Reheats Review - Another look at Reheats to Fight Building Humidity

Posted by Stuart Heisey on Friday, June 28, 2024

Frozen Reheats text, with heat and fire underneath

In 2021, Entech hosted a roundtable webinar bringing together a panel of experts in the fields of facility management, engineering, and mechanical contracting to discuss the challenges and some practical solutions related to managing building relative humidity (RH). As we once again head into the dog days of summer, when moisture is especially prone to seep into our lives and our buildings, I thought it might be helpful to focus on one key takeaway from that roundtable; reheat, and also rehash some other takeaways from that event.

Reheats 101: What is reheat and why do I need it in my HVAC system?

A cooling system reheat is quite simply a heating coil that heats air that was previously cooled and dehumidified before it is released into the conditioned space. Seems counterintuitive on the surface, but once you dig a little deeper there are a few valid (and essential) reasons for reheats.

Quick energy related note: Because of the energy intensive nature of any reheat process, energy codes limit the use of reheat to specific necessary circumstances. Essentially, reheat is permissible when you already have a variable capacity cooling system and have reduced the cooling to a minimum. Only then can reheat be used to satisfy a process need or a ventilation or humidity control requirement.

With a properly sized, designed, installed, and commissioned HVAC system, the need to enable reheat should be relatively infrequent. However, there are very few systems in which reheat is completely unnecessary. Before you install a system without reheat capability, you should be able to guarantee a consistent and long-term space use case where reheat will never be needed. Even if your space’s original use will remain unchanged, variations in the occupancy rates, adjacent spaces, outdoor humidity, accidental moisture loads, and other conditions can make reheat an essential component for problem-free space utilization. System reheats are a great resiliency strategy at a relatively low first-cost.

There is also a connection between system sizing (or oversizing) for anticipated space use change and reheat. Due to the likelihood that a space will undergo a change of use at some point in time (or even from time to time), it makes sense to provide some additional cooling capacity for your HVAC system. Unfortunately, this causes a very real problem for proper temperature and humidity control. Your HVAC system dehumidifies air anytime it is creating condensate, which is most of the time it is actively cooling. However, when it’s oversized, the system achieves the desired space temperature without running long enough to extract enough moisture from the air. This can lead to high humidity and all its associated problems . This is another way reheat can save your bacon. While it doesn’t directly dehumidify, reheat allows the system to engage the cooling longer without driving the space temperature too low.

So once again, order is restored! Occupants are comfortable; no costly mold remediation was necessary; and the HVAC system can live out its useful life while the spaces it conditions and the world outside change around it.

Reheat Related Relative Humidity (RH) Review

Our panelists covered many challenges of addressing optimal indoor temperature, ventilation, and humidity levels, particularly in climates with high relative humidities or in buildings with some systemic or operational deficiency.

One of the most effective tools in the tool bag to address those challenges, for both a short-term and a long-term fix, is reheat !

Short-term Reheating:
In the short term , you might plug in a temporary space heater to allow your cooling system to run longer and harder (sub-cool) to extract more moisture without lowering the space temperature. Reheat is critical for this approach. Simply sub-cooling the air might remove some additional moisture, but without reheat, as the space temperature drops, the relative humidity (the real enemy) will increase.

Another short-term application: Reheat might not be a strategy that we think to use when wet cleaning spaces like residence halls, since it typically coincides with warmer seasons when they are unoccupied for long periods over summer break. However, if time is short, adding temporary heat can be used in conjunction with fans to increase ventilation and space dehumidifiers to accelerate moisture removal.

Long-term Reheating:
In the long term, reheat is best provided as a permanent component of the HVAC system. (You don’t need me to tell you why plug-in space heaters aren’t ideal.) By adding reheat to the HVAC system, you can implement humidity control intelligently with data driven feedback loops to minimize unnecessary energy consumption.

Our roundtable panelists also provided some real-world examples: One example was at a college campus in an older building where an HVAC renovation replaced existing fan coil units, but flipped the heating coil to the preheat position, eliminating the ability to sub-cool and then reheat. As a result, the facilities team struggled to combat high humidity levels in the building until they completed expensive re-renovations to the system and regained their reheat capabilities.

Another first-hand example was from a production facility where air handling systems had been oversized to accommodate necessary air exchanges. The heat generated from the production equipment provided adequate reheat to the space; however, the oversized HVAC equipment struggled during periods when relative humidity was higher than average. To add misery to high humidity, the original design became especially ineffective when the production space was retrofitted for packaging and no longer contained large heat generating equipment. The solution? The facility installed alternative methods for space reheats as well as commercial dehumidifiers to regain adequate humidity control.

Lesson learned: When considering an HVAC system for a building, don’t shortchange yourself by underestimating the importance of reheats!

Takeaways – Rehashed (not reheated):

Here are some additional lessons learned from the experiences of our roundtable panel. Watch the video recap of the event to hear some of the stories that led to these suggestions:

  • Prioritize the maintenance of building envelopes: Always a good strategy, but especially in humid climates with wide fluctuations in seasonal temperatures.
  • Strategically assign spaces: If your space utilization requires humidification, interior spaces are the best choice. Humidified spaces with exterior walls often experience condensation issues when outdoor temperatures differ from the space temperature.
  • Don’t over-cool spaces: Avoid the common mistake of decreasing the temperature to solve humidity issues. A lower space temperature almost always results in higher relative humidity!
  • Use dataloggers: This is a relatively inexpensive starting point to monitor humidity levels and to put concerns into context – is excess humidity an ongoing, seasonal, or occasional issue? Dataloggers can be easily installed and provide valuable data on temperature, dew point, and relative humidity levels.
  • Don’t underestimate the importance of considering humidity control during all stages of a building life cycle; good design, quality installation, expert commissioning, and ongoing maintenance are all critical to success.
  • Create a Plan: Plan for routine adjustments during summer months, particularly in facilities with inadequate cooling systems, and make use of automated controls to minimize manual interventions.
  • Deploy dehumidifiers: This is especially applicable for vacant or low-occupancy spaces during the summer months to control humidity levels (and an obvious strategy for drying out a space after a high moisture event such as spills, wet-cleaning, roof leaks, etc.).
  • Deploy Temporary Reheat: Sometimes a permanent HVAC improvement is not financially feasible. Consider space heaters as an alternative reheat solution to prevent the more costly problems that result from high humidity.

Have questions about reheat or any of these solutions to combat humidity in your building? Reach out!


Stuart Heisey, PE Photo

Stuart Heisey, PE


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Categories: Buildings & Campus

Tagged: Mechanical  |  Engineering  |  Building Humidity

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