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I am going to replace all the old hardwired smoke detectors in my house.

In my research I have found some of the features available include 10-year batteries and dual sensor types (photoelectric & ionization) .

However the 10 year battery doesn't seem to be available in a hardwired detector.

Would it make sense to switch to the 10 year battery model, which means not using the pre-existing wiring in my house? Even with a hardwired system there are batteries to replace annually, so moving to the long life battery would be nice. But I feel like it might be silly to ignore the hardwired setup that I already have.

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    Are your smoke detectors interconnected? – ThreePhaseEel Nov 26 '16 at 15:51
  • I believe they are - during false alarms all the detectors in the house sound their alarms. – ePhrygian Nov 29 '16 at 11:57
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I strongly recommend continuing with the AC with battery back-up models. They are required by code for new construction (in my area) for a very good reason- the potential for someone to remove the battery. Also, they are available with a wider range of features, such as described in your question.

This battery removal behavior is very common in rental units. So much so that landlords now are obliged to pay for replacement batteries, to remove any financial incentive for not replacing them.

Even if you will personally be in control of the units, you might decide to remove the battery for some reason, then forget to put it back. Most of the AC units let you know (with an annoying chirp) that the battery back-up is dead or is not installed, whereas "battery only" units just sit there dead with no indication to the user that it is not functioning.

10 year batteries are available for you to install in your AC w/battery back-up units:

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To add to ThreePhaseEel's comment, the best answer here lies with whether your smoke detectors are interconnected or not. If they are interconnected, then it would be advantageous to keep that system, so that when one alarm detects smoke, all of them alarm.

If not, go ahead with the 10 year model. Note that the 10 year models are designed to be completely replaced at the 10 year point.

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    All detectors should be replaced at that point. The effectiveness of the sensors degrades over time. – isherwood Nov 26 '16 at 18:03
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Interconnectability is a huge advantage and the wiring in place will (hopefully) facilitate that. Hard wired does not necessarily mean interconnected. My electrician disregarded my instructions to interconnect but he did hard wire. Grrrrrrr In regard to battery units versus hard wired - It seems sensible to use the hard wiring in place. Battery removal is a discipline matter. You may come up against the issue of photo electric versus ionisation. The radioactivity is so small it is not an issue. Politically correct organisations and individuals will generally promote "photo electric' but their main motive (IMHO) is to avoid critisism from generally il-informed people. Ionisation and photo electric work differently but the performance is not huge. Some people simplify the difference by saying "photo electric" see and ionisation - smell. false alarms are in some circumstances less with photo electric. I believe that the odd alarm caused by genuine smoke is a good thing because it re assures you that the alarm is working. some people however feel it necessary to attack the smoke alarm with a broom on such occassions. I think if all else is equal I would go for photo electric but don't lose sleep over the choice. I know of an organisation that installed many hundreds of 10 year alarms which lasted between 1 and 4 years before they needed batteries changed. The problem was they were biased towards a cheap as possible solution. I qualify this answer by declaring - I am from New Zealand so there is a possibility of local differences.

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