So I live in a duplex with one bathroom down (this has the hot water heater) and 2 bathrooms up. All the taps are single handle type.
The kitchen gets no hot water until I run the tap in the downstairs bathroom - then it gets super hot and I can't get any cold water out of the tap. This downstairs bathroom sink tap seems to control the temp for the entire house - run it cold and the rest of the house just gets hot, run it hot and everywhere else is cold. Additionally if I run the tiniest shot from the kitchen sink while the upstairs shower is running and it is scalding for a couple minutes.
Driving me nuts - and I know NOTHING about plumbing.
What happens if you run the kitchen tap for a much longer time without running the downstairs bathroom tap first?
You need to check the taps themselves for possible internal clogging, or malfunction of antiscalding components (if any) inside.
Don't forget to turn the water off before taking anything apart.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.