DISPLACEMENT VENTILATION IN CLASSROOMS

REPORT OF VISIT TO BOSCAWEN ELEMENTARY SCHOOL
BOSCAWEN, NH

Introduction

This report concerns my visit to observe the displacement ventilation system installed in the classrooms of Boscawen Elementary School, Boscawen, NH. The Boscawen Elementary School was designed by the H.L Turner Group, Inc. of Concord, NH. Construction was completed in 1995. Its design incorporates many revolutionary and innovative design features.

I visted the site on Tuesday September 22, 1998. I was accompanied by Ms. Jane Lacasse, Principal of Boscawen Elementary. This report summarizes my observations.

Executive Summary

The Boscawen Elementary School in Boscawen, NH was visited to observe the installed displacement ventilation system, which supplies 100% outside air to classrooms, conditioned to 68°F. The conditioned air is supplied at the floor level through low velocity supply diffusers, and is exhausted from the room at the ceiling level. Air - to -air heat exchangers are used to recover energy from the exhaust air. The system is apparently able to maintain required indoor temperature and humidity levels throughout the year. According to the building users, the displacement ventilation system has improved the indoor environment, and lowered absentee rates compared to the previous building system.

At the time of our observations, the indoor conditions met typical building requirements. Noise generated by the HVAC system appeared to be less than would be generated by conventional systems.

What is Displacement Ventilation?

Displacement ventilation is a unique, innovative concept for supply of conditioned air and ventilation of buildings. It use the natural buoyancy of warm air to provide improved ventilation and comfort. First developed for industrial buildings, displacement ventilation now enjoys an increasing percentage of market share for many applications throughout the world. Although relatively new to the United States, displacement ventilation has been in use in the Scandinavian countries since the 1970s, where it is now seen as a proven technology.

Displacement ventilation systems differ from conventional HVAC systems in several important ways. In a conventional HVAC system, air is supplied at the ceiling, at a relatively high velocity, at a temperature about 20&degF below the desired room temperature. The supply air mixes with the room air to provide a nearly uniform temperature throughout the space. Because of the mixing effect, "used" room air recirculates, resulting in relatively low ventilation efficiency. In a displacement ventilation system, supply air is introduced to the space at or near the floor level, at a low velocity, at a temperature only slightly below the desired room temperature. The cooler supply air "displaces" the warmer room air, creating a zone of fresh cool air at the occupied level. Heat and contaminants produced by activities in the space rise to the ceiling level where they are exhausted from the space.

In theory, the displacement ventilation system provides the following advantages:

Improved ventilation efficiency, resulting in better indoor air quality.
Energy savings - the higher supply air temperature results in greater opportunities for use of airside economizers.
Improved acoustics - the low velocity air supply should generate less noise than a typical supply diffuser.

Several practical difficulties exist for displacement ventilation systems, including:

In some cases, larger quantities of supply air may be required.
Because of the high supply air temperature, indoor humidity must be carefully controlled.
Displacement ventilation systems may not be appropriate when contaminants are heavier than air, or not associated with heat sources.
When very high loads exist, a displacement system will require uncomfortably cold supply air. Therefore, displacement ventilation may not be appropriate in extremely warm climates.
Performance of the displacement system is dependent upon ceiling height. Displacement ventilation may not be appropriate in spaces with low ceilings.

Because of these advantages and limitations, displacement ventilation is typically seen as appropriate for schools, classrooms, lecture halls and theaters with high ceilings, and large ventilation loads relative to the sensible cooling load in the space. However, it may, in theory, also be used to great advantage in office spaces in temperate climates such as New England.

Observations

The Boscawen Elementary School is a single story complex, consisting of classrooms, gymnasium, cafeteria, library, and school offices, surrounding an open interior courtyard.

The office area is conditioned by a conventional VAV reheat system, and is not of interest for our purposes.

The individual classrooms use the displacement ventilation concept. Each classroom is approximately 1000 square feet. A low velocity supply air grille is located at each corner of the room. (See Fig. 1) These grilles supply a constant volume of air to the room at a temperature that varies between 68&degF and 80&degF depending on room temperature. Air is exhausted from the room through a single grille in the ceiling. (See Fig. 2) Each classroom has a set of operable windows. Below the window is a piece of hot water fin-tube radiation.

Multiple classrooms are served from a single air handler. Temperature control for each classroom is achieved by a hot water reheat coil, which modulates in response to a signal from the room thermostat.

Classroom air handlers are designed for 100% outside air. Air - to - air heat exchangers are used to recover energy from exhaust air by preheating or precooling outside air. (See Fig. 3) A hot water heating coil provides additional heat to the air as required. Approximately 40% of the airflow passes across a "dehumidification coil". This coil is a standard direct expansion cooling coil connected to an oversized condensing unit. The dehumidification coil sub-cools a portion of the supply air to a temperature far below its dewpoint. (See Fig. 4) Although the classrooms are not "air-conditioned", per se, the system is able to maintain room temperature in the range of 75° F year round, according to the building users.

Carbon dioxide sensors in each classroom allow the air handlers to operate at variable airflow rates, although the system typically operates at full airflow during normal occupied hours.

Other areas of the school, including the library and cafeteria also use this basic system. The Gymnasium area uses a similar system, except that the space is heated and ventilated only.

Bathroom exhaust and make up is completely separate from the other air systems in the building. Each bathroom is equipped with a special ventilating unit consisting of an exhaust fan and air - to - air heat exchanger. This allows the building to recover energy from the exhaust air to pre-heat make-up air to the toilet rooms.

Ms. Lacasse expressed extreme satisfaction with the building and the displacement ventilation system. She gave anecdotal evidence of the improvement in indoor environment created by the displacement ventilation system. Ms. Lacasse told the story of a teacher who is suffering from asthma. At the previous facility, she was frequently absent due to the symptoms of her condition. According to Ms. Lacasse, she has not missed a single day due to the asthma, since the new building has opened. Ms. Lacasse also stated that the student absentee rate had fallen since moving from the new facility. Although there are many factors that may account for these improvements, the consensus of the building users is that the ventilation system is a major factor.

In our observation, the systems seemed to do an adequate job of conditioning the indoor environment. Indoor temperature and humidity were within the normal range for comfort. Acoustics in the space were excellent. Noise generated by the HVAC system appeared to be well below levels generated by a conventional system. In fact, noise levels were so low that it was difficult to determine if the system was running.