[Greenbuilding] Non-passive, non HRV incoming air

[Nick Pine]

Sacie writes:

>... I have the ability to bring in cold outside air (not HRV) via a 
>minimally ducted system... Designed as an experimental system for a 
>tight house, the machine's primary use is to dehumidify basement 
>storage in the summer.

I wonder how it would do that. With a clever control, you might move 
outdoor air through the basement when it contains less moisture, in the 
absolute sense. A less weather-dependent passive system might have 
desiccant clay bags hanging on wires in a glazed box connected to the 
basement with an air-air heat exchanger. The sun would heat the bags to 
evaporate water, and the heat exchanger would make the basement and box 
air about the same temp, so heavier drier air would flow down into the 
basement via gravity. Uline sells nice Tyvek clay bags for about $1/lb. 
They can hold 28% of their weight in water.

> I run small humidifiers in the above very dry room but am wondering if 
> I run the cold air in, if I might be more successful in raising the 
> humidity?

More likely you'd lower humidity in wintertime. To raise it, you might 
airseal the house more, with a blower door test.

>... This morning is typical.  High teens outside, RH in the 40s inside, 
>with temps in the mid to high 60s and a nice bead of water across the 
>base of every window in the house, including those that are inoperable. 
>Simple difference between indoor and outdoor temps?

No. The indoor RH matters too. Condensation happens when the indoor 
window surface temp is less than the dew point of indoor air. If you put 
a few ice cubes in some room temp water in a glass and stir it with a 
thermometer until you see condensation, you can read the dew point on 
the thermometer. Or calculate it: with room temp T (F) and RH R (%), Td 
= (460+T)/(1-(460+T)ln(R/100)/9621)-460, where ln is the natural log on 
your $10 Casio fx-260 calculator. For example,
T = 70 F and RH = 50% make Td = 520/(1-530ln(0.5)/9621)-460 = 50.5 F. 
Better windows with lower U-values have higher indoor glass temps, with 
less chance of condensation.

>Second question: if there are no vents below or above (an open, 
>therefore multi-story house with no ability to close either up or down) 
>and a small exhaust (don't know how many cfm pulled) in the main center 
>floor, what is the effect?   House tight, but many many windows.

With a few air leaks, running the fan would likely introduce fresh air 
and lower the indoor RH in wintertime.

Nick