Planning for HVAC System Renovation


IPI has recently completed a second series of Sustainable Preservation Practices workshops and is preparing for a second series of webinars. All registrants for these presentations complete both a preliminary and a follow-up survey to help us better understand the needs and interests of our audience and colleagues. Certain responses come up frequently in response to the “what do you hope to learn?” question. IPI Director Jim Reilly offers his response to two of the most common questions asked in this article.

Is a Mechanical System Upgrade Necessary?

Before contemplating a new mechanical system (and the associated cost and disruption) do what you can to determine whether the old one is truly incapable of doing the job by paying closer attention to exactly what’s wrong with the way it’s working now. It may turn out that an existing mechanical system, if maintained and operated to its full potential, could do the job that needs to be done.

There should be clarity about the capabilities and the shortcomings of existing systems before designing and buying expensive upgrades or whole new systems. Lots of systems appear inadequate because there hasn’t been sufficient attention paid to understanding the functions and capabilities of the current system. There is a tendency to assume that the problem is equipment-based when your goals for improvement, if properly defined and understood, could be met without a major project.

We suggest that a team including facilities and collection staff take a holistic view of the existing system. Study the original design plans and determine that all elements of the system are working properly. Create simple diagrams and floor plans to document which AHU serves each collection storage area. Identify current environmental concerns and set goals for acceptable performance.

You can use graphs of environmental data to help you better understand system performance. If you have a few years of data from a location it is easy to see seasonal patterns and significant changes in performance. Overlay a full year of indoor and outdoor dew point data to identify periods of humidification and dehumidification. If the system has no humidity control the curves will lie on top of each other or follow the same overall pattern. When humidification and dehumidification are taking place you will see a “floor” in the indoor dew point graph above or below which the dew point temperature will not go. A review of several years of data will illustrate seasonal changes in mechanical system operation and changes from year to year that may need to be addressed.

Full year of indoor and outdoor dew point

Optimization of existing systems is not always sufficient—sometimes new equipment is the only answer—but it’s difficult to know without a thorough review of the existing systems.

What Do I Need to Know When Working with Engineers to Plan HVAC Upgrades?

Architects and engineers often ask the client in the design process for a specification of environmental conditions in the form of a target temperature and RH, with specified tolerances around the targets. Unfortunately the answer is not that simple. Based on decades of preservation research, we are more focused on what poses the greatest threat to preservation (what circumstances we need to avoid) than with defining ideal conditions.

One important issue to consider is whether the space will be used solely for collection storage or not. For display spaces and work areas, human comfort is uppermost. This profoundly affects what conditions can be specified. For many different kinds of collections, human comfort will prevent design engineers from creating the most energy-efficient or preservation friendly environments, so it’s best, whenever possible, to separate storage from human-occupied spaces.

Another important consideration is the material makeup of the collections that will be affected by the new environmental conditions. Collection objects respond to the environment in different ways. Optimal preservation conditions vary depending on the component materials and composite structure present. Organic materials (such as paper, textiles, wood and leather) which are vulnerable to chemical change, like cool conditions. Metals like it dry. Composite materials react badly to wide and frequent fluctuations in temperature and humidity. Another way of putting it might be: what are you worried about? Which forms of deterioration most threaten the majority (or most important subset) of objects in your collection?

Broadly stated, environmental specifications for the majority of collection materials include:

  • Make it as cool as possible while avoiding mechanical damage due to dryness (below 20% RH) or high levels of moisture (above 65% RH)
  • Keep excursions below 20% RH or above 70% RH as short and infrequent as possible to avoid mechanical damage
  • Keep excursions above 65% RH to a few days or less to avoid mold and other biological decay
  • Keep summertime dew points as low as possible
  • Keep wintertime dew points from being too low

The cooler temperatures necessary for preservation of organic materials are not necessarily costly to maintain, provided the HVAC equipment and room architectural characteristics (insulation, etc.) are designed for the purpose.

Simplicity of system design and maintenance are also important to overall success. Quite often, a relatively simple design for new equipment and controls will be the cheapest to build and operate. HVAC systems and especially modern automated control systems are complex enough without adding unnecessary features in the name of collection preservation. Most collection objects can tolerate a wider range of relative humidity than the classical museum recommendation of 50% +/- 5% RH. Likewise, a modern view of temperature ranges for collection storage permits a wider range of acceptable temperatures than formerly assumed. Other than human comfort, there is little reason to hold temperature within a very narrow band, especially from season to season. Keep the following facts in mind when specifying operating or design conditions for collection storage areas:

  • The most significant drivers of mechanical decay are seasonal—the extremes of seasonal dryness and dampness are more important than short-term variations in RH
  • Short-term events are also less significant because objects take a considerable amount of time to equilibrate fully to a change in room RH
  • Room temperature settings are too warm for safe storage of organic objects that are vulnerable to rapid chemical change

Finally, it is important to consider sustainable operation and energy efficiency. One way to save money in new HVAC projects is to reduce the size or capacity of equipment serving collection areas. Equipment is typically designed not only to maintain conditions within the specified temperature and RH range, but to do it on all but the most extreme weather days. This means that most of the time the system will operate at well below its design capacity and that means less efficiently. A lower capacity system may not be able to hit the target range on the most extreme days, or be able to pull the conditions into range quite as rapidly during startup, but it will likely offer less operating cost and still be just as good for collections. Other recommendations for improving energy efficiency during the design process without negatively effecting preservation quality include:

  • Design equipment to be efficient at low loads (the majority of time when it isn’t the warmest or most humid outside)
  • Design to keep outside air to code minimums
  • Consider a design that dehumidifies only the outside air not the total air
  • Plan to keep collection storage areas separate from human occupancy spaces
  • Install occupancy sensor control of storage area lighting
  • Design ducts to reduce fan energy

A final thought on the planning process is to view it as an ongoing collaboration, not a one-time thing. If you don’t understand in detail how the new system will work, there can’t be much of a discussion later on when the system is installed. Nor will there be much opportunity to change things. A lot rides on the initial statement of desired conditions, but it doesn’t stop there. For many reasons, collection care professionals need to be aware of both the cost implications and the preservation soundness of the conditions they specify. Getting involved and staying involved in the planning process is the best way to do that.