Do single-handle faucets pose a problem on Shabbos?
Single-handle faucets have lately gained popularity over two-handle ones. Whereas two-handle faucets have dedicated hot and cold water handles that control the hot and cold water, single-handle—or lever-style—faucets control both the hot and cold water flow with the same handle. Turning on the single handle faucets releases a mixture of cold and hot water unless pushed all the way to one direction. This poses a problem on Shabbos.
We may not turn on the hot water on Shabbos since doing so causes additional water to be heated to replace the hot water that was used. There is a greater likelihood of the hot water being turned on by mistake with a single-handle faucet. Some might forget to push the handle all the way to the right; others may not know how a single handle faucet operates. In addition, some models might release hot water even if the handle is pushed all the way to the right.
To avoid these potential problems, a faucet with two handles should be used or the hot water valve should be turned off before Shabbos. When faced with a one-handle faucet in an unfamiliar home, there is room to be lenient and assume that pushing the handle all the way to the right is okay.