block up all air vents and use a dehumidifier to fight mold?
 

Hi this is my first post here

Its winter here and I have been sealing up all my doors with foam tape.
They are realy tight now and there is no draft. it got me thinking about ventilation. I bought a relative humidity meter and it says its 61%RH, which i think means its time to buy a dehumidifier.

Do you think its better to block up all air vents and use a dehumidifier to fight mold?

why do ecobuilders use large heat exchangers when they could just use a dehumidifier?

I air sealed my house and use a centralized dehumidifier to keep the RH at 40%. The benefit is that the dehumidifier produces water, which then goes towards flushing toilets,and it also helps heat the house.

yes the energy would not be wasted.

would i need a big expensive one or could i do it with a 600ml/day one?

My dehumidifier can extract up to 12 litres per day, and normally pulls out about 4 litres in spring and autumn, more in winter as the tumble drier is on.

Because you exhale more than just water vapor. Building codes require a minimal rate of replenishing the house's air, for example 50% of the house's volume per hour per person. Water vapor is the least of your worries, what you want to get rid of is CO2, carbon monoxide, vapors from chemicals, plastics, etc.

A dehumidifier will help if you live in a particularly humid area, or if your house has an old ventilation system which can't keep up with your water production (many occupants, lots of cooking). A heat exchanger can also help dehumidify the incoming air.

Relative humidity is 61% but what is the air temperature of that space?
Dew point is more important than relative humidity alone, if it's above 55 degrees F, you need to remove moisture to prevent mold because your ground temperature could be that cold which would mean the basement slab and walls will be that temperature with walls and flooring against it which can create condensation trapped between those materials and develop mold.

If it's cold outside, and humid inside, then you definitely need some ventilation. You can leave one or two windows open an inch or so. This works, but is not energy efficient plus it's drafty. Or you can put in a ventilation heat exchanger. The advantage of the heat exchanger is that it's both energy efficient and not drafty.

A friend lived for over ten years in a tight house with a noisy dehumidifier grinding and groaning all winter long. He suffered from asthma the entire time. We finally persuaded him to put in a ventilation heat exchanger. No more need for the dehumidifier, and his asthma problems went away.

Outside air is cleaner than inside air. And healthier.

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.

i have put this on a spread sheet and played with the numbers. its quite easy to see situations where the window open is much worse but difficult to see situations where the dehumidifier is worse. when the temperature gets below zero centigrade opening the window is really bad.

Regarding two posts ago, it might be a good idea to combine this with a laundry spinner(water extraction spinning device) which spins the laundry much faster than a washing machine can to get even more water out prior to hang drying.

I don't have one of these, at least not yet. ..but when I do my laundry both the washing and electric dryer use a little under 3kwh and I dry my clothes until I can't feel any moisture on them at all, including the socks and thicker sections of jeans.

I'd imagine with a laundry spinner that I'd be under 2kwh with the washer and dryer energy usage combined because of how much faster my clothes would dry with it. This would likely then be less energy than your dehumidifier would use. Either that or use the laundry spinner and the dehumidifier as you were going to. My personal experience with dehumidifiers is that they suck power and it seems like I don't get my electricity's worth.