Surprising HVAC Lesson: Air Conditioning Costs in Philly vs. Vegas

In summary, the conversation revealed that it is significantly more expensive to air condition a building in Philadelphia compared to Las Vegas due to the difference in design conditions. In Philly, the design conditions have a higher enthalpy of 39.0 compared to 33.0 in Vegas. This means that it is 18% hotter in Philly and requires 57% more energy to cool the outside air. The closer the dry and wet-bulb temperatures are to each other, the more energy is needed to condition the space. This information was shared by someone who recently took their Mechanical PE exam and is still knowledgeable about the subject.
  • #1
russ_watters
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One of the guys I work with is taking a course in HVAC and since my boss used to teach it, we got a little lesson today (helping with the homework). The result of the conversation was the realization that it costs significantly more to air condition a building in Philadelphia than it does in Las Vegas. Surprising, huh? Here's the math though:

Indoor design conditions are for 75F and 50% relative humidity. That corresponds to an enthalpy of about 28.5 (btu/lb). In Philly, the design conditions (from NOAA data) are 92F dry bulb and 75F wet bulb (that's about 48% relative humidity). That corresponds to an enthalpy of 39.0. In Vegas, the conditions are 112F wb and 68F db (about 8% rh). Thats an enthalpy of about 33.

So the difference between the two enthalpies means its 18% hotter! in Philly than in Vegas on a typical August day and requires 57% more energy (10.5 vs 4.5 btu/lb of air) to cool outside air (your house is more complicated since much of the load is actually solar).
 
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  • #2
Not bad on your explanation. When I design systems and calculate annual energy costs, a good rule of thumb to use is the closer the dry and wet-bulb temperatures are to each other, the more energy you'll use to condition the space. Air conditioning systems cool 'sensibly' first, ie they change the dry bulb temperature. The last thing that they do is remove moisture, ie they drastically change the enthalpy, humidity, etc. Here in Houston, the design dry bulb is 96 and the wet bulb is 79 (depending on the source)--a resulting enthalpy over 42 btu/lbm!

Let me know if you have any further questions on the matter. The subject is still pretty fresh in my mind--I just took my Mechanical PE exam a few months ago.
 
  • #3


Wow, that is definitely surprising! I never would have thought that it would cost more to air condition a building in Philly compared to Vegas. It's interesting to see how the different design conditions and humidity levels can make such a significant impact on the energy required for cooling. It really shows the importance of considering all of these factors when designing and installing HVAC systems. Thank you for sharing this information and giving us a glimpse into the world of HVAC. It's great that you were able to learn from your boss and help with the homework - it sounds like a valuable and eye-opening experience.
 

1. How do air conditioning costs in Philly compare to those in Vegas?

The surprising lesson is that air conditioning costs are actually higher in Vegas than in Philly. This is due to the extreme heat in Vegas, which requires more energy to cool buildings.

2. Why do air conditioning costs differ between cities?

Air conditioning costs can vary based on a number of factors, such as climate, energy prices, and building size and efficiency. In the case of Philly and Vegas, the extreme heat in Vegas is the main factor contributing to higher costs.

3. Are there any other cities where air conditioning costs are higher than expected?

Yes, there are likely other cities with high air conditioning costs, especially those located in regions with extreme heat or high energy prices. It is important to research and compare costs before moving to a new city.

4. How can individuals reduce their air conditioning costs?

There are several ways to reduce air conditioning costs, such as using a programmable thermostat, sealing air leaks, and keeping up with regular maintenance. It is also important to use energy-efficient appliances and consider alternative cooling methods, such as ceiling fans, to reduce overall costs.

5. Is there a significant difference in air conditioning costs for residential vs. commercial buildings?

Yes, there can be a significant difference in air conditioning costs for residential vs. commercial buildings. Commercial buildings often have larger spaces to cool and may require more powerful and energy-intensive HVAC systems, resulting in higher costs. However, factors such as building size and efficiency also play a role in determining costs for both residential and commercial buildings.

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