We've just had a our pressure regulator replaced, and a new recirculating pump installed. All 3 showers used to get hot water reasonably quickly (15-20 seconds). Now the shower furthest from the water heater won't heat up unless a shower or hot faucet in another bathroom is run. It just runs luke warm at best. The hot faucet in a sink in the same bathroom is the same - just luke warm, but no problem with flow rate.
What is stopping hot water reaching the shower, unless another bathroom's hot water is on? Doesn't sound like an air blockage, and it seems that water is still flowing in the shower through the hot pipe, because it gets colder when the temp control knob is turned to "cold". Weird and annoying!