Temperature In Junction Box Buried Under R49 Insulation (heat Detectors/smoke Alarm) - Electrical - DIY Chatroom Home Improvement Forum

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Old 01-16-2012, 03:34 PM   #1
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Temperature in junction box buried under R49 insulation (heat detectors/smoke alarm)

Ordered a pair of Kidde HD200F / 602 heat detectors that go off at 200F. They're designed for unfinished attic usage.

House is one story with a basement and an attic.

Also ordered a pair of relay / power supply modules for these. These heat detectors only act as switches, they don't have alarms themselves. So, I've also purchased a single hardwired Kidde smoke alarm, which these detectors connect to, to sound an alarm. (Existing smoke alarms are just 9v, not hardwired, we aren't going to replace them all right now.)

The relays are going to be near the smoke alarm, not the detectors. This will keep the relays at lower operating temperatures.

Near the smoke alarm, but not necessarily in the box the alarm is connected to, I am going to have:
* Unswitched 12/2g cable supplying power
* (3) 18AWG wires pre-attached to smoke detector
* (2) relay modules (about 1" x 3" with 7 18AWG pre-attached pigtail wires that are about 8" each, even though in my situation I'm only using 3 wires.)
* (2) 12/2g cables, one going to each heat detector in the attic

Pretty sure that's WAY too much to fit in a nice circular mount box for the smoke detector. Not sure what my best option is.

I was thinking that I could have a regular sized circular ceiling mount box for the smoke detector, and have a large enough junction box within a foot or so of it within the attic containing everything else.

The product literature from Kidde for the relay/power supply says: "Do not exceed the temperature or humidity limits of +40F to 100F and 90% relative humidity for either the relay module or the alarms."

I called Kidde product support, and the representative seemed to know what he was talking about. I asked him if he thought the temperature in a junction box in the attic under R49 insulation would be an OK place to keep the relay modules, or if I needed to mount them in a wall instead, which wouldn't look as nice because the smoke detector wouldn't cover up the sized junction box that I would need, so I'd wind up probably having an oversized metal cover on it.

He said that attic temperatures didn't actually affect the relays, and that I could mount it in the attic like I was planning without an issue.

[If it helps point you in the right direction for temperature requirements, each relay is going to be using three wires: a black hot comes in from the heat detector when it trips, there's a white neutral, and there's an outgoing gray wire that the relay sends 5-10V DC out to the smoke detector to sound the alarm. So, the relay basically converts 120V AC when it receives it to 5-10V DC.]

I'd love to go with what he said, but someone wrote the literature the way they did, saying don't go above 100F. (Maybe they wrote it wrong, or too conservatively...)

My question is what you guys would recommend doing, and if anyone had a good idea what temperature a junction box would be that was going to be buried under R49 insulation. I'm in Southeast Michigan. All my other junction boxes in the attic are well above where the insulation will get to, but I figure this one might need to be buried to give it lower temperatures.


Last edited by darlingm; 01-16-2012 at 03:39 PM.
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Old 01-16-2012, 03:43 PM   #2
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I don't know about your attic, but mine gets up to around 140 on a nice warm Summer day. I would go by what the manufacturer suggests. If the equipment is not made for use in that space, do not use it. BTW, you really do not need a heat detector up there, which really is overkill. Save them for use in your basement or garage.


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Old 01-16-2012, 03:55 PM   #3
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Yeah, I figure the attic will probably get up to around 140F. Just trying to figure out if a junction box under R49 insulation in a 140F attic would stay at a low enough temperature. I am pretty heat sensitive so run the A/C hard. House never gets above 70-72F.

I know it's definitely overkill. My wife's grandparents house had an attic fire. My father-in-law's house had an attic fire. Our own backyard was on fire for 72 hours last summer due to a primary transmission line being down from a storm combined with the power company's incompetency to get the line turned off quicker than 72 hours. So, we're happy spending the $65 in this situation and the time to hook it up to do the overkill.

A different possibility that I just thought of is that the smoke detector is going to be right near our hallway closet. Behind the closet is easy to access right now because it's a void above stairs going to be basement -- so it's an open cavity up in the attic. Going to be sealing that off once electrical inspection is done, but I could always put everything in a box behind the closet and leave a blank cover on it. Should keep it cool, and remain accessible, without being ugly.
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