Sat Jun 22, 2019, 09:16 AM

A bit of trivia

Summertime temperatures are soaring, but another factor, the so-called "heat index" is also on the rise here in the south.

Weather people are quick to tell us a higher heat index makes the weather more dangerous to our health.

Does the "heat index" affect your air conditioner? Why, or why not?

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Reply A bit of trivia (Original post)
oflguy Jun 2019 OP
TexMexNext Jun 2019 #1
oflguy Jun 2019 #2

Response to oflguy (Original post)

Sat Jun 22, 2019, 09:24 AM

1. So called heat index?

The heat index exists. The heat component of the index definitely affects your AC. A typical unit can only hold temps 20 degrees lower than the outside temperature.

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Response to oflguy (Original post)

Sun Jun 23, 2019, 09:06 PM

2. To answer the question, it is necessary to have an understanding of what the heat index is.

Last edited Sun Jun 23, 2019, 10:04 PM - Edit history (1)

heat in·dex
1. a measure indicating the level of discomfort the average person is thought to experience as a result of the combined effects of the temperature and humidity of the air.

So, we know that the heat index is a measure of the air’s temperature, combined with the level of humidity in the air. Raise either one of the components, and you have raised the heat index.

There is always a heat index of air. We don’t pay attention to it when it is low, but in the summer when the humidity gets high, the weatherman starts to report the “feels like temperature.” The higher the heat index, the more uncomfortable it is.

We measure moisture in air two ways. One we call relative humidity, which is the amount of moisture in the air as compared to how much moisture the air can hold at that temperature. 50% relative humidity means the air is holding half the moisture it could hold at the specified temperature. Heat the air without affecting the moisture content and the percent of relative humidity goes down, even though the same amount of moisture is present. It goes down because now the air can hold more moisture at the increased temperature. Heat the air with the same amount of moisture and the heat index goes up, even though the relative humidity goes down.

The other way to measure moisture in air is by measuring its dew point. Dew point is the temperature of air whereupon moisture will begin to condense out of the air if you cool the air down to that temperature. Once the dewpoint is obtained, the air is called saturated, or 100% relative humidity. At this point the air is 100% full of moisture for that temperature.

Our bodies sweat in order to keep it cool. Moisture in the air reduces the effective cooling effect of sweat because the rate of evaporation on our skin slows as the level of moisture in the air increases. A low level of humidity in air facilitates faster evaporation on our skin, and we feel cooler.

The technical terms for heat in air are sensible, meaning the temperature of the air, which can be measured with a dry thermometer, such as the thermometer in your thermostat. The other portion of heat content is the moisture in the air, called latent heat, sometimes called “hidden” heat because your thermostat cannot measure it. The combination of sensible and latent heat determines what the total heat in air is. The technical term for total heat in air is enthalpy.

OK, that’s people, but the question asked was “Does the "heat index" affect your air conditioner? Why, or why not?”
High levels of humidity compared to low levels of humidity do have significantly different effects on air conditioners. Air conditioners cool a house by passing air over a cooling coil, or evaporator. As air passes through the coil, both sensible and latent heat are removed. Sensible heat is removed through the reduction in temperature of the air. Typically, that reduction is about 18 degrees F. The other component of heat, which is its latent portion, is removed as some of the cooling capacity is used to condense moisture from the air. This moisture is drained away from the coil through a drain pipe.

High humidity results in a high heat index. That means more of the A/C’s cooling capacity is used to remove the higher moisture content, or latent heat. That means less of the A/Cs capacity is available for the sensible load. The ratio of sensible to latent cooling in a coil is called the sensible heat factor.

So, the definitive answer to the question “does the heat index affect your air conditioner?” is an absolute yes. Less sensible cooling means your unit runs much longer to pull the house’ air temperature down. The extra moisture consumes more A/C capacity, and results in higher energy costs and longer running time.
The key to remember is it is the combination of sensible and latent heat that determines the total amount of heat you’re A/C unit must remove. High summertime dew points not only make us feel hotter, given similar dry bulb temperatures, they make our A/C units work more to remove the extra latent heat.

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