Tony Hwang wrote:
Hi,
Test button test is not real test. Use real smoke and flame(candle) to
test it.
Or spray made for testing. Commercial guys use it.
I replace them every 10 years. Mine are hard wired with 10 year Li.
battery back up. I do same with gas detectors.

So, do you replace them because:
a) It's been ten years, or
b) Because they failed the smoke-in-a-can test?