Electronic Alarms and Home Security Devices - Smoke detector problem

The smoke detectors in our home are 6 years old. We have a chirping problem.
I have replaced all the batteries but still have the chirping problem.
The detectors are hard wired with 9V battery back-ups.
I currently have them off at the breaker panel. ( not good I know).
When I push the test button, not all of them will chirp, some do and some don't. If, when I push the test button should the alarm chirp and if it doesnt should I replace the detector? Also, I was under the impression that if one alarm began to chirp all of them should begin chirping....true or not?
I don't know anything else to do to trouble shoot this problem. Other than the test button on the detector is there any other way to determine if the detector has failed?
Any help will sure be appreciated.
Thanks.

Normally the chirp is low battery, but some models also do this when the sensing chamber is dirty. Only the unit with the low battery/dirty chamber chirps. The interconnnect only kicks in if they actually alarm.

At 6 years old, especially if they haven't been getting at least an annual cleaning, I would put it in my budget to replace them all. The replacements will run 15-20 $US at any of the big box stores.

Should all smoke detectors be replaced at the same time?
Should they all be the same kind?
Thanks

Unless your detectors are connected together within a system brands could be different but basically you do need to replace them all.There is no advantage to mixing brands so I'd shop around for a good price and buy all the same.

This is a common problem with smoke alarms.Six years is on the short end of normal.Often you can get ten or so but not always.If someone smokes,you cook alot especially foods with high grease content etc or have a heating system that uses oil or gas these all shorten alarm lifespans.Significant fireplace or kereosene heater use also can contribute.Anything that creates residue that has the ability to build up.

If you do use oil or gas or heaters you might want to change to combo smoke and CO2 detectors.

Any suggestions on the most user friendly detectors to buy?
For example those that we currently have must be taken down to replace the battery and it is not always easy to replace the battery because they don't always fit in properly without a lot of hassel to get them in.
As long as I'm replacing them I'd want to find one that is most efficent.
:):).
Thanks...

Take a look at the Kidde and Firex models. Both brands have several models with top and side load battery compartments.