Teaching An Old School New Tricks
In this overview of a three-school hydronics project in Texas, the author demonstrates how control is critical and why it’s worth putting some thought into alternate design scenarios along the way. He also offers a couple of tips for retrofits in constantly occupied spaces.
In 2005, the El Paso Independent School District, in EL Paso, TX commissioned Alegro Engineering, L.P. to implement IAQ improvements to two elementary schools and one middle school. The schools’ existing (and original) A/C systems, installed in the ’70s, included central evaporative coolers, hot water reheat systems, and constant flow pumping systems. With this system set-up for use during the cooling season, there was no way to provide individual classroom control.
One of the goals of the project was to provide individual room control during the heating and cooling seasons, as well as to maintain relative humidity (rh) at no more than 60% year round.
Control Is KeyIn order to achieve the owner’s requirements during the schematic design phase, Alegro Engineering designed a system with central AHUs and variable flow terminal units with hot water reheat coils to each space and classroom. Not only had this system been used with success in other school projects, but individual room control could be achieved through this design. The hydronic portion of the systems included air cooled chillers and constant flow primary pumping systems. On the heating side, the system consisted of water boilers, a primary pumping system with constant flow, and a secondary system with variable flow pumps. All AHUs were provided with pre-heat coils, and most terminal units were provided with re-heat coils.
All secondary pumping systems were provided with two-way valves throughout except for one AHU, and both the chilled-water coil and the hot-water coil were designed for constant water flow. We used a three-way valve to avoid a pump working on a dead-head situation. Each selected coil would require 10% to 15% of the maximum flow.
When the opinion of probable construction cost for each project was prepared, we found out that there was not enough money to provide each school with the required upgrade. In a team effort, and working with the owner, areas that could remain with evaporative cooling systems for all three schools were designed and planned as alternate design solutions.
Design ConsiderationsBased on the sequences of operation planned for the project, the AHU preheat coils were selected to increase the temperature from the local winter drybulb to 90°F. We planned this in order to have a morning warmup sequence that can rapidly bring the building temperature up to a comfort level.
During building warmup, all outdoor airflow is blocked, and the supply air temperature increases rapidly. During this time, the reheat coils at the terminal units remain closed. Once the building is at operating temperature, the outside air (OA) dampers open, and the OA is brought into the buildings as required by ASHRAE Standard 62 and the International Construction Code.
All reheat coils were selected to increase the temperature from 53°. At all times, the AHU supply air temperature, either by pre-heating the air or by bringing enough OA to maintain such temperature during the winter months, is at 90° to maintain space temperature.
With the energy management and control system (EMCS), the mixed air temperature can be monitored, and increase OA in an economizer mode during the heating season to maintain a 53° at all times.
Alternate PlanningBecause of the alternate designs, the design engineer must evaluate the hydronic system under all possible scenarios (e.g., with only one alternate design accepted, all alternates, and no alternates).
With the help of equipment representatives, we evaluated the situation and ensured that we were selecting a pump that could perform under all alternate designs scenarios, giving the owner the ability to make upgrades to those areas of the building that had fallen under alternates at a later time, without requiring major changes to the pumping systems. All piping was designed with the whole building load (e.g., all alternates had been accepted and could be installed at any time). During construction, all three schools were occupied; only partial areas were renovated at a time, and the pump had to properly function at every phase of the project.
Thinking It ThroughAs the design engineer, always consider a bypass with a temporary circuit setter and to have it removed before moving to the next phase. Flowmeters in different zones of the building represent another consideration that the design engineer commonly ignores (and/or the contractor fails to install).
In the case of a multistory building, flowmeters should be provided at each floor. This meter helps regulate the actual flow through the zone, aiding during the test and balance phase and commissioning of the project, as well as at a later date when building requirements change.
When planning phases and alternates on any given project, the design engineer should always think through each phase as if the alternates are not accepted. With that extra thinking and minimal extra work from the designer, a lot of headaches and problems can be avoided during the construction phase.
When coordinating installations during a retrofit, several issues may spring up to attention during construction, and a lot of surprises can show up. In some instances, this means older alterations to the building, like replacing roof drain piping, where the retrofit might have been done using PVC pipe in a plenum return. When installing the new system, a PVC pipe exposed in the plenum may not comply with current codes and/or building occupancy.
For all three projects, the existing systems had to be operational while the new systems were installed and commissioned. During such time, bypasses shall be provided at each phase in the old system as well as in the new system.
All new piping must be coordinated with existing installations (ductwork, existing piping, electrical, communications, etc.), and the structure has to be evaluated to ensure that it can handle the weight of old and new piping and equipment. To a certain degree, planning a design in an existing facility can be much easier if enough time is spent upfront investigating and documenting existing conditions to reduce possible conflicts and issues during construction. ES