View Single Post 09-30-12, 02:23 PM #8 ecomodmonster Lurking Renovator   Join Date: Sep 2012 Location: England (stoke) Posts: 14 Thanks: 0 Thanked 2 Times in 2 Posts I am interested to see whether using a dehumidifier with the window closed is better than just opening the window to air out moisture from clothes drying. I have just done a rough calculation on this and think that dehumidifiers are slightly better that opening the window. Here is my reasoning based on robs great work, data from without hot air, and my own dehumidifier experience . There are lots of assumptions in it though: -Wet laundry has 2kg of water in it [1 page 54] -Latent heat of vaporisation (at 15ºC) = 2500kJ/kg [1 page 54] -Specific heat capacity of air is 1.2 kJ/m3/ºC [1 page 289] -Air changes per hour with window open = 4 [estimated based on 1 page 289] -Air changes per hour with window closed = 0.5 [1 page 289] -The laundry is put in a small room (23.2m3) over the radiator and next to the dehumidifier.  -My dehumidifier uses 2kwh and gives me 2kg of water. COP = 1.7  -Gas is 4p/kwh -Electric is 12p/kwh -It takes 12 hours to dry the clothes with the window open and the radiator on  -The temperature difference between inside and outside is dT = 10ºC -Gas boiler is 75% efficient Laundry on radiator (common to both scenarios) 2kg of water in laundry is evaporated by radiator E = 2 * 2500 / 3600 = 1.38 kwh Cost = 1.38 * 4 = 5.52p With dehumidifier 2kg of water is collected by dehumidifier and it uses 2kwh Cost = 2 * 12 = 24p Heat from condensation = 2 * 2500 / 3600 = 1.38 kwh Heat generated from dehumidifier = 3.38kwh With the window closed, energy lost to ventilation is: E = Volume * air changes per hour * time * Cp * dT E = 23.2 * 0.5 * 12 * 1.2 * 10 / 3600 = -0.46kwh Total heat generated = 2.92kwh Total cost = 5.52 + 24 = 29.52p With window open The air changes per hour increases to 4, energy lost to ventilation is: E = Volume * air changes per hour * time * Cp * dT E = 23.2 * 4 * 12 * 1.2 * 10 / 3600 = -3.71kwh To compare like with like the radiator must match the 2.92kwh of heat generation from the dehumidifier. The thermostat would ensure this. Total radiator heat used = 2.92 + 3.71 = 6.63kwh Cost of heat = 6.63 / 0.75 * 4 = 35.36p Total Cost = 5.52 + 35.36 = 40.88p So the dehumidifier is slightly better, but it is very close. Depending on assumptions you make. The heat exchanger would be a better option. Please feel free to point out the mistakes I will probably have made. Last edited by ecomodmonster; 09-30-12 at 02:32 PM..  The Following User Says Thank You to ecomodmonster For This Useful Post: