It depends.
If the dehumidifier is not permanently attached to the property and only keeps the property is good working condition, then you will be able to expense the cost as a repair in the current year.
Also, if the dehumidifier costs less than $500 then you will also be able to expense it whether or not the item is considered a repair or an improvement (the De Minimis safe harbor rule).
However, if the dehumidifier is a betterment to the property and provides this benefit for more than one year, you will need to capitalize these expenses. You will be able to claim an annual depreciation expense over the life of the capitalized assets.
According to the IRS, expenses that may result in a betterment to your property include expenses for fixing a pre-existing defect or condition, enlarging or expanding your property, or increasing the capacity, strength, or quality of your property are considered expenses that must be capitalized.
If your dehumidifier is fixing pre-existing moisture damage, then it will be considered a betterment of the property and you will need to capitalize these expense.
Please refer to this link for more information about rental income and expenses: ttps://www.irs.gov/taxtopics/tc414.html
Additionally, click Depreciation of rental property for more information about depreciating a capital improvement.
Just a clarification: The threshold for depreciating is that it can be deducted in one year if it is $200 or less. If you QUALIFY for the 'De Minimis' election, that could allow you to deduct in one year items up to $2500 or less.
While I realize this is an old question, I find it interesting that both answers neglected to mention the Safe Harbor for Small Taxpayers which allows you to expense repairs that result in betterment (subject to restrictions of course). See this link for an explanation of it, along with the income, property value, and expense limits:
<a rel="nofollow" target="_blank" href="https://www.nolo.com/legal-encyclopedia/small-taxpayer-safe-harbor-for-repairs-improvements.html">https://www.nolo.com/legal-encyclopedia/small-taxpayer-safe-harbor-for-repairs-improvements.html</a>