Window AC's and dehumilification......................(Starch)

John Starcher_4

....who works at a refrigeration and hvac supply house, is telling me that window air conditioners do not dehumidify as effectively as central air. I'm having a hard time believing this.

Obviously, if a 5K btu window shaker is trying to cool a room with 12K btu load, it will not be effective in cooling or dehumidifying. If, however, that same 5K unit is cooling a room with a 5K cooling load, then why would it not provide dehumdification equal to central air?

Starch

Brad White

AC is AC

On a space by space basis and all other things being equal, so long as a window AC unit cools the air below the target dewpoint of the space served, both will dehumidify; as you state the question, "how well?"

To be fair, most window AC units have 3-row coils. Central air coils can have 3, 4, 5, 6... the more rows the better the dehumidification.

The real comparative question is, what is the sensible heat ratio (SHR) at the conditions encountered?

If a system has an SHR of 0.80, 20 percent of total capacity (2,400 BTU's out of the 12,000 total in a ton) will be reserved for latent or dehumidifying capacity.

A system with a lower SHR, say 0.75 or 0.70, will have that much more capacity available to dehumidify at a given condition.

When a system starts up in a hot and humid environment, the SHR will be lower and more humidity will be removed initially by any system. Once the space gets to temperature you are in your real rating area.

The Wire Nut

Like Brad...

... I think the performance of an AC system depends on so many variables. A properly-sized window AC with the right SHR ratio will perform better than a oversized central AC system, for example.

One of my biggest complaints is that most thermostats are still too "dumb" to make the connection between humidity and temperature (i.e. psychrometrics) and only focus on temperature. However, if the real aim is to maintain human comfort, then both temperature and humidity have to be monitored (and, ideally, controlled).

If systems can maintain low humidity levels, that translates into less heat gain (less &DeltaT), hence fewer operating hours, lower costs, etc. with no loss of occupant comfort. With many CAC systems cranking up the sensible heat ratio via large coils, we may see the emergence of central dehumidifiers. Lennox already has one, I would be surprised if other manufacturers don't as well.

This is also an argument for multi-zone systems to either have a bypass or a variable-speed fan motor and assignable air flows (by zone). Otherwise, the air will flow past that coil much too quick to get any dehumidification done.