How to Install an Anti-Sweat Valve for a Toilet

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How to install an anti-sweat mixing valve to prevent condensation from forming on the outside of your toilet tank.

If you are having a problem with condensation forming on the exterior of your toilet tank, one way to correct the problem is to install an anti-sweat mixing valve. An anti-sweat valve is a device that mixes a small amount of hot with the cold water supply before it enters the toilet tank to raise the temperature of the water slightly. Warm humid air will condense on the porcelain surface of the toilet tank during summer months and may cause damage to the walls and floor of your bathroom. The cold water supplied to your house is brought in through underground piping that lowers the temperature of the water to 55 degrees Fahrenheit or less. This is about the same temperature as the cold air supplied by an air conditioner.

Installing an anti-sweat valve is a straightforward repair if you have some basic plumbing skills. The mixing valve costs around $30 and it can be purchased at home centers, online, or at plumbing supply houses. You may also need a few feet of ½ inch copper pipe, Tees, and 90-degree elbows.

Before you start you need to determine how you will install the valve and where the hot and cold supply lines are located. On toilets located on the first floor, you can install the valve in your basement or crawlspace. For a second floor installation, or for homes without a basement, you may be able to install the valve right at the toilet or inside a vanity beneath the sink. In this case you will need to run a new supply line from the valve to the toilet either through the side of the vanity or run a new pipe inside the wall to the toilet.

Tools and Supplies

Anti-Sweat Mixing Valve

½-inch shut off valves, optional

Copper pipe

Copper fittings

Teflon tape

Pump Pliers or adjustable wrench

Safety glasses

Soldering Torch

Flux and Solder

Pipe Cutter

Emery Cloth

Note: Depending on the type of anti-sweat valve you purchase, the valve may be installed horizontally or vertically. Some valves contain internal check valves and need to be installed in the vertical position to work properly. Valves that come with check valves reduce the likelihood of cold water passing through the valve and filling the hot water lines in the house.

Bathroom Installation

Basement Installation


1. Locating and piping the valve

Determine the best location for the mixing valve and shut off the water to the hot and cold water supply lines. Cut the line and insert the pipe into the outlet port of the valve. Mark where you need to cut the other end of the cold water line and determine the placement of the Tee for the hot water line.

Anti-sweat Valve Schematic

2. Making the Connection from the outlet of the valve

You may want to add a piece of Teflon tape onto the threads on the valve body before inserting the pipe, ferrule, and compression nut into the mix port of the valve. (This is the pipe that goes to the toilet) Make sure that the pipe is pushed into the valve body as far as it will go before tightening the compression fitting.

Compression fittings

3. Connecting the pipes from the hot and cold supply lines to the valve

Install a Tee and copper pipe from the hot water line to the mixing valve. Use a compression fitting or solder. You may want to install a valve after the tee in case you ever need to make repairs to the toilet or valve. Repeat the process for the cold water supply line to the ant-sweat valve.

You may need to use a fireproof mat to protect any wood framing that is within 6 inches of the piping when soldering. Attach the pipe and fitting assembly to the valve by using the compression fitting that comes with the valve. Make sure that the copper pipe and fittings are cleaned well and flux is applied to the inside surface of the fitting and the outer surface of the pipe. Use only lead-free solder.

4. Completion and testing

Solder all new joints or tighten compression fittings. Shut off the water valve below the toilet and then slowly turn on the cold water. Check for any leaks and then proceed to turn on the hot water. If no leaks are present flush the toilet a few times to purge the air out of the lines.


You will need to adjust the amount of hot water entering the valve through a process of trial and error. It depends on the air temperature and conditions where the toilet is located and the temperature of the cold water entering your home, but the temperature of the water inside the toilet tank only needs to be at or slightly above 70 degrees Fahrenheit. If the toilet condensation problem is seasonal, you can shut off the hot water entering the valve. You may want to mark the location of the hot water valve with a marker for future reference.

Alliance anti-sweat valve needs to be installed in a vertical position



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