My boiler is flashing red light every few days. If I won't switch it on for few days its on for around two hours and then the lights is going red for another few days. I don't know what could be wrong. I had heating engineer coming and have a look a this but they are not sure what the problem is. They desc ale the pipes and has changed the little divided bottle inside the boiler( sorry I don't know what's names the part), they have cleaned the heat exchanger as well, but is still going back to red. It costed me so much money already and it still doesn't work. They want to change the heat exchanger but they are not sure if that's the problem. I know they want all the best but I need second opinion. Could you please advise? Thank you