air changes per hour calculation

[Deleted User]

it does help.

[Deleted User]

how are you guys calculating

air changes per hour today? my hoffman handbook (1935) lists one air change if one wall is exposed, 1 1/2 air change if two walls exposed, 2 air changes if three walls exposed...ive been using 1.2 as thats what hvac-calc seems to default to....any more current data..slant fins software and siggys software seem to leave it up to me to pick something.

Duncan_6

Guessing

It's pretty much a guess if a blower door test hasn't been done. Even with blower door data, there's some controversy how to interpret the data.

1.5 ACH seems high to me for a reasonably tight house.
It's probably a safe bet that today's houses, built since the energy crunch of the 70's, are tighter than they were in 1935.

If the house is wrapped in Tyvek or plastic, and has decent wiondows and doors, I'll figure 0.5 ACH. I also use 0.5 ACH if it's homeowner built, the owner/builder typically is MUCH more attentive to sealing air leaks as the house is buiilt.

If it's your basic tract home, or builder in a hurry, I use more like 1.0 or 1.2 ACH. Log homes are notorious leakers, I'll use 1.5 or 2.0 ACH as a best guess, unless there's some unique building system, like the Dovetail log system (brand name out of Tenesssee) which uses more effective chinking.

That seems to be the extent of how exactly tightness is categorized: Unusually tight construction, or typical construction.

Just to compare, I'll typically plug in 0.5, 1.0 and 1.5 ACH in a heat loss spreadsheet I developed just to get an idea of how drastically different air changes per hour affect the heat loss. When I did this stuff early on (and had more time), I'd even use two or three different techniques to calculate air infiltration, to see how the different methods compared. On new homes of decent quality construction, I usually figure the infiltration losses don't exceed the conduction losses. I figure I'm still probably oversizing the heat source.

Hope this helps.