I have had a whole house tankless hot water heater for a couple of years that is probably working as designed: once the water heats up and travels to the faucet, it will keep providing hot water forever. Unfortunately, one of the longest runs is to the kitchen sink and it's a pain waiting for the water to heat up when washing dishes. Can I put a small tankless unit under the sink? If so would I just cap off the hot feed and branch off the cold feed through the new tankless to form a new hot feed for that one sink? (I wouldn't want to come off the hot feed because that would turn on the whole house unit, right?) If that won't work, is there a better solution? Thanks.
You can do a local demand unit as you describe. They also make demand units that are designed to be used in conjunction with the whole house unit. When working in conjunction, the local unit heats water only until it senses hot water arriving from the whole house unit.