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Currently, my home's smoke detector is situated right outside our bathroom in the hall. Whenever someone takes a hot shower (usually me), the steam escaping the bathroom trips the detector, presumably because of heat.

I would like to replace this detector, but I want to be sure that what I replace it with is only detecting the presence of smoke. Not heat.

I was looking at this model at Lowes yesterday, but wasn't clear if it was focused upon just smoke or if it would also trip due to smoke. The manufacturer's text includes, "Photoelectric sensor detects smoke while minimizing false alarms."

Does this mean that the detector will only focus upon smoke or will it also trigger due to heat?

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    Are you sure it's the heat it's detecting and not the steam? Maybe what you need is a better exhaust fan in your bathroom... – brhans Oct 9 '20 at 14:34
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    Photoelectric detectors are commonly used in areas where conventional alarms trigger due to cooking / steam etc. The blurb says "Photoelectric sensor detects smoke while minimizing false alarm", so it should be OK. You may still get the odd false trigger if the steam is really bad. – SiHa Oct 9 '20 at 14:35
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    Your presumption is very probably dead wrong. Steam blocks light just as smoke does. and +heat detectors are an extra, not the standard run of the mill. You need a working exhaust fan. Even the lowest "heat" only or +heat detectors would require the dector to be at "scalding" temperatures to activate, which is highly improbable. – Ecnerwal Oct 9 '20 at 14:41
  • While there's a misconception, it's still a good question (I'm also confident it's triggered by steam, but you could test that with a hairdryer to give dry heat hotter than the air would get near your shower - so much hotter that you'd need to be gentle with it – Chris H Oct 9 '20 at 14:49
  • The detectors used to detect heat are normally based on « rate of rise » and are used in hotter environments such as some professional kitchens. – Solar Mike Oct 10 '20 at 15:43
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TL;DR Your older detector, statistically speaking, is likely ionization, so your newer detector, which is photoelectric (both because you said so and due to current market conditions as explained below) may do better because it is different or simply because it is newer. But either way, the problem is likely steam and not heat because standard $ 10 - $ 50 residential smoke detectors do not detect heat.


Most residential smoke detectors do not detect heat. That is one advantage of sprinklers (aside from the fact that sprinklers actively protect people & property, rather than just notifying you of a problem) - sprinklers have a fusible link that melts and lets the water through - no electronics involved. Sprinklers are required in many locations for new construction of all homes and/or for renovation of multi-family homes. But that is a separate topic. Back to smoke detectors. There are two basic types:

  • Ionization

  • Photoelectric

You can find a reasonably good explanation of the differences on Wikipedia.

But the short answer is that both types detect smoke, not heat, but they do it using different technology. Each one is better/faster at detecting different types of fires.

I just replaced one of my smoke detectors and the replacement is a photoelectric model. I found that unusual because for many years, ionization models have been the norm. In fact, my understanding is that the "10 year smoke detector rule" - instead of replaceable batteries, all residential battery powered smoke detectors now have a built-in maximum lifetime of 10 years - is based on two factors:

  • Practical - A lot of people take out the battery when the low battery chirps in the middle of the night and forget about replacing the battery until it is too late.

  • Technical - Ionization detectors are based on a teeny bit of radioactive material which, due to the laws of physics gradually becomes less effective. It doesn't matter whether there are any fires, ambient temperature & humidity (which affect electronics in general), or whether the unit has ever been turned on.

I am not sure how much of the current debate over ionization detectors is based on "Oh no, you want me to put a radioactive device in my house! The horror! We'll all become zombies/Godzilla! We're doomed!" - to which I can say "Let's ban bananas next!" and refer you to the obligatory XKCD (thank you @spikey_richie), vs. an actual scientific analysis of which detectors work best overall when it comes to saving lives and property. But the end result is that several US states have banned or restricted ionization detectors (or alternatively, required photoelectric detectors) and major manufacturers and retailers far prefer to manufacture and sell a single product line across the entire country, so ionization detectors are rapidly disappearing from store shelves.

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  • So any idea which type of smoke detector is less susceptible to false alarms due to steam/humidity? I think that was the root question asked. – PhilippNagel Oct 9 '20 at 15:33
  • And the honest answer is: I'm not sure. But I'll add a TL;DR. – manassehkatz-Moving 2 Codidact Oct 9 '20 at 15:43
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    Photoelectric detectors are less susceptible to false alarms from steam, and are a good choice for outside bathrooms and kitchens. – user3757614 Oct 9 '20 at 16:14
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    I had to post this, even though it doesn't contribute to the answer. As they say, there's an XKCD for everything... xkcd.com/radiation – spikey_richie Oct 9 '20 at 16:55
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    The issue is that ionization detectors struggle a bit with slow smoldering fires, and are harder to "tune" than modern photoelectric detectors, which can use multi-wavelength sensing to be more...discriminatory between alarm conditions, nuisance smoke, and other particulates or mists/fogs. (This matters partly because the latest edition of UL217 added a nuisance-cooking-smoke-resistance test to the standard test protocol) – ThreePhaseEel Oct 10 '20 at 3:04

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