The key reason for this problem is that the pipes feeding the shower and toilet are too small in diameter to provide enough pressure for both at the same time, says Hendrik Vandepoll, a plumber with more than 30 years of experience, and the co-owner of Service Force Plumbing in Rockville, Maryland.
The main cause of cold shower water when the toilet flushes is your plumbing setup. When the toilet tank needs to fill, it diverts water from your shower. There are simple fixes that cost nothing. Replacing a valve to solve the problem costs $100 and up.
To achieve your desired temperature, your shower valve is mixing hot water and cold water. However, when the toilet is flushed, cold water is temporarily siphoned away from the shower as the toilet refills. This makes the mixture of water coming out of your showerhead much hotter.
This is normal. The water used for the toilet is being robbed from the shower. As mentioned, newer valves have pressure balancing spools in them to prevent scalding in the event all cold water was lost to a shower/tub.
To do this, adjust the dial or temperature setting on your water heater and wait for at least five minutes. Use the tap closest to the heater to access this warm water. If the water is still too cold, repeat the above steps until you're satisfied with the temperature.
The most common cause of an electric shower running cold is an electrical supply fault or wiring problem. It's important to check the wiring in your bathroom and ensure that all connections are secure. You should also inspect the shower unit itself for any signs of damage or wear and tear.
The easiest and cheapest way to correct the problem is to slow the flow of water into the toilet, and the second-easiest and cheapest way is to install a pressure-balanced valve. The mystery of why flushing your toilet affects your shower temperature isn't so hard to solve.
Possible Causes
The most common cause of water backup is a clog in a sewer line. This can be the result of flushing things that aren't supposed to be flushed such as toys and feminine products or a grease buildup. Invasive tree roots can also cause blocked pipes.
When the toilet flushes while you're showering, the toilet demands a load of cold water, and because it shares a cold water line with the shower, the shower temporarily loses pressure from the cold water line. Without the cold water to temper the hot, the shower can become uncomfortably hot.
The water fixtures and pipes in any household are all well connected. In some cases, when someone will use another tap in the laundry or kitchen, you might experience loss of heating to the shower. This is because of the water supply from the hot water tank and subsequent water pipes working together simultaneously.
Basically, they work by adjusting the ratio of hot to cold water pressure. So, when your toilet flushes it pulls cold water from your shower. The pressure-balancing valve senses the drop-in cold water pressure and responds by restricting the hot water pressure.
This valve is found inside your shower's handle in most modern homes. There are a few different types of valves standard in showers. Pressure balancing valves are installed in showers and sinks, and faucets.
The water comes from the cold water supply. So when someone flushes, the pressure-balance valve senses a drop in cold water pressure. To counteract this, the valve restricts the release of hot water, resulting in a pressure drop. This minor inconvenience will continue until the water tank completely refills.
If your problem wasn't the anti-scalding device, your shower mixing valve has most likely gone bad and is preventing hot water from mixing properly into your shower water. Your shower mixing valve controls the water temperature of your shower by blending together cold and hot water.
Problems with the P-Trap
Problems with gurgling can arise if a sudden flow of water inside the pipe – such as when a toilet is flushed – creates a positive pressure that causes wastewater to back up in a drainage system. This can force water and air bubbles up the drain, resulting in that tell-tale gurgling sound.
You will likely pay around $350 to $650 to have a plumber unclog a main sewer line unless you have complications like serious damage to the line.
What's happening is the water pressure in the main water supply has gone over 80 psi, forcing water through an old seat washer whenever the toilet is flushed and water flows from the supply line to refill the toilet tank. This is possible because all the water in your bathroom plumbing is connected.
In almost all cases, no. Plumbers do not install toilet pipes directly into shower pipes. Doing so could create a health hazard for you and your family, should wastewater from the toilet seep up into the shower. However, both drain pipes eventually connect to a primary drain line in your sewage system.
OPTION #2: HOT WATER, SOAP & SOAK
Squeeze about a quarter cup (or as much as you can) as close to the hole as possible. Add some hot (not boiling) water and let sit to help lubricate and loosen the clog.
When you flush the toilet, it calls for cold water to fill up the tank. The toilet draws water from the trunk pipe. If you're showering during this time, the shower will have less cold water to draw from in the trunk pipe, and compensate with more hot water.
Installing pressure regulators can help maintain consistent water pressure, reducing temperature fluctuations. Corroded or blocked pipes can affect water flow and temperature. Repairing or replacing these pipes can resolve such issues.
If your shower valve is damaged beyond repair, you may need to replace it to prevent leaks. Removing your old valve and installing a new one can take time, but with patience, a novice handyman can do it without the help of a plumber.
Scalding Hot or Freezing Cold Water
One of the most frustrating shower mixing valve problems is when you're forced to deal with water that's either too hot or too cold. This often happens when the valve is not correctly calibrated.
If you notice that your shower isn't dispersing hot or cold water, it may mean that your shower check valves are clogged.