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Musings of an Energy Nerd

Mouse Mystery

Dozens of mice were entering our house. But how?

Mus musculus, the house mouse, is a common indoor rodent. However, in Vermont, Peromyscus leucopus, the white-footed mouse, is seen more often than Mus musculus. Most homeowners prefer their homes to be tight enough to exclude both species. [Photo credit: Wikimedia Commons]

If you live in a rural area, you’re probably accustomed to seeing the occasional mouse in your kitchen. Here in Vermont, the most common time to see an indoor mouse is in the fall, when outdoor mice are in search of a warm, dry location to spend the winter.

When I spotted a mouse in our house in late September, I wasn’t particularly concerned. I set out a few mousetraps. When I caught a mouse, I assumed the problem was solved. But I soon saw more evidence of mice, so I set out more traps. I caught more mice—yet there were still mice in our house.

After catching more than a dozen mice, we realized we had an unusual situation on our hands. What was going on?

We search for clues

Karyn and I put on our detective hats and started gathering data. Our first big break in the case came when I was standing near our on-demand propane-fired water heater, and I happened to notice a mouse running up a pipe and then disappearing through a hole in the bottom of the heater. Yes—I saw a mouse enter the wall-mounted water heater.

Was there a mouse nest inside our three-year-old Rinnai RL75iP? To investigate, I took off the cover and looked inside. No—there was no mouse nest in there. Everything looked clean. So where was the mouse headed?

The next breakthrough in our case came after Karyn suggested an ingenious way to gather data: we would use our wildlife camera—the motion-activated camera that we usually attach to a tree in the woods, in order to photograph deer, moose, bear, and bobcats. Before we went to bed, we set up the wildlife camera indoors, in hopes of photographing any unusual activity near our water…

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  1. charlie_sullivan | | #1

    Thanks for the entertaining and educational story. Congratulations on solving the mystery.

    I'm confused, though, about the hole in the water heater. Isn't it supposed to be a "sealed combustion" unit? Why is there a mouse-size hold from the conditioned space into the combustion air supply?

    1. GBA Editor
      Martin Holladay | | #2

      I tried to determine how the mice were traveling inside the water heater -- in other words, to solve the maze -- but I failed, for the simple reason that I am much larger than a mouse.

      The fact that the Rinnai technical specialists are aware of the problem, and were quick to suggest the purchase and installation of the Ubbink termination screen, is strong evidence that Rinnai engineers know that there is a pathway for rodents to travel through the air intake openings.

  2. kevin_in_denver | | #3

    Do you have a wildlife camera recommendation?

    1. GBA Editor
      Martin Holladay | | #4

      We have a simple camera made by Bushnell. Cost about $100 at Walmart.

  3. jameshowison | | #5

    I wonder if a certification of a house as "bug-proof", proved by a blower door test (Hey it's 1 ACH50 bug-proof), would be a better way to change what home buyers demand.

    After all, we need air (so is air-tightness a thing that people emotionally respond to?) but ewwwww, bugs :)

    1. GBA Editor
      Martin Holladay | | #6

      While very tight homes are far less likely to have mice than leaky homes, calling a new home "bug-proof" is an invitation to litigation. My prediction: builders won't go there.

      (An aside: most dictionaries define "bug" as "a small insect." Mice, of course, are mammals.)

      1. maine_tyler | | #7

        I have a friend that refers to cats as 'mosquitoes,' apparently because they 'bug' her.

  4. john_prospect | | #8

    Martin, I love your dry sense of humor.

    Thank-you for continuing to write articles here on GBA!


    An excellent detail that I forgot for my build and my planning of my natural gas water heater. Do you know if Rheem also has that kind of problem?

    1. GBA Editor
      Martin Holladay | | #10

      I don't know if Rheem water heaters have this problem. Two factors are necessary for the problem to exist: (1) There must be mouse-sized holes (or bigger-than-mouse-sized holes) in the air intake pipe termination, and (2) There must be a mouse-sized travel path from the air intake to a hole in the water heater cover.

      As Charlie Sullivan pointed out, since this type of water heater is advertised as being a sealed-combustion heater, the travel path taken by the mice remains mysterious.

      1. CCM_CONSTRUCTION | | #11

        Thx Martin. I will take a closer look and post it here if I found out if Rheem is similarly affected by the same problem.

  6. baking_fool | | #12

    Mr. Holladay - This is totally off topic, but I see very few options at this point. I have an account that is 5 years old, that I upgraded to GBA Prime 3 days ago, and also a new account that I created 2 days ago due to inability to post a new topic. With either account I receive one of two messages when I attempt to post a new topic - "Critical error on this website" or "Validation error you need to wait more time to post a message." I see no quoted time to wait, and I saw another user with a similar problem a week ago. Is this a bug, or because it is the weekend/holiday and someone needs to approve my account? If you know and can explain I would greatly appreciate it. I have tried posting at least 20 times and I am very aggravated because this is a tremendous resource. Thank you.

    1. GBA Editor
      Kiley Jacques | | #13

      Hi Don,

      I want you to know that I have contacted the web development team about your problem and I hope to get it resolved quickly. Feel free to contact me directly with any questions or updates on the situation: [email protected].

      1. baking_fool | | #14

        Thank you Kiley. The problem persists this morning. I hated to post this here, but was hopeful I could gain attention to the issue and possibly this could help others. You have very knowledgeable people in the community and I am eager to post my topic.

  7. bob_swinburne | | #15

    My cat catches mice outside and brings them in to torture them. Sometimes they escape.

    1. Expert Member
      MALCOLM TAYLOR | | #16

      And live under the fridge surviving on cat food.

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