Sorry to dig up an old part of the thread here, but I discovered something in my experimentation that it may be helpful for you to know. At least on our FTC6 unit, when you activate “Force DHW/Heat Now”, the system seems to ignore the “Eco” DHW setting, and instead heats in the more aggressive “Normal” mode.
My take-away from this is that it’s better to leave the DHW enabled at all times you might want it, and then to suppress it under software control by reducing the tank target temperature when you don’t want it to run. I haven’t actually implemented this yet, though!