HVAC, Plumbing, and Electrical Systems: Building Analysis

HVAC System Overview

Components of the System

Direct Expansion (DX) air conditioning serves as the main HVAC system for this building. This type of system utilizes the Refrigerant Vapor Expansion/Compression Cycle (RVEC) to allow for the direct cooling of the air supply to an occupied space. Because the building includes both split systems and one packaged system for the DX units, the components differ between configurations. The packaged system contains all four elements of the RVEC system — evaporator, condenser, compressor, and expansion device — along with unit controls, internal ducting, and fans. The split systems house the fans and evaporator within the building, while the remaining RVEC components are placed outside as separate units, allowing greater flexibility.

The system features a hot water heating coil, a DX cooling coil, and an integral energy wheel. The cooling capacity is thirty tons and uses CO₂ sensors plus a Variable Frequency Drive (VFD) to adjust the quantity of exhaust air and outside air based on building occupancy. Ductwork covering return, supply, exhaust, and outside air is constructed from galvanized sheet metal with external duct insulation. Sound attenuators are installed alongside each VAV terminal unit. The three HVAC systems consist of one DX packaged unit (located at the east end of the building, outdoors) and two DX split systems. The Mechanical Room includes ventilation (exhaust with 200 CFM capacity) and heat (1 kW wall heater).

Purpose and Function of Each Component

The heating and cooling coils warm and cool the air, while the energy wheel recovers and transfers heat. Variable Air Volume (VAV) systems can serve multiple purposes, functioning as heating, air conditioning, and/or ventilating systems that deliver a constant supply of airflow at variable temperatures. Sound attenuators act as duct silencers, reducing noise generated by airflow through the HVAC system. The galvanized sheet metal ducts offer easy fitting and flexibility, ensuring unencumbered airflow throughout the building.

How the Components Work Together

The air needed to cool the occupied spaces is directly chilled by the refrigerant housed in the cooling coil. Because the refrigerant directly cools the air — and similarly for the heating coil — the system achieves higher cooling efficiency. Refrigerant piping, however, cannot always be routed over large distances. For this reason, one unit is located on the first floor, one in the basement, and an external component is positioned to the east of the building.

Controls of the System

The controls are part of the APR Control, a capacity-control device that modulates refrigeration and air conditioning capacity to match fluctuations in the BTU load. The unit reads the heat content of the air returning through the system and then adjusts the suction gas pressure accordingly. The APR Control can sense variations via the evaporator coil and the enthalpy of returning air. Installation of an APR Control is performed by a refrigeration mechanic and typically takes four to six hours.

The Cooling Cycle

Air passes over an indoor cooling coil, heating the cold liquid refrigerant and transforming it into a warm gas or vapor. A copper tube then transfers the vapor to the compressor, which compresses it and increases its pressure. The hotter vapor travels through another heat exchanger — the outdoor condenser — where the hot gas transforms into a warm, high-pressure liquid. This liquid moves through a liquid line (a smaller copper tube) and then to a dryer/filter before reaching an expansion device, which reduces the high-pressure liquid to a low-pressure, cold liquid. Finally, the cold liquid passes through the indoor cooling coil, and the cycle repeats.

The Heating Cycle

The heating cycle follows the same process as the cooling cycle with one exception: a special valve located in the refrigeration piping reverses the refrigeration cycle. In colder weather, a defrost period is also necessary for the heat pump to maintain efficiency.

Energy Conservation Measures

The DX systems offer flexibility, lower costs, and reduced maintenance demands. Both the packaged and split systems are suitable for a variety of situations and can operate at high capacity. Lower maintenance requirements translate directly into energy savings. Sealing and insulating the ductwork further conserves energy and reduces air leaks throughout the system.

Why This System Was Chosen

The system is comprised of two DX split systems using R-410A refrigerant. Because DX systems require less maintenance than many alternative system types and remain a reliable, cost-effective choice, they are well-suited to the estimated building usage and help minimize life-cycle expenses. The DX systems include two air-cooled condensing units. One unit, with a cooling capacity of twenty tons, serves the first floor; the second unit, at half the capacity (ten tons), serves the basement level. Both units accommodate VAV applications and feature dual refrigerant circuits with scroll compressors. The design is based on the Carrier 38AUDB25 and 38AUDB12 models. The first-floor unit delivers an airflow of 8,000 CFM, while the basement unit delivers 4,000 CFM. Hot water reheat coils are provided at all VAV terminal unit zones.

Impact on the Construction Schedule

Because three total DX systems are being installed, the work may take somewhat longer than a single-system installation, as the split systems require components to be placed both externally on the building and on each floor. However, the overall installation timeline may remain manageable given that the control unit installation takes a maximum of six hours.

Plumbing System Overview

Potable Water Supply and Waste Discharge

Two full-length trench drains will replace the existing floor drains within the building. The new drains will be connected to existing underground waste lines. The trench drains are located in the latrines. Additional drain lines will serve the mechanical room and the cadre lounge, the latter accommodating ice makers.

Type and Location of Water Pipes

There are two types of water pipes: hot and cold. A firestop material covers the pipes, along with rock wool pipe insulation and sealant, and a steel pipe sleeve. The water line size is 3 inches, reducing to 2½ inches at the main shut-off valve. The water pipes run alongside the latrines and the cadre lounge.

Type and Location of Sanitary Sewer Pipes

The sanitary sewer pipes are located in four bathrooms (latrines): two male, one coed, and one female. The plumbing fixtures include six floor-mounted flush valve toilets, three showers, eight wall-hung lavatories, and five wall-mounted flush valve urinals. Additional fixtures include ten stainless steel single-bowl classroom sinks, two hose bibbs, two dual-level electric water coolers, and one floor-mounted mop sink.

Pumps in the System

A domestic hot water recirculation pump rated at 11 GPM at 1 ft of head, operating at 115 volts with a 1.4 full-load amp draw, is included in the system. An in-line hot water circulating pump also maintains the proper loop temperature at the water heater.

Controls in the System

One gas-fired water heater heats the domestic water supply through direct venting, with an input of 125 MBH and a storage capacity of 65 gallons. The design basis is the Lochinvar SNR126-065. The recirculation pumps and mixing valves also function as controls. The mixing valves restrict the maximum hot water temperature to 120°F.

Hot Water Source and Distribution

Hot water is produced by the gas-fired water heater and recirculated throughout the system by the hot water recirculation pump, ensuring consistent temperature at all points of use.

Impact on the Construction Schedule

The plumbing system is complex and involves the installation of new trench drains integrated into the existing building framework. The need to construct several latrines and accommodate multiple water uses — including water coolers and ice makers — may add time to the overall schedule, particularly if complications arise during piping installation.