What is in this article?:
- The devil is in the details
- Choose the lowest horsepower fan and vary the speed
- Eliminate the need for reheat
Eliminate the Need for Reheat
Modern VAV systems do not reheat cooled air. In the winter, ventilation air is delivered to zones at temperatures needed to serve zones calling for cooling. Heating in perimeter zones is limited to times when primary airflow is based only on ventilation demand. This requires careful zoning, added perimeter insulation, heat recovery from warm ceiling plenum air, and demand-based ventilation control.
Other tips for eliminating the need for reheat include:
- Adding enough inexpensive insulation to the building envelope until the heat loss through the envelope at outside temperature below winter design is less than the heat gain of the occupants and other internal loads in perimeter spaces. Morning warm-up heating may still be required, but this can be supplied before cooling load is present.
- Having shrunk the perimeter-heating zone as much as possible, use separate air handling units for each heating zone exposure and separate air handler(s) for the interior.
- Recover heat in the ceiling plenum using fan-powered boxes on the perimeter.
- Activate mechanical heat only when cooling flow is at its minimum to satisfy ventilation requirements.
- Use high-induction diffusers to reduce stratification. All VAV diffusers must be high induction to deal with flow variations.
- Use the lowest possible minimum-flow setpoint. It must be the higher of the minimum ventilation requirement or the lowest allowed by the VAV diffusers (10 percent of design airflow).
- Use supply-air-temperature reset. An increase in supply-air temperature will require a larger volume of air, but the higher supply temperature will eliminate (or minimize) reheat. Good zoning and perimeter insulation makes this unnecessary, but where simultaneous heating and cooling in one air system is unavoidable, use modeling to find the ideal compromise between supply temperature, reheat energy, and fan energy.
- Increasing the use of fan energy to lower the use of central plant energy cannot be justified, except when also reducing reheat. One method is to reset supply-air temperature during cool weather and size interior zones for 60°F or higher supply air. Supply-air reset may not be beneficial in warm climates and in high-humidity locations where alternate dehumidification may be required.
Small Zones Improve Comfort and Reduce Overcooling/Overheating
Try to provide a VAV zone and a thermostat for each occupant. This can be done cost-effectively with diffusers that incorporate temperature-control dampers.
For Lowest Pressure Drop in VAV System—Use Low Supply-Air Temperatures and Temperature Reset
Large components, smooth transitions, unobstructed fan inlets, and fully developed fan discharges matter. Ducts can be smaller if design supply temperatures are lower. This approach, coupled with supply-temperature reset, saves enough on ductwork to fund many other attributes of HPAS without driving installed costs too high.
Select the largest coil that can fit in the space. Coils should have a minimum waterside delta-T of 15°F and face velocities between 300 and 450 fpm.
- Use the largest filter bank that can fit in the space.
- Select low-pressure-drop extended-surface-area filters.
- Minimize fan-system effects with an unobstructed inlet and a straight run of duct or an elbow in the direction of the fan rotation at the outlet.
- Use the minimum duct liner only when necessary for sound attenuation. Avoid sound traps by selecting larger, quieter, more efficient fans and using components with low face velocities.
- Place the shafts close to the air handler.
- Deliver air from rooftop units inside an insulated roof curb to avoid ducts outside the building envelope.
- Size for 800 to 1,200 fpm closest to the air handler.
- Consider multiple air shafts for large floor plans.
- Use leak-tight, aerodynamic round duct designed using static regain to reduce fan load and duct pressure. In many cases, this means using round or oval duct with gasketed connections and fittings.
- Use multiple home runs to a discharge plenum. This saves space, money, and energy.
- Make ducts as straight as possible with a minimum of transitions and joints.
- Use large-radius elbows and low-pressure-drop fittings and takeoffs.
- Size for 700 to 1,200 fpm or pressure drop no greater than 0.08 in. wg per 100 ft.
- Limit use of flex ducting to a maximum of five feet at diffusers.
- Use duct liner only where necessary for sound attenuation. Avoid sound traps.
- Size VAV diffusers for low static-pressure drops: between 0.25 and 0.05 in.wg.
- Size VAV diffusers as large as possible, especially at the end of the duct run, for the lowest possible pressure drop at design airflow.
No balancing damper should be installed before the last VAV diffuser (alternatively that the balancing damper before the last VAV diffuser shall remain open) so that the system will be balanced at the lowest possible fan speed.
- Provide at least one return-air grille for each VAV zone, more if the zone is large.
- Size return grilles for pressure drop no greater than 0.08 in. wg.
- Use ceiling return-air plenums (except in high-humidity locations, such as DOE climate zone 1). Seal ceiling plenums for minimum air infiltration.
- Size return ducts for pressure drop no greater than 0.04 in. wg per 100 ft.