A common technique for opening a slow drain is to pour a combination of vinegar, baking soda, salt, or all three into the drain to let the reactions between the ingredients scour gunk off the sides of the drain pipe -- followed by a healthy dose of boiling water. Yet in the past several years, separate advice has come out to never pour boiling water into your drain. The cause of the confusion is simple but serious. Plus, if you have a drain that can't take boiling water, you need to change your drain-cleaning game quickly.
Pipe Material Determines Boiling Water Use
Your sink pipes will be made of one of two materials: metal or PVC (or a combination). The advice to pour boiling water into the drain to clear up a slow drain is generally sound -- there is logic to it -- but if your pipes or pipe joints are PVC, you can't use boiling water.
Boiling water helps soften grease inside the gunk. In other words, if the sink is draining slowly because of a buildup of fats, the intense heat from the boiling water will help soften that fat, and the rush of water will push the gunk through your plumbing. Even if the slowdown is due to something like hair, the boiling water can dissolve enough of the gunk that you normally get inside a drain pipe to open up the pipe and allow excess water to drain out of the sink.
So the idea that you should use boiling water -- not hot, but boiling -- is sound; not-boiling, hot water doesn't have the same oomph. However, boiling water can also be hot enough to melt material that isn't rated to handle it.
If you have PVC pipes or pipe joints -- take a look under the sink to see the material -- don't use boiling water. It can cause the joints to loosen. Instead, switch to hot, but plan to work a little more on the drain or call in a drain-cleaning service.
When You Have PVC Pipes
You can still use baking soda, vinegar, or salt with PVC pipes. Place those in the drain as usual and wait half an hour to an hour.
When time has passed, turn on the hot water faucet in your sink. If your water normally starts out cold from that tap, hold a large bowl under the faucet and run the water into the bowl (and keep draining it into a sink that isn't slow) until the water heats up. Once the water is as hot as the tap will let it get, let it run into the drain.
You may have to repeat this a couple of times over the course of the day because the hot water won't push as much through the drain as boiling water would. But you'll stop your pipes from melting. If you still can't get the drains to drain properly after that, call in the drain cleaning service. Have them clean out all the drains in your home to ensure you don't have to repeat this process for a long time.