no heat

problem with the heat (little heat). My mechanic first said there was problem with the thermostat, and he changed it, but did not solve the problem. Then he checked the two pipes under the hood, and found the heat-in pipe is hot, but the heat-out pipe is cold, and he said that means the system produces the heat but the system is clogged somewhere, and it needs a kind of power pressure machine to pressure it through. Is it correct? Any information or advice is appreciated. Lee