Thinking Ahead to a Humid Summer

March/April 2005
A version of this article appears in the March/April 2005 issue of Home Energy Magazine.
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March 01, 2005
Q. I recently read that a variable speed, two stage air conditioner can maintain 50% relative humidity (RH) even when the latent load is 60% of the total load.Would this really work?         A. What you heard exaggerates the true capability of a two-speed compressor, variable-speed fan machine. I know of no residential air conditioner made that can accomplish 60% latent cooling (moisture removal) and 40% sensible cooling with a 50% indoor RH.         Using a Carrier 38TDB060-31 with an FV4BNB006 indoor section for example, the absolute best you will get at low speed (300 CFM per ton) is 23% latent cooling (77% sensible heat ratio). That occurs only in the following unlikely scenario: 75ºF and 50% RH inside, 75ºF outside, running continuously, with no heat gains in the return duct system.         The way latent cooling increases with conventional A/C (including a two-speed machine) is that the indoor RH rises above 50%.The indoor RH rises until there is a balance between the latent capacity and the latent load. For example, at the same conditions as above except that the indoor RH has risen to 66%, the unit produces ...

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