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re: running dehumidifier with central a/c, yes or no?
19 jul 2004
timr  wrote:

>> stormy weather  wrote:
>> >if your intention is to control the humidity, then yes run your
>> >de-humidifiers. the trade off is more money. the heat added by the
>> >de-humidifiers will cause the a/c to run longer...

the heat is bad news, in summertime.

>>...just let the ac filter get dirty.

to reduce the airflow over the cold side...

>well, it is wrong, but filters isn't the answer.  (although they may
>help with the constant volume problem.)

i disagree. (what's the "constant volume problem"?)

>i don't think his intention is to control humidity, i think it is to
>save energy.

we might use night air for cooling, with ac mainly for dehumidification.

>...normal household ac does not control humidity.  it controls temperature,
>and reduces humidity by happy thermodynamic accident.

it's no accident. if c cfm of 70 f air at 50% rh with pa = 0.374" hg enters
a box with a 20 ft^2 40 f coil and mixes well before leaving, we might have
something like this (viewed in a fixed font): 

      1/c       1/40
70 ---www-------www--- 40
            t = exit air temp = 40+30c/(c+40).

cfm     t     rh@70f = 100e^(17.863-9621/(t+460))/(2x0.374)

100     61.4  50%
 50     56.7  50%
 20     50.0  49% |
 10     46.0  42% | dirty filter 
  5     43.3  38% |

>one way to get better control is to use a humidistat and add reheat. 

reheat is bad news, in summertime.

>a dehumidifier could sort of function that way.  i think you would want
>to change your basic ac parameters if you wanted to do that though.

we might add a freezestat...


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