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.
By Jerry Bauers and Ryan T. Evans October 28, 2015

Jerry Bauers, PE, Outcome Construction Services LLC, Kansas City, Mo. Courtesy: Outcome Construction Services LLC

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. 

Ryan T. Evans, PE, LEED AP, Henderson Engineers Inc., Lenexa, Kansas. Courtesy: Henderson Engineers Inc.

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?

Bauers: Depending on the construction of a particular wall, insulation can be blown into a wall using penetrations at the top and bottom between studs. Removing drywall for insulation purposes only—although it’s the most effective renovation—is hard to justify with energy savings. Removing drywall (demolishing to the studs) may provide for an opportunity to substantially improve the building’s air barrier (see Ryan’s comments regarding air barrier and wall insulation design). As Ryan noted in his presentation, reducing infiltration may have the most significant impact on energy and may well deliver the collar benefit of reducing mechanical system costs in the renovation.

Q: What about 5 cfm/person and 0.06 cfm/sq ft?

Evans: We believe the question stems from the ASHRAE 62.1 sections defining a people-basis and an area-basis for code-minimum ventilation air. Unfortunately, we didn’t have time to get in to the nuances of the code, so we presented a more simplified model of 15 cfm/person, or 20 cfm/person, etc. The point we were attempting to make is that there are documented advantages to labor productivity when increasing air supply. Using a strategy like demand controlled ventilation (DCV) could actually give you the best of both worlds by marginally increasing your people-basis number (to something above 5 cfm/person in this example), but only maintaining the area-basis number when the zone or space is unoccupied. Nevertheless, we must first better understand the impacts of increased ventilation to lifecycle cost. We know that it will have an impact on energy, but it may be more than offset by productivity gains of the workers.

Q: How can freelance/retired engineers continue to participate in the HVACR industry in providing consulting engineering services to customers both during construction projects and continued optimization for the operation/maintenance of their mechanical/HVACR systems?

Bauers: While I’m neither freelance nor retired, but I am close. The challenge that we have in the winter of our careers is dedication to projects. If we can commit to projects from start to finish, including what is often today, unreasonable schedules, there is a place for the experience of more senior engineers. In fact, the generation immediately behind baby boomers is very small. The numbers of engineers available to provide engineering management and project leadership is smaller than the workload. Finally, if your skills are really inside buildings optimizing system operations, there is much more work available than there are people to do the work.

Q: Actually, 75 yr seems to be a stretch with overall building life and improvements in technology, or is the total time period variable?

Evans: It’s the latter. The lifecycle is variable. I had another example of an LCCA we were going to share, but cut for the sake of time. In that example, we evaluated envelope components (roof insulation, windows) against a 30-yr building. Office buildings tend to get modeled against a 30-yr life, whereas institutional buildings are in the 75-yr range. It all just depends.

Q: How do you clearly explain the difference between equipment testing and commissioning testing to a client and her operations and maintenance staff?

Bauers: In the commissioning world, equipment testing is most often what we refer to as "pre-functional testing." It is equipment startup focused on determining whether a component—both simple and complex components—function as the manufacturer intended. The challenge a commissioning agent faces is demonstrating and adjusting these components to operate in a real world integrated system-a building. Commissioning testing is about determining whether the components, strung together in a system, will operate in a reliable fashion to support the business function of that building. Equipment testing can establish that a component can be started and stopped using enabled setpoints. Commissioning testing will determine whether that equipment will actually shut down in a fire condition or loss of power. As importantly, it will determine if it will restart when the emergency condition is either cleared, or it requires operation of that equipment, for example, smoke evacuation purposes.

Q: You mentioned heat load by a formula for only sensible heat. How about the latent heat?

Bauers: If latent load in a space is significant, you can use the same technique to measure space load by using both dry bulb temperature and relative humidity, or dry and wet bulb temperature. The formula you use is either the dry bulb formula or the enthalpy formula. Typically, when we are working in an existing building, we have a very good sense of whether the latent load of the space is a factor in coil selection. Remember that it is always necessary to either test your space in neutral conditions or calculate the impact of envelope loads. In reality, I’ve most often used this technique where I have high internal loads, where electronic equipment is a significant factor in the internal loads, and the envelope load is relatively easy to define (low glass and solar loads).

Q: Can you talk about ventilation requirements and how it relates to a "sick building" as building construction creates tighter building envelopes?

Evans: With the inclusion of air barriers, no doubt buildings today are much tighter than their historic or near-historic counterparts. However, the infiltration of yesterday’s building was not necessarily a good thing at all. For example, I’ve seen many 75-yr-old buildings where the highest relative humidity we could maintain when it was 0 F outside was 3%. If you try to humidify the spaces, condensation takes place in the walls where you cannot see it, which can lead to mold growth. If left at 3%, viruses thrive. Further, the dry air can crack your nasal passages, further complicating matters. This explains why it’s easier to contract the flu in the winter. The key then is to stop/limit infiltration and then deliver the appropriate amount of ventilation air. Code-minimum requirements are based on scientific studies of air quality, so we recommend starting with that and going up from there. (Remember, studies show increasing ventilation can improve worker productivity in certain situations, generally by improving human health and reducing absenteeism.)

Q: Consulting engineers usually like to gut all the ducts, HVAC units, and redesign new equipment. What do you think of that? We are assuming the equipment is still usable.

Bauers: Wholesale replacement is easier than an effective evaluation of existing components. I believe that a lot of building renovation projects are often driven by an engineer’s reluctance to establish existing equipment performance. On the flip side, in cases where the owner directs the engineer to reuse equipment, we see a significant number of performance deficiencies because engineers use manufacturers’ data, old drawing information, or assumptions that may have never accurately represented the installed performance capability of systems. Barring substantial age and documented problems, retaining existing equipment is often reasonable and cost effective. A detailed analysis of the real performance capability of these existing components delivers both the benefit of leveraging sunk capital costs and avoids unanticipated cost surprises during the startup and acceptance phases of renovations projects.

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