In a large pot, cover diced potatoes with cold water by about 1-inch. Bring to a boil then reduce heat to a simmer. Cook, uncovered, for about 5-7 minutes, until potatoes are fork tender. Drain the potatoes and rinse with cold water. Set aside to cool to room temperature, then transfer to a large bowl.
Meanwhile combine the mayonnaise, mustard, hummus, vinegar, salt & pepper in a small bowl or large measuring cup. Whisk together until smooth.
To the bowl with the potatoes, add the celery, red pepper and relish. Pour the mayo mixture over top and stir until everything is evenly coated. Add more salt & pepper if desired. Serve immediately or refrigerate for up to 2 days in advance.