who pays the costs of a dehumidifier?

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The cheaper dehumidifiers go on trying even at low humidity and can waste electricity. You can get them with a humidity control but they cost more and would require a certain intelligence to adjust.
I am trying one of the continuous ones on a segment timer for two hours twice daily in one flat and would calculate the electricity to be about 5 pounds monthly. As the tenants are saving on heat by not ventilating and heating this should be cheaper than living in a damp fug with water dripping off the windows!
I would guess that usually tenants with condensation are drying laundry indoors with the windows closed. You can get away with that in a drafty house but not in a flat!