plumbingavoidance

I have this same problem... no hot water pressure in the shower... first it was the bathroom sink, now the sink is fine. The faucet setup is two knobs. There is no discernable information on them. They look pretty old - definitely nothing fancy. What is a mixing valve? I live in a condo... is this important in the scheme of things? The water pressure is fine in the rest of the condo. Though it seems the water pressure has been generally inconsistent over the four years I have lived here and in different areas. Thanks! Tracey