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I'm planning to install several temperature sensors in my hot-air furnace's ductwork, specifically in the return duct and in the supply plenum of each of our two furnaces. The sensors I plan to use are 7mm in diameter; I'm looking for a way to get them into the ducts without a) leaking air and b) looking crappy.

So far, my best thought is a 5/16" or 3/8" hole drilled and sealed up with foil ("real") duct tape; are there other ways that might be cleaner-looking?

This is mostly a "because it seems cool" project; planning to use a Raspberry Pi to capture the data and add it to a small home status dashboard I have set up. It will also give some insight into how the furnaces and A/C are performing.

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2 Answers 2

up vote 2 down vote accepted

If you can't get to both sides, you could drill a 1/2" hole (which may be tricky) and then use a knock out plug that is designed to snap in from one side.

You could then use something like the strain relief described in gregmac's answer to mount the thermo couple on the plug and then snap it in and seal the edge with foil tape.

Knock out plug

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I like that it would be easy to remove the sensor and replace it with a blank knockout. I can experiment with using gregmac's suggestion to make the seal around the cable airtight (I'm guessing I may need a little silicone under the edge of the knockout to make sure it's tight, too). –  TomG Feb 7 '13 at 2:43
    
Next challenge -- drilling a 1/2" hole. I'll have to experiment on some spare stock to see if I can get a clean hole. –  TomG Feb 7 '13 at 2:44
1  
@TomG You might try a step bit: amazon.com/Neiko-Titanium-Step-Drill-Bit/dp/B000FZ2UOY/… –  Josh Bush Feb 7 '13 at 14:26

I would probably use a strain relief connector (aka dome connector):

enter image description here

There have a rubber grommet in the middle that compresses down around the wire when you tighten it, forming an air-tight seal. Some also have a gasket around the nut at the bottom. These are really easy to install (just require a 3/8" hole), pretty cheap, and available at the box stores.

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As I understand how these work you need access to both sides of the hole which may not be easy if you are installing into an existing sealed duct. –  Craig Feb 6 '13 at 18:27
    
@Craig good point, I hadn't thought of that. If the thread pitch is right it may be possible to just screw it in, but I have doubts that work work really well with sheet metal. Perhaps it would work combined with some silicone to seal it (rather than a gasket). Ideally of course, you have access to the other side to put the nut on it. –  gregmac Feb 6 '13 at 19:24
    
+1 Great idea, but I don't have access to the back side. I may end up combining this with Craig's knockout plug idea. –  TomG Feb 7 '13 at 2:41

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