Boiler Hot Water Issue

We are in a 7 unit coop experiencing lack of hot water, low water pressure and dirty water. Also, top floor seems to have air in pipes with faucets sputtering when turned on – they also have the most complaints about low water temp and pressure. We believe this may be a problem with the mixing valve which we are prepared to have serviced. Q: Is the air in pipes connected to hot water issue? We called the city regarding the sludgy water but they found nothing in the area. We have a Smith cast-iron 19A-6 Dual tankless heater.