[wxperson]

I just had a whole house dehumidfier installed. It has a fresh air intake and a return from the house as input.

I notice today (damp day..temps and dew points near 60.. Atlanta) the humidity in the house was in the low 50s and not going down very quickly.

I checked and the damper for the fresh air intake was wide open. The question I have is how much fresh air should I be allowing into the unit? Seems to me it has just about as much returned house air as outside air coming into it. Does that sound right or should I damper the outside air down a bit?

I will also ask the installer this question but wanted some opinions first.

Thanks,

George

 

[tonys]

V. Little fresh air is needed (relative to the unit size)

ASHRAE standard recommends approx. 15-20 cfm per person.

 

[wxperson]

Thanks for the info..

I had read at http://www.efi.org/articles/erv.html that a 3000 sq foot home needed about 150 cfm. Do you think that is too much? (I only have 3 people at this residence).

Thx,

George

 

[cem-bsee]

shoot for 1 air change ea 3h; or 15-20 cfm per bdrm + 15. ( one considers each bdrm to have one person, master to have 2 )

 

[teddy bear]

Thanks for the info..

I had read at http://www.efi.org/articles/erv.html that a 3000 sq foot home needed about 150 cfm. Do you think that is too much? (I only have 3 people at this residence).

Thx,

George

50 cfm of fresh air when occuppied will control the CO2 and replace the oxygen used. If this is the only ventilation, 50 cfm will change the air every nine hours. That's a little show for purging pollutants. But there are other ventilating mechanics like bath fans, clothes drier, and the most powerful, wind. More ventilation equals more moisture during the wet time of the year(@70^F dewpoint), 1 pint of moisture per day 1 cfm while maintaining 50%RH. During cold dry weather more ventilation makes your home drier. 50-75 cfm sounds good to me. Only ventilating when occupied is a conservative move. How big of dehumidifier did you get? Never bring in more air than you can dry. <50% RH is more important than a specific amount of fresh air.

 

[wxperson]

Thanks for the information.

I ended up going with a solution from HealthyAir. They use a locally built unit.. I spoke with the manufacturer (10 miles away) and several other references. It claims it removes 160 pints/day... and also has a fresh air option. I went for the larger capacity because I am also dumping air into a rather large crawl space. After 3 days of showery weather with outside dew points around 55-60.. it is holding the humidity at around 48-50% in the house and 55-60 in the crawl space.

There is a little more info on their unit at http://www.healthyairusa.com/pr_proffered_humid.asp .

It would be nice to have an outside air vent that would close when the humidity outside was over 50%. However... around here that is probably most of the time

George

 

[teddy bear]

Thanks for the information.


It would be nice to have an outside air vent that would close when the humidity outside was over 50%. However... around here that is probably most of the time

George

Be concerned about dew point not the %RH. 50^F, 100%RH(raining outside air is 50^F dew point when warmed to 70^F is 50% RH. 90^F, 50%RH is 69^F dew point, when cooled to 70^F is 98%RH. 100 cfm requires 100 pints of dehumidification per day. Ventilate the minimum required when occupied and forget it.