Today’s DIY project: sun shade for heating system temperature probe

By | June 11, 2023

My heating system has a little temperature probe wired from it to the outside of the house:

Small, black wire coming out of a hole in the white vinyl siding of a house, inserted into a small, grey metal bracket that is screwed into the siding.

I don’t understand exactly why this is needed, but my impression is that it has something to do with maximizing humidity inside the house without causing condensation to form on the windows.

Unfortunately, the installers put the sensor on the south side of the house, so it’s in direct sunlight for over half the day in the winter, completely throwing off its readings. We’ve seen it read more than 50°F hotter than the correct temperature.

I wanted to fix this by installing some sort of shade over the probe, but I could not find a prefabricated cover of the right shape and size. Then I had an astounding conversation with Inflection AI‘s “personal AI”, Pi:

Me: I have a weird question that I am wondering if you might be able to help with since I am coming up empty in traditional search engines. We have an outdoor thermocouple thermometer wired to our home’s thermostat. Unfortunately the people who installed it put it in a location where direct sunlight shines on it for half the day, making it read many (like often more then 30) degrees hotter than the real temperature. I know I need to install some sort of little shade over the thermometer to keep the direct sunlight off of it, but I can’t figure out what product name to search for to actually find the shade in online stores. Any suggestions?

Pi: That’s a really interesting problem! You’re right that direct sunlight will distort the reading of the thermometer. I think you might be looking for something called a radiation shield. It’s a little cover that goes over a thermometer to protect it from sunlight, rain, and other environmental factors. Does that sound like what you need?

Well, that term gets me closer than any other term I was able to come up with on my own, so thank you, but none of the search results look like they would work in the particular location I need to install the shield.

I hear you. It can be tough to find the perfect solution to a problem like this. What about just rigging up a little shade using some kind of inexpensive material, like a piece of fabric or cardboard? It might not look as nice, but it would get the job done and wouldn’t be too expensive.

It’s going to get rained and snowed on so fabric and cardboard won’t work.

True, those wouldn’t hold up well to the elements. You could try something like a piece of sheet metal, aluminum, or even plywood. Those would stand up to the rain and snow, and you could cut them to size and attach them with some basic hardware. Do you think that might work?

I suppose I could buy a small piece of sheet metal and some outdoor spray paint, bolt a couple of angle brackets onto the sheet metal, spray paint it all to match the siding, and screw it into the wall above the thermometer. But I don’t know how it where to buy a pre-sized piece of sheet metal with the edges turned under to avoid injury, and I don’t think I have the right equipment to cut sheet metal to size or turn under the edges properly.

Those are all valid concerns. I wonder if a hardware store might have a small piece of sheet metal that’s the right size, or if they would be able to cut one to size for you? As for the edges, you could try using some kind of tape or self-adhesive strip to cover them and make them safe. Would that work?

Maybe. Though I think tape or self adhesive strips would have the same weathering problem as fabric or cardboard.

You’re right, those wouldn’t last very long in the elements. I have another idea that might work. You could try using a piece of aluminum flashing. It’s thin, flexible aluminum that’s commonly used for roofing and waterproofing. It would stand up to the weather and it’s relatively easy to cut to size. Would that be something you could work with?

Actually, that might do it. Bend the flashing into a half cylinder with a lip on each side and use outdoor adhesive to glue the lips to the siding above and below the thermometer so that it is stable and there is shade but also air circulation. Thanks.

That’s a great idea! The half-cylinder would provide the shade you need, while still allowing for air circulation. I think you’ll need to use something stronger than outdoor adhesive, though. I would recommend using silicone caulk to attach the flashing to the siding. It would create a stronger bond and it’s waterproof, so it would stand up to the elements. Does that sound like a good plan?

Yeah actually I was thinking of silicon caulk.

Great minds think alike! I think you have a great plan for protecting your thermometer. As for the silicone caulk, you should be able to find that at most hardware stores. Once you have it and the aluminum flashing, all you’ll need is a few basic tools. Do you have a pair of tin snips and a caulking gun?

I do.

Perfect! This plan should be a piece of cake for you, then. Just cut the flashing to size with the tin snips, then apply the silicone caulk to the back and attach it to the siding. Make sure you follow the instructions on the caulk to ensure it cures properly. Once it’s dry, you should have a sturdy, waterproof cover for your thermometer!

Thanks for your help.

You’re very welcome! I’m always happy to brainstorm solutions to problems. That’s part of why I was built! I hope this solution works out for you.

I want to emphasize that the conversation above was between me and a computer program. We’re well beyond the Turing test at this point, folks. I’ve been working in tech for more than 30 years and I have no f’ing idea how an AI can be this good.

Anyway, off to the hardware store I went, to acquire caulk, aluminum flashing, and spraypaint. Here they are together with the tools needed for the job:

A can of semi-gloss white spray-paint, a 6" × 10' roll of aluminum flashing, a squeezable tube of indoor/outdoor weatherproof caulk, a chisel, a pair of tin shears, and a hammer.

I bought a squeezable tube of caulk because I didn’t want to deal with the caulking gun. And I didn’t end up needing the chisel shown in the photo; I thought I was going to need it to score the flashing to make it easier to fold, but it turns out it’s soft enough that wasn’t necessary.

Step 1: Cut open the flashing and cut a piece about 10″ long off the end of it with shears, yielding a piece that’s about 6″× 10″:

Flat piece of aluminium flashing with the remains of the mostly peeled off product label visible on it.

You can see here the remains of the product label which I peeled off the flashing. It’s OK that there’s a bit of paper and adhesive left because that’s going to be the inside surface of the cover so no one will see it.

Step 2: Fold over about 5/16″ of aluminum on each side, using a hammer to pound each side flat after folding it over:

Aluminum flashing with the edges folder over and pounded flat with a hammer.

This is necessary because the edges of the flashing are sharp so we want to protect our fingers when working on the project and the fingers of anybody else who has to come into contact with this cover in the future.

Step 3: Form the flashing into a half cylinder along its long axis and bend outward the doubled-over edges so that they will rest flat against the siding when the cover is mounted:

Step 4: Cut open the tube of caulk at the smallest line on the tip (to make a small bead) and apply caulk to the flat edges of the cover:

Inside view of cover with beads of caulk applied to both bent-out edges.

Step 5: Stick it onto the siding above and below the temperature probe:

Cover stuck to the side of the house over the temperature probe.

Step 6: Spray with spray-paint:

You might think I’d be done at this point, but alas, nope. I went back a half hour later to check on the cover and it had fallen off. Apparently the caulk didn’t get tacky enough fast enough to prevent even the slight weight of the cover plus paint from pulling away from the wall.

So, Step 7: Put on some masking tape (alas, I don’t have any blue painter’s tape) to hold the cover on until the caulk sets enough to do the job:

If I had realized this was going to be necessary, I would have held off on the painting until after the caulk had set and the tape was removed.

Step 8: Wait a day for the caulk to set.

Step 9: Remove the tape and tug on the cover to make sure it’s secure. Alas, when I removed the tape some of the paint came off with it, which wouldn’t have been a problem if as I mentioned above I’d waited until after the caulk was set to paint:

Therefore, Step 10: Get out the spray-paint again and do touch-up:

Step 11: Wait until the next sunny day to confirm that the cover is working as intended to prevent the temperature probe from getting much hotter than the actual outside temperature.

Success! Right now, with the full sun beating down on the house and the new cover, my phone says it’s 79°F in my area, my wireless digital thermometer says it’s 77° in the shade, and the thermometer connected to my thermostat and covered by the new cover says it’s 85°F. It’s not surprising that it’s as few degrees high—air circulation under the cover isn’t quite good enough to eliminate all the heat build-up—but that’s 20°–30° lower than it have been without the cover.

If you stumbled upon this article and it helped you solve a similar problem for yourself, add a comment and let me know!

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2 thoughts on “Today’s DIY project: sun shade for heating system temperature probe

  1. Tom

    Mostly commenting in respond to your Mastodon request for comments 😀

    Odds are your flashing will fall off in winter. If it does, I recommend trimming the folded edge at the top (including the top half inch of the side-folds) so you have a sharp edge there, and jamming it up under the siding that it’s now abutting. Folded, it’s likely too thick to do this. Alternatively, a single short (1/2 or 5/8) screw would do the trick.

    Glad it’s working so well!

    1. jik Post author

      For the benefit of other readers, I asked Tom on Mastodon why he thought the flashing would fall off in winter, and he said:

      Thermal contraction of the metal will strain the adhesive, and depending on how wet the snow is some may stick. The space underneath the flashing will be marginally warmer, which will help stick snow when the temperature is close to freezing. And mostly I don’t trust adhesives! So wait and see, and see what happens. I could very well be wrong.

      To which I replied:

      I don’t think contraction will be a significant enough issue to matter, both because the metal won’t contract enough and because silicon adhesive is designed to expand and contract. You may be right about the sticking, though. We’ll have to wait and see!


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