23 81 00 DECENTRIALIZED UNITARY HVAC EQUIPMENT

The Professional shall obtain permission from the University before designing packaged rooftop units for University projects.

Air cooled packaged air-conditioning equipment shall be equipped with low ambient controls to permit operation to 0°F.
Rooftop package air conditioners 5 tons and larger shall be mounted on structural steel channel curbs with curb isolation rails. Smaller units may be mounted on manufacturers' prefabricated curbs.
Submit details and catalog cuts of unit prior to design. Units must be manufactured for that application.

Use of air cooled packaged heat pumps on University Park Campus are not permitted.
When considered for use on Commonwealth Campus, prior approval must be received and 100% auxiliary heat must be provided.

Evaluate and select systems and equipment for lowest 30 year life cycle cost. Refer to Design Phase Submittal Requirements, “Design Phase” Section, paragraph B.1. Also refer to Design and Construction Standards, Introduction, Paragraph K and 23 00 01.01 Owner General Requirements and Design Intent.

Consider extra high efficiency units with 2 stage compressors and ECM fans whenever appropriate to achieve energy efficiency goals and improved part load performance, including reduced cycling of compressors.
Select refrigerant type for least environmental impact and best long term economic benefit.
Where close dehumidification control is needed for the application, use technologies that avoid simultaneous heating and cooling mechanical energy. Hot gas reheat or wrap around heat pipe may be viable options.
Minimize pump energy use with variable flow pumping controls whenever justified by lowest life cycle cost.

Systems shall be designed to include means to ensure proper flow to each unit within allowable ranges as overall system pressure and flow fluctuates without objectionable noise or maintenance nuisances.

Large quantities of decentralized terminal units with DX refrigeration systems and filters are undesirable in large scale projects due to extensive, multiplied maintenance requirements.
Dedicated outside air systems to supply preconditioned and dehumidified fresh air are required to adequately maintain zone relative humidity within acceptable ranges for indoor air quality and to minimize risk for mold growth.

Experience has shown that areas served by terminal cooling units with constant volume and occupied continuous fan operation and supplied with untreated OA as a rule have problems with inadequate dehumidification. When compressors cycle off when space temperature is satisfied, any moisture condensed on the coil during the on cycle is re-absorbed back into supply air. This problem is worsened by short cycling due to cooling loads lower than design maximum, which is most of the time. Very serious high humidity problems occur when space has low cooling load and outside air conditions are cool and humid.
Refer to economizer requirements elsewhere for spaces that will have year round cooling requirements.

Mechanical equipment requiring routine access for inspection and maintenance such as fans, compressors and filters shall be located in mechanical spaces with sufficient working clearances maintained. Refer to Coordination requirements in 23 00 01.06.
Free delivery type units with compressors and fans located within areas served are prone to objectionable noise levels. Therefore, they are not acceptable in noise sensitive areas such as classrooms, conference rooms, and sleeping areas. Refer to Vibration and Sound Control requirements in 23 05 01.05.
Maintain at least a minimum deadband of 20 degrees in the condenser water temperature control(per IECC) between minimum setpoint(enabling heat addition) and maximum setpoint (enabling heat rejection).
Be sure to insulate piping that will carry fluids below 55 degrees F or otherwise where condensation may occur due to transporting fluid at temperatures lower than ambient indoor dewpoint.
With low temperature loop temperature, the use of high efficiency condensing boilers is a viable option. However, special care must be taken to ensure acidic condensate will be neutralized and operating staff must be properly trained to keep it maintained in perpetuity in order to not harm plumbing systems.

Preferred method of heat rejection is open cooling tower (induced-draft type with VFD fan speed control), remote indoor sump, tower loop pump, plate and frame heat exchanger (or shell and tube with marine type headers that allow easy end removal for inspection and cleaning without disturbing the piping system) and a constant pressure, centrifugal solids separation system.

The first cost of this combination is relatively close to the cost of the closed circuit fluid cooler with all required freeze protection methods. In addition, operating costs are lower because no heat is lost from the loop in the winter, winterization/freeze protection is minimized and less power is required for cooling tower fans.

Ground-Source Heat Pump systems shall, in addition to all of the above, meet the requirements listed below:

Refer to associated ground coupled heat exchanger (well field) requirements in 23 21 00.06 and 33 20 00.

Test wells: In addition to the geological information required, the test well data shall include empirical thermal conductivity values that can be used to optimize the well field design.

Design and installation of ground-source heat pump systems shall comply with industry best practices in accordance with the following publications:
ASHRAE: Ground Source Heat Pumps – Designing Geothermal Systems for Commercial and Institutional Buildings, current edition.

Antifreeze solution, if required, shall be non-toxic and have low environmental impact to minimize risk in the event of uncontrolled fluid loss through the well field to the ground/groundwater. Ethanol formulated for commercial antifreeze solution and Propylene Glycol appear to be relatively non-hazardous and are presently the only options acceptable to the University. Ethanol has slightly better heat transfer and lower pump energy characteristics and estimated lower solution costs of those two options.

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