Putting hot water from the tap in the pot before you boil it can save quite a bit of cooking prep time, but is the time saved worth the risk?
Some nights I think “yes!” but then I remind myself of the fact that hot water is much more likely than cold water to leach heavy metals from your plumbing pipes. If your house’s plumbing was installed before 1986, you should be particularly conscious not to use hot water from the tap, as lead was used to solder most plumbing pipes before this date. However, plumbing parts legally considered lead-free today are allowed to contain up to 8% lead. So either way, you likely leave lead in your plumbing (not a fun fact, I know).
Why should this matter to you? Lead can cause serious damage to the brain, kidneys, nervous system, and red blood cells (it is especially bad for pregnant women and children).
For this reason, the EPA recommends that you do not use hot tap water for cooking or drinking, and especially not for making baby formula. Furthermore, it recommends that anytime a faucet has not been used for 6 hours or more, flushing all the water that has been sitting in the pipes (i.e. simply letting the water run down the drain). When the water stops getting colder (anywhere from a few seconds to a few minutes), the pipes are likely flushed. You may decide to collect the water that would otherwise be wasted during flushing for watering plants so you don’t feel wasteful!