ASHRAE Journal - March 2011 - 78

HVAC MytHbuster

Do Furnaces Dry Air?
by Jim Crawford, Fellow ASHRAE; and roy Crawford, Ph.D., Member ASHRAE
80% 60% RH = 40% 0.02 20% 0.00 0.06

nitions for a myth. The one that fits best here is a half-truth. Most technical myths have an element of truth to them, but the facts are somehow twisted to give an erroneous conclusion.
This month’s myth: furnaces dry out the air. This myth is usually attributed to homes with forced-air furnaces versus homes with hydronic baseboard or radiant heating systems. In fact, hydronic heating systems are often called “wet heat” systems. To verify that this is indeed a commonly held belief, we did a Google search on “furnaces dry air.” Here are some quotes we found on various websites: • “I’ve moved into an apartment with a gas furnace, and now I wake up four to five times a night because my entire skull is a desert.” • “Forced air systems can also overly dry the air...” • “Hydronic heat is wet heat; it doesn’t dry out the air.” At the outset, it should be said that furnaces do not dry out air. To facilitate discussion, we need to agree on terminology: to dry the air means to remove moisture from the air. This moisture is in the form of water vapor, which is present in some quantity in all naturally occurring air. Heating the air with a furnace, heat pump, radiator, electric space heater, solar panel or any other device, raises the temperature of the air. But heating it neither removes water vapor from the air nor adds it. If you plot a typical heating process on a psychrometric chart, it is simply a horizontal line moving from left to right with no change in humidity ratio (Figure 1). However, there is a decrease in the relative humidity of the air. This
78 ASHRAE Journal

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60 70 80 90 100 110 120 change in “relative” humidity Dry-bulb temperature (°F) has nothing to with a change in the moisture content of the Figure 1: Typical heating process. Another common justification for this air; it is simply that warmer air can hold more water vapor. Since the quantity of myth is that older models of forced-air moisture in the heated air is unchanged, furnaces use indoor air for combustion, heating the air will have no impact on the which results in more outdoor air being pulled into the house. This seems to make humidity level within the heated space. It is obvious that blowing hot air on a sense, except that hydronic heating sysmoist material (wood, wet clothes, skin, tems typically use gas-fired boilers, which etc.) will cause it to “dry out,” by absorbing also draw combustion air from inside the water vapor given off by the moist material. house. Thus, you would not expect to see Perhaps this commonly known effect leads much difference in indoor humidity levsome people to believe that the air is dryer. els between houses with these two types Is there any truth to this myth? Houses of systems. Furthermore, the additional typically have lower indoor humidity infiltration due to the combustion air is levels during the heating season (winter) on the order of less than 0.1 air changes when gas furnaces are typically used, than per hour, so it probably has little effect on during the cooling season (summer) when indoor humidity levels anyway. The only possible support for this they are not. But that is because outdoor air is significantly drier (lower water myth that we can propose, is that houses vapor content) during the winter and air with forced-air systems may have higher exchange with the outdoors (infiltration infiltration rates due to duct leakage. or ventilation) has a major impact on Hydronic systems would not have this indoor humidity levels. Thus, furnaces issue. One might experience lower winter might suffer from guilt by association; indoor humidity levels if they changed they are only used during dry conditions. from hydronic to forced-air and blamed The previous explanation does not the furnace for this effect. So we claim that the statement, “furnaces explain why some people think that forced-air heating systems dry the air as dry the air,” is completely false. Leaky ductcompared to hydronic heating systems. work may result in lower indoor humidity Perhaps the use of the term “wet heat” levels, but it is not due to the furnace. Jim Crawford is an independent consulcontributes to this misunderstanding. It seems obvious that “wet” refers to the tant in Tyler, Texas. Roy Crawford, Ph.D., water inside of the hydronic system, is director of research and technology which hopefully never comes in contact development for TxAIRE at the University of Texas at Tyler in Tyler, Texas. with the heated air.

ashrae.org

March 2011

Humidity ratio, lbw/lbda

his is the first of a series of columns intended to dispel some common myths about HVAC. There are many defi-

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ASHRAE Journal - March 2011

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