You may have noticed that your boilers and other heating equipment had to work a little overtime this year. As the data shows, May 2002 was an unusually cold month for much of the country.

In some cites, the resulting increase in heating and humidification energy costs were actually large enough to measure, which is unusual for May. Take Minneapolis for example, where the May heating degree days were close to 50% higher than normal. If 100,000 cfm of outside air was heated and humidified to maintain 70 degrees F and 30% rh space conditions and the fuel costs were $0.85/therm, the ventilation heating and humidification costs would have been $3,513 higher than normal.

On the positive side, the cooler weather resulted in a significant increase in economizer cooling hours, resulting in much lower-than-normal May cooling costs. In fact, there were enough economizer hours in cities like Minneapolis, Denver, and Detroit to justify the operation of waterside economizer systems, also very unusual for May.

Creating Your Own Heat Recovery Savings Calculator

Last month, in Steps 1 and 2, we created the file "HRCalc" using the hourly weather data text file for Boston. We are now ready to start setting up the input data. As mentioned previously, if you would like to follow along and still have not obtained a copy of the Boston text file, feel free to drop me a note and I'll send you a copy of the file.

Step 3 - Move DOW column: You will notice that the day of the week column (DOW) is located between the wb and enthalpy columns. If you haven't already done so, insert it in Column D, next to the rest of the time-related data.

Step 4 - Setting up the input data: Name the worksheet with the hourly data on it "Data" by right clicking on the tab at the bottom. Create a new worksheet called "Report." This worksheet will be a clean, presentable summary sheet containing all of the input required to perform the calculations as well as the results.

In the "report" worksheet, fill in the input data cells. As you can see, cells with titles are shaded in grey. Cells that will contain formulas are shaded in yellow, and cells that will require a manual input will remain white. Some borders were also added to distinguish between blocks. Obviously, you can use any color or border scheme you wish, but remember to use the same cell addresses because the formulas we will be using will reference specific cells.

In Figure 2, input block #1 is self-explanatory, project-related data. Blocks #2 and #3 are the design indoor winter and summer space conditions. The db and rh are manual inputs. The humidity ratio (W) and enthalpy (H) could be either obtained from the appropriate psychrometic chart and manually input or can be calculated using the optional psychrometric calculator. The psychrometric calculator will calculate W and H for any given altitude.

Although the formulas look a little cumbersome, I think the calculator is worthwhile to incorporate because altitude can have a significant impact on the results and not all projects are at sea level. If you decide not to use the calculator, make W and H manual inputs for winter and summer and skip Step 5.

Step 5 - The psychrometric calculator: To take advantage of the calculator, type in the formulas shown in Figure 3 just as you see them (without the quotation marks). The formulas are shown for row 13 (winter). For row 14 (summer), just copy E13 through I13 down one row and replace all winter db and rh references in the formulas (cells F5 and F6) with summer db and rh (cells I5 and I6).

When finished, compare your results to Figure 2. If they don't match up, check your cell references. For a more in-depth description of the formulas and methods used to generate these calculations, refer to Chapter 6 in 1997 ASHRAE Fundamentals. Good luck!

EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue. ES