Hot Gas Reheat Coil: Unlock Top HVAC Efficiency & Comfort

Here’s something that can be huge in terms of transforming your indoor climate: the hot gas reheat coil. Post: You ever walk into a building on a humid day and feel that clammy air, the sticky air, even if it’s technically “cool”? Or perhaps you know the experience of being in a spot where the air-con is cranked up and you’re freezing, and yet you feel damp? That’s exactly the kind of frustration that a hot gas reheat coil wants to snuff out.

It’s all about finding that sweet spot: perfect comfort, but with just the right amount of humidity, without making you feel as if you are trapped inside a walk-in freezer.

Hot Gas Reheat Coil Your magic cure for perfect air

Okay, so what is a hot gas reheat coil? In other words, a key hairpin shape in HVAC systems, which many are DX (Direct Expansion) cooling. Its main job? It uses air that your system’s evaporator coil is already cooling and dehumidifying, then adds a caressing touch of warmth from recycled heat generated by its refrigeration process. That means you get comfortable air that is not so chilly, and, critically, not so humid.

How These Coils Are Working: A Peek Behind the Curtain

Think of your HVAC system as a delicate conductor of air. Here’s the play-by-play with a hot gas reheat coil in tow:

1. Cooling And Dehumidification that happens simultaneously: air from your space moves through an evaporator coil. It’s super cold, this coil, pulling heat out of it. As the air is cooled, moisture is deposited on the surface of the coil, in the same manner that dew forms on a cold glass. This moisture then evaporates and escapes, and the air becomes drier. The air that leaves this coil is usually quite chilly, frequently in the low to mid-50s Fahrenheit. If you directed all that straight into your space, you’d be grabbing a sweater in July.
2. The Reheat Magic: That’s where the hot gas reheat coil comes in. Instead of all the hot refrigerant gas from the compressor flowing straight to the outdoor condenser to dump its heat, some gets deflected. Where does it go? To the reheat coil which is located downstream thereof in the air stream.
3. Heat Transfer: The Smart Way: Instead this warmer hot refrigerant gas flow carries the heat it picked up in the process into the reheat coil where it dumps its heat into the cool – but no longer damp air directly after it left the evaporator. It’s a warm hug for the air, heating it as it enters the space and before it’s blown out of the vent.
4. Controlled Comfort: The true brilliance here is in the control. Balancing out the amount of hot gas sent to the reheat coil the system instantaneously adjusts the supply air temperature and humidity level. No longer shall you overcool to try to eliminate the stickiness. The air then goes to the supply fan, and then into your space, sometimes even through the system’s main heating coil if more warmth is required. These coils can be built in to your system as a part of the initial system construction, or installed as a retrofit at a later date.

Why Your HVAC System’s MVP is Hot Gas Reheat

Consider this your HVAC system’s “cheat code” for comfort and efficiency. The advantages are huge, particularly for facility managers, but also for anyone who is concerned about the performance of buildings.

Heavy Energy Efficiency: We’re not talking feeling good, but saving coin. Hot gas reheat systems do not require a separate, energy-hogging heat source (such as electric resistance heaters) to heat the air again. And they do it by capturing some of the waste heat that’s already part of the refrigeration cycle. It’s like getting “free” reheat. That translates into lower energy bills (and, a happier bottom line over the unit’s long life).

Accurate Humidity Control (No More “Sweaty Greg”): This is the real winner. Hot gas reheat means you can regulate humidity, and therefore more control over it compared to with temperature. In humidity – think the Midwest, where it can be muggy but not soul-melting hot – this is a big deal. You can take the humidity out of the air, to the level you need for air quality, without causing the room temperature to drop precipitously. It stops the clammy sensation and reduces the risk of your “freezing the snot out of Greg in accounting”.

Improved Occupant Comfort and IAQ: Comfort is more than just the temperature of your space, it’s also the quality of the air you’re breathing. By keeping humidity in line, these systems enhance indoor air quality, and avert the ill effects of air that’s too cold. Less humidity just feels more comfortable in the space, that “stickiness” we don’t like. And it can help keep your building structurally sound and your furnishings immune from moisture damage.

Longer Compressor Life: It’s hard to believe, but this clever system can help extend the life of your compressor. In 100% outdoor air units, the refrigeration system may also run continuously at above a pre-determined conditions to not subject the system to excessive on-off cycling.

Microchannel Coils: A few of today’s hot gas reheat coils use microchannel technology, and these are a looong. They have a few neat advantages over old-style, fin-and-tube coils:

• More heat transfer: They do more work for their size.
• Less refrigerant: Fewer pounds mean more savings and it can contribute in earning credits for LEED “charge per ton.”
• Stronger: Less susceptible to corrosion than older models.
• Streamlined system: They usually don’t require receivers or oil flushing cycles.

Cleaner System Design: You could potentially even take out less effective components like face and bypass dampers, which would make the system design more streamlined.

Where You’ll Meet These Climate Control Heroes

Hot gas reheat coils are working hard in different applications, when pinpoint climate control is necessary, from industrial to commercial settings.

Commercial HVAC: This is their sweet spot especially with things like supermarket refrigeration and large commercial buildings.

Rooftop Units (RTUs): They’re found in package rooftop systems as well as air handlers.

Dedicated Outdoor Air Systems (DOAS): Required for the treatment of 100% outdoor air before it is introduced to a space, creating neutral supply air for terminal units such as fan coils or VRV DX systems.

Specialized Environments: Consider the locations in which the air needs to be perfect:

• Hospitals Surgical Suites: A clean space and patient comfort.
• Laboratory and Clean Room Environment: Essential for process control and avoiding contamination.
• Museums: To save priceless art and historical artifacts from becoming moisture destroyed.
• Outpatient Operating Suites.
• Pharmacies.

Large Single-Zone Applications: Areas with lots of people, where humidity can also become a problem fast, such as churches, theaters and gyms. These types of systems typically don’t dehumidify well at part-load without reheat.

Variable Air Volume (VAV) Systems: particularly those with high dehumidification requirements, where they provide a dehuidification safety factor in humid climates, and “free” reheat.

Humid Climates: As noted, they’re clasp in areas (hello Midwest!) where humidity can be high even on cooler days.

How They Plug In: Modalities and Modes

Adding a hot gas reheat coil into a refrigeration loop is not one size fits all. There are several major methods and control strategies, each with its own flavor.

Primary Integration Methods:

System Types and Control Strategies

And while you can talk until you’re blue in the face about how the coil is plumbed in, how it’s controlled has a big impact on how it performs and how precise it is:

ON/OFF Hot Gas Reheat: It’s the easy way out. The reheat coil and condenser are usually piped in series, so that refrigerant either passes through the reheat coil (active) or bypasses it (inactive). It’s an on-off — when the system cycles, you will see some temperature swings. Though it does the trick for humidity control, it’s not as accurate.

Modulating Hot Gas Reheat: This is precision. This is equipped with a three-way modulating valve. This is the master valve, controlling between 0% and 100% hot gas diversion to the reheat coil. What’s the result? A more uniform discharge air temperature.

• Standard Scroll Compressor on Air Cooled Model: Ideal on applications requiring closer temperature control such as a clean room or laboratory; leaving air temperature (LAT) is maintained within ±1.0°F.
• With Digital Scroll Compressor: This is how you flex. Its modulating reheat and digital scroll compressor provide significantly greater degrees of modulation, particularly at part-load. Hot gas bypass may also be eliminated with this arrangement so even more energy may be saved by the ability to completely modulate compressor operation down to 10% capacity.

Series vs. Parallel Reheat Systems:

• Heat Rejection by Series: Some of these systems are designed for only partial recovery of refrigeration circuits heat, with the remainder rejected to the main condenser. They’re generally simpler.
• Parallel Reheat: This is where the party starts. Parallel systems are available and sized to accommodate 100% of the overall heat of rejection. In such an arrangement, refrigerant may have the entirety of the condenser coil completely bypass the refrigerant circuit in a reheat mode. This configuration is frequently matched with microchannel coils. It’s a great maneuver for achieving air that’s higher than neutral in temperature, from the concentrated heat.

Hot Gas Reheat with Liquid Sub-cooling Coil: Neat little add on. Sub-cool Coil~ A coil between the evaporator and hot gas reheat coils. Its purpose is to sub-cool the liquid refrigerant and it reduces the leaving dew point by 2-3°F with an insignificant energy penalty. Then it absorbs heat from the liquid refrigerant, which then gets dumped into the airstream as “free” reheat, increasing the supply air temperature by 8-10°F. Win-win!

What Makes Up These Coils? The Building Blocks

Carefully designed steam reheat coils, with moderate tube spacing, are utilized to meet the special requirements of refrigerant heat transfer.

• Fins and Tubes: Most come with aluminum fins and copper tubes for normal operating pressures.
• High-Pressure Options: If you have tougher, high-pressure case coils are available with copper alloy tubes.
• Ultra-High Pressure: If you are working with pressures above 1740 PSIG then both 304 and 316 stainless steel tubes are available.
• Construction Quality: A good manufacturer will have certified welders and brazers to ASME Section IX code, and may even offer internal pickling and passivation of your stainless steel to maintain its resistance to corrosion.

Hot Gas Bypass vs. Hot Gas Reheat: Not One In the Same

This can get confusing, so let’s try to clarify. They both use hot refrigerant gas, but serve two completely different functions.

• Hot Gas Reheat: Purpose is to control the humidity and temperature downstream of the evaporator coil. It does so by heating up already cooled, dehumidified air to a comfortable level, without overcooling.
• Hot Gas Bypass: This is only a safety feature. Its purpose is to keep an evaporator coil from frosting by pumping hot gas into the evaporator when the suction temperature is low. It’s keeping the system in good working order, not necessarily making air for comfort.

So, even though they both contain “hot gas,” they’re sort of like two different tools for two different jobs.

Being a Good Sport: Industry Norms

In HVAC-ese, particularly when you are in a commercial building, the ASHRAE standards are the law of the land. Hot gas reheat plants are often an important part of achieving these criteria, especially in relation to indoor air quality and energy consumption.

ASHRAE Standard 62.1 (Ventilation and Indoor Air Quality Acceptable):

• The 2019 and 2022 revisions require a maximum zone dew point temperature of 60°F during occupied and unoccupied periods when the outdoor air dew point exceeds 60°F.
• In other words, many systems can’t just dehumidify “passively” anymore; they have to control humidity actively, and that means bringing hot gas reheat into the picture.
• There is one exception: the 60°F dew point limit does not apply during overnight unoccupied periods (up to 12 hours) if the relative humidity does not exceed 65%.

ASHRAE 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) – Commercial Buildings Only:

• This code usually restricts the use of reheat for dehumidification. However, there are important exceptions in which hot gas reheat is an acceptable and wise decision:
• When at least 90% of reheat energy is recovered on site in the form of condenser heat (which includes hot gas reheat). This is a biggie for energy efficiency.
• For certain zone types with specialized dehumidification requirements, such as museums, surgical suites, pharmacies.
• For other fan power limitations within VAV systems—reheat systems where greater than 50% of design airflow or minimum outdoor airflow are served through reheat.
• The standard also says that humidistatic controls shall not permit humidity to be reduced to less than a 55°F dew point or 60% RH in the cooler zones but allows the lower humidity if a direct result of mechanical cooling for temperature control.

It’s imperative to understand these codes because they have direct bearing on system design and operation, and in many cases drive the need for active humidity control with hot gas reheat, instead of it just being a nice-to-have solution.

Making the Call: Design and Installation Tips

Designing to use hot gas reheat coils, or not, is based on a few critical factors.

Upfront Cost vs. Long-Term Savings: So yes, there’s an upfront cost. No sugarcoating that. But for the great majority of units, which may have a useful life of 10 to 15 years, the energy savings (from eliminating electric heating elements) could result in substantial cost savings over the unit’s useful life. Think of it as an investment with a swift payoff. Although in some occasions annual energy costs will be higher, the significant advantages of active humidity control, improved comfort, and protection against moisture damage easily make up for the higher energy costs.

Technical Design Nuances:

• If you are going with the Hot Gas Method you need to make sure that the design keeps the refrigerant velocity high so you don’t cool the gas off too soon.
• The Parallel Condensing Method calls for even less flow rate than the other one and the coil construction must condense the refrigerant entirely.

Integration Options: You may integrate these systems to new Rooftop Units (RTUs) at factory level. But if you want to upgrade an existing system, there are retrofit options. Air conditioning modulation can be incorporated cheaply with a field installed device, such as a Rawal valve etc.

Modeling is Your Friend: When developing complex structures, designers are working with strong tools for load and energy modeling. They can be used to simulate how hot gas reheat this is going to perform, to properly size components as well as to even forecast an estimate of energy costs. They can also run complete 8,760-hour analyses to predict zone humidity levels year-round. That’s just what you get for being smart and strategic in your design.

Single-Zone vs. Multiple-Zone VAV Systems:

• In a one-zone system, hot gas reheat reheats the cold and dry air directly to avoid overcooling it and to reach a desired calculated discharge air temperature setpoint.
• Hot gas reheat might not be necessary for simple humidity control in multi-zone VAV systems, because these systems typically dehumidify the air to a low dew point anyway. But if there is an extreme rise in humidity, the system can modulate lower the evaporator leah in air tempSo the dehumidification increases and then the hot gas reheat coil reheat the air back to the discharge temp you desire without the extra energy expense at the fan or zone. It’s a next-level flex for cool comfort, all while transitioning between multiple zones and activities.

The Bottom Line

So, here’s the scoop. The hot gas reheat coil is not mere component; a hot gas reheat coil is a crucial component for today’s HVAC systems. It’s your ticket to:

• Same comfort: No more clammy air or shivering indoors.
• Real energy savings: It’s efficient, because it uses waste heat instead of generating new warmth.
• Compliance with stringent standards: It assists buildings in meeting ASHRAE’s current air and energy requirements for indoor air quality and humidity control.

When comfort, efficiency and compliance are no longer up for discussion, hot gas reheat coil is no longer an option, but rather a mandatory solution for that vision of just-right climate.

FAQ: A Few Quick Hits on Your Hot Gas Reheat coil

Q1: What is the primary purpose of a hot gas reheat coil? A: Its main job is to remove moisture from cool air without cooling the space too much. Instead, it takes air that has been chilled and dried by your system’s main cooling coil, and then warms it back up to a comfortable temperature, reusing heat that would otherwise get blown outside.

Q2: How is energy saved using a hot gas reheat coil? A: It is a cool idea (ahem) in part because it recycles “waste heat” from the very refrigeration cycle it serves. ELECTRICAL ENERGY Extraction, Production, and Consumption Energy Conversion Unlike typical designs, which may use electric resistance heater elements to heat the air, this system does not waste additional, ‘fresh’ energy simply to reheat.

Q3: What are some common applications for these coils? A: They exist in a majority of commercial HVAC applications, including grocery store refrigeration, larger commercial buildings, rooftop units (RTUs), and Dedicated Outdoor Air Systems (DOAS). They are essential in environments that require accurate humidity control such as hospitals, laboratories, clean rooms, surgical suites, and museums.

Q4: What is the distinction between modulating and on/off hot gas reheat? A: On/off systems are easier; reheat is all in or out, and there may be temperature swings. Modulating systems are more sophisticated. They have a special valve that modulates hot gas flow to the coil, so can achieve more consistent and stable discharge air temperatures and better humidity control.

Q5: Is hot gas reheat the same as hot gas bypass? A: Nope, not at all. Hot gas reheat is used to control humidity and temper air after it’s been cooled by the evaporator, so that it is suitable for the space. Hot gas bypass is, on the other hand, a control technique commonly used in order to prevent the evaporator coil from freezing under low loads by supplying hot gas to the evaporator. Different jobs, different solutions.