What is wrong when the bath tub fills with water when you run the washing machine?

Answer 1

This is caused by the main drain pipe (into which all of your home's drain pipes converge before draining out to the municipal sewer line) is partially clogged and not allowing sufficient volume of drainage to pass through quickly enough. The problem is often not noticeable when relatively small amounts of wastewater drain away, say from a flushing toilet or using the kitchen sink. The washing machine dumps by far the largest amount of wastewater in the shortest period of time compared to any other Plumbing fixture in your house. If the main drain line is partially clogged, the wastewater from the washing machine cannot pass through it fast enough, and the wastewater backs up into the rest of the drainage system in your house (called the "drain-waste-vent," or DWV system). The DWV system in your house is not under pressure, as is the freshwater supply system. Thus, when there is a blockage in the DWV system, the blocked wastewater seeks its level and eventually appears backing up through the drains of your home's fixtures, starting with the lowest one first. This is usually a bathtub on the lowest floor of the house, because it has the lowest (closest to the floor) drain in the house, compared with all the other fixtures (e.g., kitchen sink, laundry tub, dishwasher, and bathtubs on higher floors).

Assuming the blockage is simply accumulated stuff that can be cleaned away either by jetting it away using water pressure, or by using a plumber's snake to scrape it out inside the pipe, this is the simplest way to solve the problem. You can buy devices at hardware stores and home centers that use either of these methods to clean out your drain line.

To prevent recurrence, avoid disposing of stuff in any of your drains, say via toilets or the kitchen sink, that tends (i.e., is known to) build up inside your drainage lines. This includes, typically, human hair, paper towels and facial tissue not intended for disposal in toilets. (Toilet paper, by contrast, is engineered to disintegrate quickly in the drain, so that it won't get stuck in your drain pipes like other types of paper). Also avoid disposing of grease and food waste in the kitchen sink. Instead, dispose of them in your trash. Be aware that you cannot depend on a garbage disposal to adequately disintegrate all manner of waste material. Corn silk, celery, fats and oils, etc., may disappear through your disposal, but they may also be secretly building up in your drain line to cause future backup problems in your bathtub months or years later.