Your questions answered: Retrofitting HVAC systems in existing buildings
While the mechanical engineering team has much more control over how HVAC systems are designed and how they fit into the facility for new construction, retrofitting HVAC systems to accommodate existing buildings is considerably more challenging.
Jerry Bauers and Ryan Evans present additional information about retrofitting HVAC systems. Engineers must determine which HVAC system components should be included in a retrofit, how new equipment and supporting infrastructure will fit existing spaces, and how to bring systems in existing buildings up to local code.
The Oct. 15 "Retrofitting HVAC systems in existing buildings" webcast presenters addressed questions not covered during the live event.
Question: In a building retrofit, how do you decide what parts of an existing control system can be reused and what needs to be replaced? If controls are to be replaced, how is the work coordinated for occupied buildings?
Jerry Bauers: The most appropriate method of evaluating whether any device—and particularly control system devices—can be reused in a renovation is to complete a thorough test of those portions of the existing control system that can be reasonably retained for future use. In our experience, conducting field calibration tests of analog sensors, operational tests of safeties and interlocks, stroke testing of analog outputs, and sequence testing of controllers and control loops is a necessary part of evaluating existing controls. Some of this testing can be conducted at the control system front end after the sensor inputs are verified to be properly mapped and reporting. In a recent project that we completed, this on-site evaluation of the existing controls was eliminated from our work on the advice of the on-site control service provider. At start up, we found that the existing controls had substantially failed and required extensive repair and replacement work.
This unanticipated condition caused several months of delay in completion of the project, both to convince all parties of the failures and, only after everyone was on board, to complete the work. The second part of this question addresses control system replacements in existing buildings. Generally, control devices can be installed parallel to existing controls and burned in while the existing controls continue to work. When the new system is complete, we typically bring systems on one or two at a time in a closely-defined window. At times, these systems are operating in hand during the transition. But, the most important strategy to consider to minimize negative impacts on building operations is to complete the startup and commissioning of the controls as a team. Control contractor personnel, commissioning agents, and TAB professionals should be on-site together to initiate controls; validate all normal, transition, and emergency operating modes; and validate control loop tuning during fully automatic integrated operations. When complete, the system should be trended for a minimum of 7 days to identify any operational conflicts that may compromise building operation. This commissioning team must remain available during this period to address poorly performing controls as quickly as possible.
Q: Based on lifecycle cost analysis, which systems typically "win" for different types of facilities? In other words, which system types have the lowest lifecycle cost?
Ryan Evans: A fair question, but one in which the answer is quite involved. It depends greatly on the type of facility and its operation schedules, the utility rate structure, and the perceived building life. In the example we provided during the webcast, the building life was 75 yr per the building owner/operator. Because it was a governmental entity, they could not take advantage of the relatively lucrative tax incentives at that time for a ground source heat pump (GSHP) project. This caused the simple payback to go from about 6 yr to about 30 yr. However, the net present value of that project at a 3% discount rate was about $1.5 million—nearly double the DX system with heat recovery. In this scenario, GSHP was king because of the anticipated savings over 75 yr, but if the building life was only 30 yr, it would have never costed out in our model. We need more years to get the building to start paying back. Therefore, on institutional structures with long building lives, we want to consider the more expensive, but efficient systems. On retail facilities or for more temporary structures, code-minimum systems are perhaps the best alternative, especially if electricity and gas are cheap.
Q: How do you assess and treat existing components that will connected to new work? For example, when connecting new ductwork to existing, does existing need to be cleaned? If it has internal insulation how is that evaluated for cleanliness, air quality issues, etc.?
Bauers: Great question. Typically, existing ductwork is retained wherever reasonable because of both the cost of the ductwork and the related renovations. In this case, we do recommend a couple of strategies. First, the ductwork should be inspected for dirt, biological growth, and existing internal insulation. If any air quality issues have been reported in the building, conducting air quality testing is prudent to protect both designer and owner by establishing a baseline. It is worth noting that when possible, increasing air velocities in the ductwork that will remain often can shake lose problems that may not be present until the renovated systems are started. In some projects, an internal duct liner is unacceptable. If it exists, as a minimum, we want the owner to be advised of the liner and explicitly accept it with any caveats that are appropriate for the project. Don't assume that because an owner has a liner, that you will be free of any problem that liner causes later.
Do a visual inspection of the ductwork, particularly in tight spaces, for bad fittings. Pressure profiles can also identify these fittings. Replace bad fittings. They are problems that never go away. Finally, take pressure profiles (air flow and pressure drop) of ductwork mains that will remain. This information, provided by qualified TAB or commissioning professionals, is the best information available about future performance of the ductwork that is to remain.
Q: Do you suggest tearing out drywall to upgrade the wall insulation if necessary? Is it worth it to the owner? For a typical renovation? Or does it depend on the lifecycle cost?