja which applies if the maximum temperature matters to you.

Other guide defaults: Dehydrate starts at 140°F and runs 1 to 12 hours in 15-minute steps; Bake runs up to 4 hours; Reheat defaults to 250°F for up to 1 hour. The guide's Roast page lists a 250°F default and up to 1 hour, but its steps repeat the Reheat wording, so confirm Roast settings with SharkNinja.

Some Amazon fields also mention a transparent viewing window on the baskets. SharkNinja's SL451 page as read does not mention a viewing window, so do not buy on the expectation of seeing food through the drawer without checking with the seller.

Headroom: 6 inches on top and sides

The Amazon listing says the stacked design fits under standard cabinets, and its details repeat that claim. The owner's guide's troubleshooting section says that an O-H message means the unit is overheating because it is in a corner or constricted space, and that it requires at least 6 inches of space on the top and sides for air circulation.

Neither source gives a cabinet height, so measure the gap between your counter and the cabinet above against SharkNinja's 17.72-inch height plus 6 inches. The guide's fix for O-H is to turn the unit off, unplug it, let it cool for 30 minutes and move it somewhere with that clearance.

Other placement rules from the safeguards: use a worktop, not near the edge; put nothing on top while in use; do not cover the air intake or outlet; and do not move it while it is running.

Power rules and the zone timer

The guide says not to use an extension cord, never to use a socket below counter level, and not to connect the unit to an external timer or remote-control system. The listing gives 1,690 watts at 120 volts, matching SharkNinja.

Zone 1 is the top basket and Zone 2 the bottom, each with its own heating element and fan, and no preheat is needed. The panel offers Smart Finish, which uses a HOLD mode so both zones finish together, and Match Cook. Removing a basket pauses its timer, and it must go back within 10 minutes or the cook is cancelled. The unit goes into standby after 10 minutes without input.

Cleaning and handling hot baskets

The guide names the baskets, crisper plates and stacked meal racks as the only dishwasher-safe parts and recommends hand washing to extend their life. The main unit is wiped with a damp cloth and must never be immersed or put in a dishwasher.

To remove food, the guide says to tip it out or use silicone-tipped tongs, never to set the baskets on top of the unit, and to put hot baskets and crisper plates only on heat-resistant surfaces.

Warranty: one year from an authorized retailer

SharkNinja's SL451 page lists a one-year warranty. The guide's terms cover the original owner, for purchases from authorized retailers, under normal household use; SharkNinja repairs or replaces at its discretion, and a replacement unit is covered for six months from receipt or the rest of the original term, whichever is later.

Racks, pans and other wear parts are excluded, as are commercial use, misuse, unauthorized repairs and products used outside North America. Claims start by phone with the receipt. SharkNinja pays for sending the unit in but charges a return-shipping fee, stated in the guide, when it sends a repaired or replacement unit back.