food heats unevenly in this microwave, some parts are boiling and others are cold.

Diagnosis

The Breville Smooth Wave BMO850BSS is designed as a "turntable-free" microwave. Unlike traditional units that rotate the food through a stationary beam of microwaves, this model uses a series of internal antennas and a specialized waveguide to bounce microwaves throughout the cavity. This creates a "stirrer" effect intended to distribute energy evenly. However, when food heats unevenly—where some areas are boiling and others remain frozen or cold—it is typically because the standing wave patterns are creating localized "hot spots" and "cold spots" (nodes) that the internal antenna system cannot fully neutralize for the specific density of your food. This happens most frequently with dense, high-moisture foods (like stews, pasta, or thick cuts of meat) where the outer layers absorb the majority of the microwave energy, shielding the center. Because there is no physical rotation to move the food through different wave phases, the energy remains concentrated in specific geometric coordinates. You can confirm this by placing a thin layer of shredded cheese on a flat plate and heating it for 30 seconds; if the cheese melts in distinct patches while other areas remain solid, the wave distribution is functioning as designed, but your food placement is the issue.

How to Fix It

To overcome the lack of a turntable and eliminate cold spots, you must manually manage the energy distribution and food geometry. Follow these precise steps:

1. Utilize the "Ring" Arrangement: Never place food in a dense pile in the center of the microwave. Instead, arrange items in a circular pattern around the perimeter of the glass tray, leaving the center of the plate empty. This places the food directly in the path of the most active wave reflections.
2. Select the Correct Power Level: Avoid using the default "High" setting for items taking longer than 3 minutes. Press the 'Power' button and use the dial to drop the power to Medium (50%) or Medium-Low (30%). This allows the energy to penetrate the core of the food without overheating the exterior.
3. Implement the "Halfway Pivot": Set a timer for half the total cooking time. When it beeps, open the door and physically rotate the dish 180 degrees. If you are heating a liquid, stir it vigorously from the outside in toward the center.
4. Use Shallow, Wide Glassware: Swap deep bowls for wide, shallow Pyrex or ceramic dishes. Increasing the surface area reduces the distance the microwaves must travel to reach the center of the food mass.
5. Apply a Moisture Barrier: Cover the dish with a microwave-safe lid or damp paper towel. This traps steam, which conducts heat into the "cold spots" via convection, supplementing the microwave energy.

If That Didn't Work

• Check for Wave-Blocking Containers: Ensure you are not using "microwave-safe" plastic that is excessively thick or contains metallic decorative rims. Even a thin gold or silver rim on a plate will reflect microwaves away from the food, creating a massive cold spot on one side of the dish. Switch to plain borosilicate glass.
• Verify Cavity Cleanliness: Inspect the waveguide cover (the small rectangular panel on the right interior wall). If there is a buildup of grease, splatters, or burnt food residue, the microwaves will reflect off the grease rather than passing into the cavity, leading to erratic heating patterns. Clean it with a mixture of mild soap and water.
• Test with a "Water Load": Fill a large glass bowl with water and heat it for 2 minutes. If the water is boiling uniformly, the microwave's magnetron and antenna are functioning correctly, and the issue is solely with the density and placement of your specific food items.
• Reset the Control Board: In rare cases, a glitch in the power-level logic can occur. Unplug the BMO850BSS from the wall outlet for exactly 60 seconds to discharge the capacitors, then plug it back in to reset the internal timing and power sequences.

Heads Up

• Avoid Overloading: Do not crowd the cavity with multiple containers; this disrupts the wave bounce and creates significant cold zones between the dishes.
• Avoid Metal Foil: Never use aluminum foil to "trap heat" in this model; the antenna system is highly sensitive to metal, which can cause arcing or permanent damage to the waveguide.
• Mind the Center: Remember that the center of the Smooth Wave cavity is generally the lowest energy zone; always prioritize the edges of the tray for the densest parts of your meal.
• Clean Regularly: Because this unit relies on wave reflection, a dirty interior wall can actually change the heating pattern of the machine.