Hi, moved into a house that has 6x  Ei141RC smoke alarms,  1x Ei144RC heat alarms and also 2x Ei261ENRC CO alarms. The are all wired for L, N and IC. However testing the CO by the push button, none of the smoke alarms sound (The unit itself alarms as if it is standalone). Are the models compatible or just the IC not working? All other alarms work when testing by the push button. 

I would suggest you check the date on all of your detectors, as they are no longer made, and may have passed their working life of 10 years.

If you do decide to change them, the good news is that all Aico smoke alarms use the same easifit base, except for the CO alarms which will need the new easyfit base. (The same as the other bases you already have)