ASHRAE Journal - May 2011 - 14

letters

Do Furnaces Dry Air
March HVAC Mythbuster Column
OK. I give up. How do “Leaky ducts ... result in lower indoor humidity levels...”? Carl H. Jordan, Fellow/Life Member ASHRAE, Berkeley, Calif. The Authors Respond The leakage of concern is between the conditioned space and unconditioned space, including the outdoors. This can occur through normal infiltration or due to leakage of ducts in unconditioned space such as attics, crawlspaces or unsealed stud spaces. If duct leakage is on either the supply or return side, the net effect is that more outside air will enter the conditioned space. During the heating season, the absolute humidity outdoors is generally lower than that indoors. When that drier air enters, due to leakage, it is warmed reducing its relative humidity. This will be perceived as drying out the air in the house. Jim Crawford, Fellow ASHRAE, Roy Crawford, Ph.D., Member ASHRAE, Tyler, Texas In the case of the old convection gas furnace, I usually get my combustion air, and a lot of my fan draw, from air that is pretty close to outside air; and it is always at a lower temperature than inside (the furnace is in the shed, which is attached to the house in a lean to). Therefore, it usually has less water vapor to start with than the inside air, because it can’t hold as much water vapor. So, if I take one slug (the old English slugs, so to speak) of this air and shove it into the house, and push out the hot air at the top (all houses leak, except for the new net zero ones), and I keep doing that, I will eventually (Arrhenius rate of e to the minus alpha × time) get to a point where I have warmed up the air that had less water vapor to start with and driven out all the air with more water vapor. It seems to me that the net result will be that my house, since it is using a supply of “less water vapor” containing air, will dry out. Where is my thinking wrong? I am looking at the whole house, fed by this shed-based furnace. It seems to me, that in my case, the explanation on Page 78 won’t work. Let me know where I have gone wrong. David J. Thomas, P.E., Fairfax, Va. The Authors Respond The clue here is that Mr. Thomas says that a lot of the air being drawn into the house by the furnace is “pretty close to outside air.” Since outdoor air in the winter is cold, it is unable to contain much moisture in the form of water vapor. Surplus water vapor comes out as frost or snow, or dew if the temperature is above freezing. As a result, when it is cold, the outdoor air is normally drier than the indoor air. When
May 2011

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ASHRAE Journal



ASHRAE Journal - May 2011

Table of Contents for the Digital Edition of ASHRAE Journal - May 2011

ASHRAE Journal - May 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Seismic Restraint
Plug Load Design Factors
Technology Award Case Studies:
State-of-Art School
Green School Lab
Eco-Friendly, Affordable, School
Building Sciences
Special Section
InfoCenter
Solar NZEB Project
Emerging Technologies
IAQ Applications
Washington Report
International Column
Products
People
Classified Advertising
Advertisers Index
ASHRAE Journal - May 2011 - Intro
ASHRAE Journal - May 2011 - ASHRAE Journal - May 2011
ASHRAE Journal - May 2011 - Cover2
ASHRAE Journal - May 2011 - 1
ASHRAE Journal - May 2011 - 2
ASHRAE Journal - May 2011 - Contents
ASHRAE Journal - May 2011 - Commentary
ASHRAE Journal - May 2011 - 5
ASHRAE Journal - May 2011 - Industry News
ASHRAE Journal - May 2011 - 7
ASHRAE Journal - May 2011 - 8
ASHRAE Journal - May 2011 - 9
ASHRAE Journal - May 2011 - 10
ASHRAE Journal - May 2011 - 11
ASHRAE Journal - May 2011 - 12
ASHRAE Journal - May 2011 - 13
ASHRAE Journal - May 2011 - Letters
ASHRAE Journal - May 2011 - 15
ASHRAE Journal - May 2011 - 16
ASHRAE Journal - May 2011 - 16a
ASHRAE Journal - May 2011 - 16b
ASHRAE Journal - May 2011 - 17
ASHRAE Journal - May 2011 - Meetings and Shows
ASHRAE Journal - May 2011 - 19
ASHRAE Journal - May 2011 - Seismic Restraint
ASHRAE Journal - May 2011 - 21
ASHRAE Journal - May 2011 - 22
ASHRAE Journal - May 2011 - 23
ASHRAE Journal - May 2011 - 24
ASHRAE Journal - May 2011 - 25
ASHRAE Journal - May 2011 - 26
ASHRAE Journal - May 2011 - 27
ASHRAE Journal - May 2011 - 28
ASHRAE Journal - May 2011 - 29
ASHRAE Journal - May 2011 - Plug Load Design Factors
ASHRAE Journal - May 2011 - 31
ASHRAE Journal - May 2011 - 32
ASHRAE Journal - May 2011 - 33
ASHRAE Journal - May 2011 - 34
ASHRAE Journal - May 2011 - 35
ASHRAE Journal - May 2011 - State-of-Art School
ASHRAE Journal - May 2011 - 37
ASHRAE Journal - May 2011 - 38
ASHRAE Journal - May 2011 - 39
ASHRAE Journal - May 2011 - 40
ASHRAE Journal - May 2011 - 41
ASHRAE Journal - May 2011 - 42
ASHRAE Journal - May 2011 - 43
ASHRAE Journal - May 2011 - Green School Lab
ASHRAE Journal - May 2011 - 45
ASHRAE Journal - May 2011 - 46
ASHRAE Journal - May 2011 - 47
ASHRAE Journal - May 2011 - 48
ASHRAE Journal - May 2011 - 49
ASHRAE Journal - May 2011 - 50
ASHRAE Journal - May 2011 - 51
ASHRAE Journal - May 2011 - Eco-Friendly, Affordable, School
ASHRAE Journal - May 2011 - 53
ASHRAE Journal - May 2011 - 54
ASHRAE Journal - May 2011 - 55
ASHRAE Journal - May 2011 - 56
ASHRAE Journal - May 2011 - 57
ASHRAE Journal - May 2011 - Building Sciences
ASHRAE Journal - May 2011 - 59
ASHRAE Journal - May 2011 - 60
ASHRAE Journal - May 2011 - 61
ASHRAE Journal - May 2011 - 62
ASHRAE Journal - May 2011 - 63
ASHRAE Journal - May 2011 - 64
ASHRAE Journal - May 2011 - 65
ASHRAE Journal - May 2011 - 66
ASHRAE Journal - May 2011 - 67
ASHRAE Journal - May 2011 - InfoCenter
ASHRAE Journal - May 2011 - 69
ASHRAE Journal - May 2011 - 70
ASHRAE Journal - May 2011 - 71
ASHRAE Journal - May 2011 - 72
ASHRAE Journal - May 2011 - 73
ASHRAE Journal - May 2011 - 74
ASHRAE Journal - May 2011 - 75
ASHRAE Journal - May 2011 - 76
ASHRAE Journal - May 2011 - 77
ASHRAE Journal - May 2011 - Solar NZEB Project
ASHRAE Journal - May 2011 - 79
ASHRAE Journal - May 2011 - 80
ASHRAE Journal - May 2011 - 81
ASHRAE Journal - May 2011 - 82
ASHRAE Journal - May 2011 - 83
ASHRAE Journal - May 2011 - 84
ASHRAE Journal - May 2011 - 85
ASHRAE Journal - May 2011 - 86
ASHRAE Journal - May 2011 - 87
ASHRAE Journal - May 2011 - 88
ASHRAE Journal - May 2011 - 89
ASHRAE Journal - May 2011 - Emerging Technologies
ASHRAE Journal - May 2011 - 91
ASHRAE Journal - May 2011 - 92
ASHRAE Journal - May 2011 - 93
ASHRAE Journal - May 2011 - 94
ASHRAE Journal - May 2011 - 95
ASHRAE Journal - May 2011 - IAQ Applications
ASHRAE Journal - May 2011 - 97
ASHRAE Journal - May 2011 - 98
ASHRAE Journal - May 2011 - 99
ASHRAE Journal - May 2011 - 100
ASHRAE Journal - May 2011 - 101
ASHRAE Journal - May 2011 - Washington Report
ASHRAE Journal - May 2011 - 103
ASHRAE Journal - May 2011 - International Column
ASHRAE Journal - May 2011 - 105
ASHRAE Journal - May 2011 - 106
ASHRAE Journal - May 2011 - Products
ASHRAE Journal - May 2011 - 108
ASHRAE Journal - May 2011 - People
ASHRAE Journal - May 2011 - Classified Advertising
ASHRAE Journal - May 2011 - 111
ASHRAE Journal - May 2011 - Advertisers Index
ASHRAE Journal - May 2011 - Cover3
ASHRAE Journal - May 2011 - Cover4
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