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humidity question

is there a humidity range that my A/C should keep the house at?
yesterday it was about 45% inside (about 80% outside).
usually when the outside humidity is less than about 50%, the inside is about 3% to 10% lower than outside. the difference is smaller when outside humidity is low, and bigger when the outside humidity is high. does this sound right?
the reason for my question?
we have a whole house humidifier (turned off now) that is set at approx. 35% in winter. i'm just wondering why the forced air heat would dry the house out more than A/C. i only know the very basics of HVAC, and know nothing about theory.

BTW: whole house/ central forced air heat and air
2 story brick house in western PA

Re: humidity question

I will be watching this post as I to would like to see the answer.


Re: humidity question

Desired home humidity could vary by climates, but a rule of thumb is between 40% and 50%.

As for your second question, hot air dries the home faster than cold air. Just like your hair dryer and clothes dryer, which work faster when hot.

Re: humidity question

There are a lot a variables here. A lot of the variation in humidity reduction is due to the size of your AC unit. The size determines the % of run time. If the AC is running nearly all the time, there will be more reduction in humidity than if it is running say 30% of the time.

Another factor is the air infiltration rate of your house, more infiltration, more outside humidity.

To answer your question, you have to understand what relative humidity is. There are three terms to understand, relative humidity, absolute humidity and dew point. Absolute humidity is the measurement of the amount of water in a cubic meter of air. It is expressed in grams per cubic meter or g/m3. The warmer the air, the more water it can hold.

Relative humidity is the ratio of the absolute humidity at a certain temperature compared to the maximum amount of water the air can hold at that temperature. For example, at 68°F, air can hold 17.3 g/m3. If the actual amount of water in the air is 8.7 g/m3, then the air has a relative humidity of 50%

The dew point is when temperature where that absolute humidity just exceeds 100% relative humidity, in this case, somewhere between 41°F and 50°F.

Here is a table that shows the values of relative humidity (RH), absolute humidity (AH) and dew point.


From looking at this table, you can see that when your outside RH is lower, there is less absolute humidity getting into your house through infiltration, therefore there is less condensation on your evaporator coils, so the RH inside your home does not change as much as it does on high humidity days.

As for the heating side, cold air holds so much less water in the air, even at high RH, that when it infiltrates your home, the sudden rise in temperature really drops the RH inside your house. Unlike the evaporator coils of the AC, the heater itself does not have much, if any, affect on the AH.

Here is another link that illustrates what happens in winter to RH


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