Your questions answered: HVAC: Cooling systems

The May 12 "HVAC: Cooling systems" Webcast presenters addressed questions not covered during the live event.

By Rodney V. Oathout and Jason Atkisson May 19, 2016

HVAC systems consume 25% to 30% of the energy in a typical commercial building; functionally intensive buildings with controlled environments and large process loads can consume more than this. Providing cooling comfort and energy efficiency in nonresidential buildings is an ongoing challenge for mechanical engineers. Emphasis on building performance and adoption of energy-efficient building codes is increasing.

Rodney V. Oathout, PE, CEM, LEED AP, Principal, Regional Engineering Leader, DLR Group, Overland Park, Kan.

Jason Atkisson, PE, HBDP, Project Manager, Affiliated Engineers Inc., Madison, Wis.

Rodney V. Oathout and Jason Atkisson present additional information about designing efficient HVAC cooling systems.

The May 12 "HVAC: Cooling systems" Webcast presenters addressed questions not covered during the live event.

Question: In displacement ventilation systems, what supply air temperature should be used?

Jason Atkisson: Displacement ventilation systems should be designed with a supply air temperature of 63 to 68° F.

Q: Are zero energy buildings truly worth the extra initial cost, even considering a 7-year payback and all the maintenance?

Rodney V. Oathout: In my opinion, yes. A 7-year payback for a zero energy facility would be a very attractive business proposition for most building owners. I am not aware of significantly different maintenance costs for zero energy facilities compared to other facility types. Zero energy facilities usually have passive systems that typically require less maintenance. It is true that zero energy buildings can be different to operate and occupy. Just different, not more.

Q: What determines whether you choose an active versus passive chilled beam?

Atkisson: The primary factor in choosing an active versus passive chilled beam is the cooling requirement within the space. Because passive chilled beams use convection only, their capacity is limited compared to active chilled beams. Passive chilled beams can be used in spaces with a cooling load up to 20 to 25 Btu/h/ft2, whereas active chilled beams can be used where the cooling load is below 40 Btu/h/ft2.

Passive beams should not be used where their placement requires them to be located directly above an occupant because the space immediately below the beam has the highest velocity and could result in drafts. This is not the case for active beams which actually work well directly above the occupant(s). Care also should be taken with passive beams to ensure adequate space is provided for air to flow across the beam.

Q: Do you have lifecycle cost analysis (LCCA) for each different HVAC system?

Oathout: We choose to not estimate lifecycle costs for particular HVAC systems because these values can vary significantly due to changes in construction cost, utility costs, use patterns associated with facility type, and other unique features that may be present for a particular application.

Atkisson: LCCA for each system should be based on your specific project type and location. The LCCA will vary—particularly on the energy usage—based on your specific project, location, and utility rate. In addition, the cost of maintenance can vary greatly depending on an owner’s maintenance staff and their capabilities. As stated, there is no one-size-fits-all solution. You need to compare systems based on the specifics of your particular project.

Q: (a) Can evaporative cooling be used in climates other than arid? (b) Is cold air distribution practical (45° F, for example)?

Oathout: (a) Evaporative cooling can be used in nearly every climate. However, it is most effective in drier climates. Indirect evaporative cooling has more applications in non-arid climates and can significantly improve cooling performance. Direct evaporative devices are also effective as humidifiers in most climates. Maintenance is often a concern when considering direct or indirect evaporative cooling.

(b) Cold air distribution may be practical for some specialized applications, but I would not recommend it for most common HVAC systems. There are obvious reasons, such as frequent condensation issues on diffusers that make this approach problematic. The lower supply air flow rates associated with the colder distribution temperatures can also cause complications maintaining minimum ventilation requirements.

Q: What is your opinion of using chilled beams in a data center application?

Atkisson: Generally, the cooling load density within a data center exceeds the capacity that either an active or passive chilled beam system can provide. Although I have never specifically analyzed it, I would also be concerned about air turbulence in and around the equipment cabinets from local heat rejection fans.

Q: Please clarify Path A and Path B prescriptive compliance of ASHRAE 90.1. Is there more efficiency at peak, or partial load?

Oathout: Path A has a stricter part load efficiency requirement than Path B. Path B has a stricter full load efficiency requirement than Path A. The values in these minimum efficiency tables are intended to be used for the prescriptive approach to energy code compliance.

Q: Discuss condensate removal in variable refrigerant flow (VRF) systems.

Atkisson: Condensate removal for a VRF system is no different than condensate removal from any unitary terminal cooling device. Condensate must be collected at each terminal unit individually and discharged appropriately.

Q: Please clarify the difference between site and source energy.

Oathout: A detailed definition of site and source energy can be found at A simplified definition is site energy is the amount of energy used at the site. This would be the value reflected on the utility bills. Source energy is the site energy plus losses associated with transportation of the energy to the site. In the case of natural gas, the losses tend to be small. The U.S. Environmental Protection Agency (EPA) estimates the loss for natural gas distribution to be 5%. EPA estimates the losses associated with electricity to be 314%.

Q: Which of these HVAC systems presents the smallest duct-system and piping-system pressure drops for energy efficiency operation?

Atkisson: As with any system, the pressure drop and relationship to energy efficiency depend on how you choose to size your ductwork and/or piping. In general terms, if you generously size your distribution system (either airside or hydronic), any system can be designed to minimize pressure drop and save on fan/pump energy. This comes, of course, at the expense of the cost of the system and often the required space. In terms of overall energy efficiency, systems that decouple the sensible and latent loads, such as provided by a hybrid system, provide the most opportunity for improved energy efficiency. They result in smaller air distribution systems, which reduces one of the largest energy uses within the building: fan energy.

Q: How difficult is it to keep radiant panel water temperature above dew point temperature in humid areas?

Atkisson: If you are controlling the space temperature and humidity within the space where you are using a radiant panel, generally with a dedicated outside air system of some kind, then you should be able to ensure the water temperature stays above the space dew point because you are directly controlling the space dew point. Radiant systems of any kind should not be used in a space if you are unable to control the space humidity (i.e., in spaces directly connected to an uncontrolled space: lobby with no vestibule to the outside).

To ensure condensation does not become a problem, you can use one of the following condensation prevention strategies:

  • Add a condensate sensor on the chilled water supply piping that closes the individual control valve when condensation is detected.
  • Use a relative humidity sensor in the space or return/exhaust air path that closes the individual control valve when the relative humidity reaches the sensor set point.

Q: What is the average energy savings of VRF systems over baseline ASHRAE 90.1 systems?

Atkisson: Energy savings over the baseline ASHRAE 90.1 system is not something that can truly be defined without analysis, even as an average. It is directly related to the design of the system, how ventilation is treated, and project type because the ASHRAE 90.1 baseline system is determined based on building type and available utility (fossil fuel, electric, etc.).