Ghana

My daughter has an Apollo water system. No hot water in the winter at the faucets. I turned the mixing valve, no change. We have turned the water tank temp up, but no change. When I close off the heat exchanger valve, then we get hot water at faucets. What is the problem? My daughter has a plumber coming in Friday to look at it, but she can't afford it. I am trying to fix on my own before the plumber gets here.