Problems With Plumbing a Toilet Into a Tub

Siphoning

• A flushed toilet sends a large amount of waste and wastewater into the drain waste vent. This can cause a vacuum in the waste line that siphons the water out of the trap in the bathtub. If you hear gurgling sounds coming from the tub drain whenever the toilet is flushed, or when the bathtub is drained, you might have a blocked or undersized vent. When water is siphoned out of the trap, potentially dangerous sewer gases such as methane can enter your house.

Blockages

• Blockages in wet vent pipes can occur when pipe sizes are too small. According to the Centers for Disease Control and Prevention, you should never use vent pipes smaller than 1 1/4 inches in diameter because they are likely to become clogged. The 2009 International Plumbing Code includes tables that provide information on vent sizes and the types of fittings that are required. Section 916.2 of the International Plumbing Code states that vent diameters must be at least half the diameter of the drain it serves.

Backups

• Backups can also occur when toilets and bathtubs share plumbing. To keep toilet water from backing up into your bathtub, you need to install an anti-siphon toilet fill valve. U.S. and Canadian plumbing codes require that these valves be installed on all gravity-flow toilets, but it’s not easy to tell if the refill valve complies with local plumbing codes once it’s installed because they aren’t usually marked. Look for “anti-siphon” or “code-approved” marking on the packaging when you purchase a new refill valve.

Codes

• The International Code Council has developed codes and standards so toilets and bathtubs that share plumbing don’t develop problems. According to the ICC, only certain fixtures can be vented with a wet vent. These include toilets, bidets, washbasins, bathtubs or showers and emergency floor drains. The 2009 code permits two of each type of fixture – up to a total of eight fixtures – to share a wet vent. The fixtures must all be located on one floor and be in the same bathroom.