Why is my shower providing instant hot water but the sink next to it takes forever to heat up or suddenly turns cold, and how do I resolve this issue without losing water pressure?
Asked by Christoffer Kristensen 
3 months ago
Last Updated: October 7, 2024
I’ve noticed that the shower gives me hot water right away, but the sink next to it takes forever to warm up. And sometimes the sink water even goes cold while I’m using it. The pressure stays fine though. Any ideas why this is happening and how I can solve it?
If you own a tankless water heater, your faucet may not be delivering enough flow for the heater to function correctly
Mentioned that most Instant Hot Water Heaters are activated by flow. The system measures the flow to determine when to turn on. I say most because if I claim all, someone will likely find an exception and use it against me. Turn the faucet all the way to hot and fully open. Does the IHWH activate? Does the output pipe heat up? Does the hot water supply on the sink warm up?
I agree with Rob, the sink may not be drawing enough pressure when you turn on the hot water. It might be a good idea to have it checked out.
Yesterday I was at a house that had a tankless water heater too. It took a bit of time for the water to get hot. I noticed that if I turned it to cold and then back to hot, I had to wait all over again for it to get hot. If you don’t have a tankless heater, the newer single-handle faucets typically have pressure-balanced cartridges. Although I believe you might notice a change in volume when you switch to hot water.
Explained that their tankless water heater had a similar issue, but it turned out there was a timer set by the builder for instant hot water during peak hours. Maybe something similar is happening with yours.
Shared that she struggled to find warm water – it was either too hot or too cold. It seems like the faucet has a specific spot that the owners are used to.
Mentioned that we had to turn on the bathtub to get hot water elsewhere during off-peak times or wait for more than 5 minutes. Quite frustrating!
Hey Julie from 1bidpro! So the issue you’re having with inconsistent hot water supply between your shower and sink could be due to a few things related to your plumbing system like clogged or corroded pipes, a faulty mixing valve, or pressure balancing valve problems.
Have you made sure that the hot water is fully turned on under the sink when checking for the problem on the sink?
If the issue is with a single handle faucet, it might be the mixing valve.
Did you try taking out the aerator from the sink faucet to see if that helps? Also, have you made sure that the shut off valves, especially the one for hot water under the sink, are completely open? And have you looked for any kinks in the supply line? If this is a single pull faucet, check if there’s a temperature limit set on it. Some newer faucets have a stop on the hot water side to prevent scalding. You can use an instant read thermometer to test the water temperature at the kitchen sink after letting the water run for a few minutes. Kitchen sink faucets usually don’t have a limiter. Compare that temperature to the tub and bathroom sink. If there’s a difference, it could mean the limit is too low or the mixing valve is failing. You can adjust the temperature limiter or replace the cartridge for the mixing valve if needed.
On a tankless water heater, there needs to be a certain amount of water flowing to activate the switch. It’s recommended to have them serviced annually.
When we remodeled, we replaced the tank with an instant hot water system. The plumber mentioned that it takes around 1 second per meter to travel the distance. In our case, it took about 15-20 seconds to go from one end to the other in our relatively small house. He also suggested we could add another one at the opposite end to split the workload in case it became a problem.
You need to fully turn it on to begin, and then you can lower it. There must be a flow for the heater to be activated. It’s also important to ensure that the shut-off is completely open.