I have recently received a number of inquiries regarding VLI (ventilation load index) data and how it should be applied. In short, the VLI represents the energy required "at the coil" to heat, cool, and humidify 1 cfm of OA to a defined constant condition for one month. That defined leaving air condition used to calculate the VLI data reported in the "Weather Report" each month is 55 degrees F, a commonly used leaving air temperature setpoint.

Individual sensible and latent cooling and heating VLI components are calculated hourly, and then totaled for the month. The sensible cooling ventilation load index (VLIcs in Ton-hrs/cfm) is calculated for every hour that the outside air dry bulb temperature is above 55 degrees.

The latent cooling index, VLIcl, is calculated for every hour that the outside air humidity ratio is above the humidity ratio at 55 degrees saturated. The sensible heating load index (VLIhs, in Therms/cfm) is calculated for every hour that the outside air dry bulb temperature is below 55 degrees.

The last component is the humidification load index (or HLI). This is the amount of water required to maintain a constant space rh of 30% at 70 degrees space temperature. It is calculated when the outside air humidity ratio is less than the humidity ratio at the desired space condition. Note that the HLI is given in Gallons/cfm and not in units of energy because not all humidifiers require heat energy (or steam).

Now that we know what it is, the following examples illustrate how VLI data can be used to evaluate the cost impact of cooling, heating, and humidification of outdoor air;

Ventilation cost analysis (VAV fume hood exhaust example): VLI and HLI data can be used for quick and accurate ventilation energy and water consumption and cost analysis. Since VLI data is based on hourly calculations, it's much more accurate than bin data because the error associated with averaging is eliminated.

For example, let's say you need to quickly determine whether or not a VAV fume hood control system is worth considering for a 100,000-cfm laboratory exhaust and makeup air system located in Boston. The assumptions are: 55 degrees supply air, 30% space rh at 70 degrees, $0.08/Kwh electric rate, $0.70/Therm for gas, 80% boiler efficiency, 0.8/kw/Ton cooling efficiency, total supply and exhaust fan brake horsepower (bhp) = 75, normal annual cooling, heating, and humidification VLI data for Boston = 6.31, 0.92, and 4.35.

So for a constant volume system, the operational costs would be:


Cooling 100,000 cfm x 6.31 Ton-hrs/cfm x 0.8 x $0.08 = $40,384
Heating 100,000 cfm x 0.92 Therms/cfm x 1/.80 x $0.70 = $80,500
Humidification 100,000 cfm x 4.35 Gallons/cfm x 8.34 x 1,050 x
$0.70 / 100,000 = $26,665
Fan Cost 75 bhp x .745 x 8,760 x $0.08 = $39,157

The resultant total annual cost for the constant volume system would be $186,706. If we assume we can save about 50% ($93k) with VAV fume hood controls and the owner is looking for a three-year payback, VAV fume hood controls would make sense if they can be installed for $280,000.

Utility bill analysis, accounting for latent loads: VLI data is also very helpful in determining the impact that the weather has on utility bills each month. Traditionally, owners would plot utility bills against degree days each month and make comparisons to previous years. For example, if utility costs were up but degree days were down compared to the same month from the previous year, then perhaps the weather is not to blame.

However, as discussed in previous columns, degree days are based only on dry bulb temperature and do not take humidification or dehumidification into account. VLI data can be used to account for these latent loads. Figure 2 shows Boston cooling degree days vs total (sensible + latent) cooling VLI data for 2002, 2001 and normal. Note the discrepancy between the two for the month of August. When tracking utility costs, VLI data can be a valuable tool in providing the complete weather impact picture. ES

EDITOR'S NOTE: The images associated with this article do not translate to the Internet. To view the images, please refer to the print version of this month's issue.