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Why Dishwasher Soap Matters

Ask This Old House plumbing and heating expert Richard Trethewey demonstrates what happens when dish liquid is used in the dishwasher instead of dishwasher detergent gel.


  • To test how much suds and foam form with dish liquid, put a few squirts into a cup and fill it with water. The top of the glass should form a substantial foamy head, similar to a beer.
  • Put a few squirts of dishwasher detergent gel into a second glass and fill it with water. This demonstrates how the gel forms far less foam and suds than the dish liquid. The dishwasher detergent gel is meant to go in the dishwasher, not the dish liquid.
  • To see what happens when the wrong soap is put in the dishwasher, fill the soap dispenser with the dish liquid.
  • When the soap dispenser door is released during the wash cycle, foam and suds begin to encompass the basin.
  • Soap clogs the wash arm in the center of the dishwasher basin.
  • Suds begin to drip out the sides of the dishwasher and onto the floor.
  • If this mistake ever happens, clean out the entire dishwasher with a wet/dry vacuum cleaner before running another cycle. Otherwise, the same thing could happen because the soap is so concentrated and will remain in the dishwasher.