EcoRenovator - View Single Post - block up all air vents and use a dehumidifier to fight mold?

ecomodmonster · 09-30-12, 02:23 PM

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[1], and my own dehumidifier experience [2].

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. [2]
-My dehumidifier uses 2kwh and gives me 2kg of water. COP = 1.7 [2]
-Gas is 4p/kwh
-Electric is 12p/kwh
-It takes 12 hours to dry the clothes with the window open and the radiator on [2]
-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.

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[1], and my own dehumidifier experience [2]. 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. [2] -My dehumidifier uses 2kwh and gives me 2kg of water. COP = 1.7 [2] -Gas is 4p/kwh -Electric is 12p/kwh -It takes 12 hours to dry the clothes with the window open and the radiator on [2] -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..