Out door dew point determines inside dew point

[teddy bear]

Unoccupied home in FL is wet (+55%RH) or dry depending depending on the outdoor dew point.
The a/c is off and the home is unoccupied. Following quickly indicates an air change in 3-5 hours. Adding occupants raises the indoor humidity. Raising the indoor temperature lowers the indoor %RH. Significant cooling lowers %RH. Ultrimate control of the indoor %RH requires a good dehumidifier. Second graph shows same house occupied and dehumidified
Regards TB

Nov FL %RH dehu.pdf
FL DEC 12 no ac occupants RH.pdf

[waregl82]

One could also put in reheat coil or strips in duct to control humidity.

[BadgerBoiler MN]

Unoccupied home in FL is wet (+55%RH) or dry depending depending on the outdoor dew point.
The a/c is off and the home is unoccupied. Following quickly indicates an air change in 3-5 hours. Adding occupants raises the indoor humidity. Raising the indoor temperature lowers the indoor %RH. Significant cooling lowers %RH. Ultrimate control of the indoor %RH requires a good dehumidifier. Second graph shows same house occupied and dehumidified
Regards TB

Nov FL %RH dehu.pdf
FL DEC 12 no ac occupants RH.pdf

Thank you for sharing.

[teddy bear]

One could also put in reheat coil or strips in duct to control humidity.

Of course. This is common in commercial systems. A 2-3 ton a/c coil collects 2-3 lbs. of moisture before dripping to the drain. Removing 2-3 lbs. of moisture lowers the humidity 10%RH in a typical home. The dehumidistat satisfies and a/c shuts off. The moisture on the a/c coil revaporates back into the space humidifying the space +10%RH. This triggers the reheat cycle all over again. This is an energy intinsive cycle that is illegal in most building codes. An effecient dehumidifier will maintain the desired %RH for a fraction of the cost.
Thanks for the post.
Regards TB

[waregl82]

Thanks for the input teddy bear. Always learning. Was thinking commercial when I made the statement. I have never seen a dehumidifier in a commercial system, only reheat. I live in the south (Alabama). Do they use dehumidifiers in commercial buildings? Perhaps depending on the size. Just curious and learning.

[teddy bear]

Thanks for the input teddy bear. Always learning. Was thinking commercial when I made the statement. I have never seen a dehumidifier in a commercial system, only reheat. I live in the south (Alabama). Do they use dehumidifiers in commercial buildings? Perhaps depending on the size. Just curious and learning.

The larger models from Ultra-Aire handle a 7.5 ton a/c or about 5,000sqft. of conditioned space. They can also handle the make-up air ventilation needed meet most codes. They remove upto 8 lbs. of moisture per hour. The lbs. per KW are +7lbs. While a/c removes 2 lbs. per KW. ultra-aire.com
Regards TB

[waregl82]

Sounds a heck of a lot more ecomomical than reheat!!!! I wonder why I haven't seen it in commercial. Could be a lot of the AHU's I've worked on are older technology? Maybe before dehumidifiers became more economical, or perhaps the large tonnage of the hydronic systems don't lend themselfves to the technology. Gonna check out their website and learn a little more.
Thanks, waregl82

[Shophound]

Larger commercial dehumidification needs can opt for desiccant wheel technology vs. merely DX dehumidification. Especially appealing if waste heat can be used to reactivate the desiccant wheel.

[waregl82]

Now that I think about it, most have outside air units (Desert Aire for one). Everything control wise we do is moving toward "demand ventilation". The CO2 in the building is monitored, and the system calls for outside air only when necessary. All of the outside air units I've worked on have energy wheels.

[jpsmith1cm]

moved to tech to tech discussion