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Old 02-28-2012, 08:55 PM   #1
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How hot does it typically get in the main plenum of a furnace?


And can it melt electronic components/plastic? I'm working on this thermostat project that will have probes around the house and I'm thinking of putting one inside the furnace plenum. Just not sure if this would be safe or not. The sensor can measure up to 125C. I'm also thinking I could maybe just put it right up against the plenum, though that is not as accurate as putting it inside.

I'm also wondering what would be the best bonding/insulation material for the components. Basically it's just the probe itself with 3 legs and a resistor. I can probably arrange so only the probe itself sticks into the plenum, but I need a bonding material that is non conductive so I can fill the gap between the "legs" so to speak. I'm guessing caulk wont do. What about heat resistant caulk? I'm even thinking cement but I doubt that will bond well on the plenum. Whatever I use, it only needs to be like a square cm worth, this is going to be a tiny hole. I'm even thinking of just using electrical tape as insulation for the legs then duct tape to hold the sensor in place (the real duct tape, shiny stuff). Any other ideas?

I sticked a thermometer inside one of the ducts that's the closest to the furnace (literally like a 3 foot run directly from the plenum) and I'm getting a reading of around 52 degrees, so I'm thinking I should be fine.

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Old 02-28-2012, 09:10 PM   #2
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How hot does it typically get in the main plenum of a furnace?


Typical supply air is 50-70 degrees hotter than the return. Silicone is good up to 400 degrees.

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Old 02-28-2012, 09:13 PM   #3
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How hot does it typically get in the main plenum of a furnace?


Wow, so silicone should do fine in there then. That will make life easier. Now is it considered safe, or could it put off hazardous vapors or have other safety issues? I think what I'll do is drill a small hole, put silicone on the probe and pop it through said hole, using duct tape to hold the wiring in place on the outside.
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Old 02-28-2012, 09:27 PM   #4
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How hot does it typically get in the main plenum of a furnace?


Maybe we should ask red squirrel what age and style of furnace he has just in case its old (with plenum temp potential beyond what his sensor is rated for.
The other bit of advise is not to put anything into the plenum that your not prepared to have fall off into the exchanger.
Nothing like the smell of melted rubber or plastic throughout the home for months to remind one of what a mistake that was..
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Old 02-28-2012, 09:49 PM   #5
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How hot does it typically get in the main plenum of a furnace?


The furnace is a Bryant high efficiency (two PVC pipes going outside) that is fairly new. Under 10 years I'm pretty sure. I can probably open it up and get the full model number if needed.

I was also thinking of the possible danger of having it fall into the heat exchanger. Would be a pain having to retrieve that. I might stick it in one of the main ducts that way it has nowhere to fall.
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Old 02-28-2012, 10:02 PM   #6
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How hot does it typically get in the main plenum of a furnace?


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And can it melt electronic components/plastic? I'm working on this thermostat project that will have probes around the houset

OK..you have got me a bit curious, since I love electronic projects....what exactly are you working on, if I may ask?
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Old 02-28-2012, 10:05 PM   #7
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How hot does it typically get in the main plenum of a furnace?


I would use RTV caulk. Yes, a lot of caulks can are "rated" to 400* but they harden too.

RTV is used to make head gaskets on muscle cars. And it does not harden at high temps.
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Old 02-28-2012, 10:08 PM   #8
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How hot does it typically get in the main plenum of a furnace?


Just wanted to make sure it wasn't an older atmospheric. Your plenum temp will not be a problem. Getting out something dropped into the exchanger will involve cutting open the plenum. Something small and meltable in the exchanger though could just be a nightmare.

In the main duct beside the plenum sounds trouble free to me.
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Old 02-28-2012, 10:50 PM   #9
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How hot does it typically get in the main plenum of a furnace?


Good to hear, it sounds like it should be good. I'll see if I can find this RTV caulk, if not, I might just settle for the silicone, I can afford for it to harden, as long as it does not come apart. Another thing I thought of is if ever I get my ducts cleaned they might knock the sensor, though I can always just pull it out before.

As for the project, this is what I'm working on. It's almost done, I just need to run the system through some more simulation testing (I put the sensor and a light bulb in a bucket) and ensure that it runs trouble free before I put it "live". I will probably run some of the wiring tomorrow.
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Old 02-29-2012, 04:23 AM   #10
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How hot does it typically get in the main plenum of a furnace?


If the sensor assembly is rated up to 225C. Then no problem in a plenum.
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Old 02-29-2012, 05:18 AM   #11
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How hot does it typically get in the main plenum of a furnace?


Get RTV at any auto parts place. They have many varieties marketed for various applications but any of them should work for you. Permatex is one major brand.

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Good to hear, it sounds like it should be good. I'll see if I can find this RTV caulk, .
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Old 02-29-2012, 07:23 AM   #12
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How hot does it typically get in the main plenum of a furnace?


put it downstream from the plenum. if in the plenum and in sight of the heat exchanger you will get a false reading from radiation/radiant heat from the exchanger. if you want the true air temp then it should be out of sight of the exchanger. I had that on a zoning system where I ended up putting the discharge air sensor downstream as it would not sense properly and tech support at Lennox advised that 2. Can Tire has small tubes of Permatex RTV silicone in the auto supplies area.
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Old 02-29-2012, 01:08 PM   #13
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How hot does it typically get in the main plenum of a furnace?


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put it downstream from the plenum. if in the plenum and in sight of the heat exchanger you will get a false reading from radiation/radiant heat from the exchanger. if you want the true air temp then it should be out of sight of the exchanger. I had that on a zoning system where I ended up putting the discharge air sensor downstream as it would not sense properly and tech support at Lennox advised that 2. Can Tire has small tubes of Permatex RTV silicone in the auto supplies area.
That's a good point, thanks. I'll put it in one of the main ducts maybe 2-3 feet away from the vertical plenum.
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Old 02-29-2012, 01:22 PM   #14
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How hot does it typically get in the main plenum of a furnace?


another point is that it can take up to 10 mins B4 the supply air temp peaks as the ducts are heating up etc. on that furnace I would doubt it will hit 140 F. probably in the 130-135F range. if higher then you can damage the heat exchanger. if it works you can develop your own niche market and sell the plans.

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Old 02-29-2012, 02:05 PM   #15
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How hot does it typically get in the main plenum of a furnace?


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Originally Posted by hvac5646 View Post
I would use RTV caulk. Yes, a lot of caulks can are "rated" to 400* but they harden too.

RTV is used to make head gaskets on muscle cars. And it does not harden at high temps.

you dont use any kind of sealer on head gaskets.

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