Parallel wiring for dehumidistat in bathroom

**tomtbone** · 08-04-2010, 04:41 PM

Now we just need a mathematical equation to show that my bathroom fan is going to kill our children. Lol

**jwelectric** · 08-04-2010, 06:47 PM

Originally Posted by tomtbone

Now we just need a mathematical equation to show that my bathroom fan is going to kill our children. Lol

To address a couple of the issues you mention in such an impressive yet tentative post let me start with the issue of moisture trapped on the interior of your bath walls.
This would lead some to believe that someone is spending way too much time in the shower. How big is your water heater?

Then again if we are talking about moisture on the interior of the outside wall due to bad aim then the exhaust fan won’t help remove much of this moisture but having a mop handy will help a lot.

I hope that you understand the term efficiency or Efficiency = (output / input) * 100

The efficiency of a dehumidifier over an exhaust fan is at least 300%. This means that the dehumidifier will cost about 1/3 that of the fan over a length of time.

You can go to this link and enter the correct numbers to see what we are doing to our off spring and the air that they will have to breath. This is not including the natural resources we are using up that they won’t have. But I guess it will be alright just because I am trying to fix something that ain’t broke or I just don’t want to close the door till the order goes away.

http://www.greenprogress.com/carbon_...calculator.php

**nukeman** · 08-05-2010, 05:40 AM

Basically it boils down to what the conditions are as far as what is best to use:

- Say it is winter. Typically, the indoor RH is low during this time and often people will run a hunidifier to increase humidity. In this case, it is much better just to open the door. You increase the humidity in the rest of the house and not pump the conditioned/humid air out that will just need to be replaced with heating and adding humidity to the air that replaces the air that you just pumped out.

- Now say it is summer. What option is better depends on your location. In the east, it will tend to be humid outside, you'll probably have AC running to control heat as well as indoor humidity. Turning the fan on will pump out conditioned air (replaced with warmer/humid outside air). If you open the door, you won't lose the conditioned air, but the AC will have to work to remove the mositure from the bathroom in order to bring the temperature down further (humid air holds more heat than dry air).

-In the spring/summer where your indoor RH is okay and you are not using AC/heat, it might be better to use the fan. It really depends on what the RH is in the rest of the house.

One thing that is interesting is looking at what happens during the shower. Say you start out at 50% RH in the bathroom. You turn on the shower and the RH quickly approaches 100% RH. As you approach 100% RH, the rate that the shower produces the vapor decreases. Nature tries to balance things by going from a higher concentration to a lower concentration. At the water droplets coming from the shower head, the RH is 100%. At first, when the shower just starts, the water vapor transfers the quickest. As the RH of the room goes to 100%, the vapor generation goes to zero (that is why you have a hard time staying cool in humid weather..evaporation pretty much stops). Once the room hits 100% RH, the vapor that is replaced from the shower is only to replace what is lost out of the air (condensation on walls/mirror, air lost under the door, air lost with fan). So in fact, running the fan may help decrease the RH in the bathroom (depends on water temperature, droplet surface area, air RH, bathfan real CFM, RH of makeup air coming in from under the door, etc.), but the fan will also increase the rate of evaporation (more water being put into the air).

Unless your fan or dehumidifier can keep RH below 70% with the shower running (doubtful). Trying to extract moisture during the shower is pretty much a lost cause. Best bet would be to open the door or turn on the fan after the shower is done. Best way to control bathroom mositure is to take shorter showers and/or run the shower at a lower temperature. Drying off shower walls and drying off walls/mirrors that have condensation after each shower will help a bunch in controlling mold/mildew/moisture damage.

Personally, I like timers for fans. With a switch, you may forget it and leave it running all day (especially with newer/quiet fans). Running it automatically may cause it to run when it isn't really needed.

Technically, once the room hits 100%RH you will still be able to evaporate water, but that is because the air temperature is also increasing (warmer air can hold more water).

**jwelectric** · 08-05-2010, 11:47 AM

Well put Nukeman

The idea of having the exhaust fan blow for any extended amount of time is a waste of resources. It does absolutely nothing to improve our existence as a human race. It does nothing to help preserve our framing members. It does nothing to prevent mold and if it stinks that bad then we might think of joining our friend the bear and using his bathroom. How about doing math like adding a little water to subtract a little she it.

The idea that this extra blow helps preserve our home goes into the same category as dentures for chickens, I think that sun glasses for bats would be a better idea. Why not open a shop selling scuba gear to fish.

**chris8796** · 08-05-2010, 12:54 PM

It sounds like you better turn your AC off so you don't cost your grandchildren any more grief.

On a side note, I'm not a fan of the humidty controlled fan idea. Although I do use an exhaust fan in the summer. The best solution I have found to the problem is to run the HVAC with the fan on continously during periods of likely shower use (6-8 am, with programable thermostat with independent fan control). It also moderates the temperature with all the different activities going on. Although the solution to your situation is dependent on the local variables and not something that can be generalized.

Another issue I've had with humidity sensors is they are less than reliable and have a poor response time.