I have 3 smoke detectors that are hardwired. One upstairs, one downstairs and one in the garage. All 3 were quite old and the upstairs started chirping every 90sec. Tried new battery and it still chirped. So I bought a a new Ei146e and replaced it. It still chirped. So I bought another Ei146e for downstairs thinking it may have a problem causing the upstairs to chirp, but he upstairs still chirped. So I thought I will just disconnect the garage and still upstairs chirped. So I then swapped upstairs with downstairs and upstairs still chirps and I also noticed both Ei146e red led flashes every 38 sec. I have no idea what to do next. All advice would be helpful. Thanks

Chirping every 90 seconds means low battery. If you have swapped two detectors (Upstairs with the downstairs detector) and the upstairs one still chirps every 90 seconds there is ANOTHER DETECTOR near by.

To prove this, remove both detectors from where they are and just place them on a surface in another room (Kitchen worktop, lounge table etc) These will not chirp as when an Aico detector is removed from its base, the battery is disconected. (Even if its not an Aico brand still carry on)

Go back upstairs and wait, you will then have to search for the chirp, it could be an old smoke detector, maybe a forgotten one or even a CO alarm. Maybe one in the loft?

Over the years there have been several cases just like this where detectors have been forgotten about, and a minority of people have come back to say that yes, they did find an old detector. I have been called out for the same thing and the common place is on top of a nearby cupboard or in the loft.