How to Choose Recirculated Air Heating Unit?

When the weather warms up and the air conditioner comes on, there is nothing quite like the relief of cool air on a hot summer day. While a window AC unit cools one room and leaves the others warm, a central air conditioner circulates the air through the entire house using your duct system. This process allows the whole house to be cooled, rather than concentrating the cooling in one area.

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Whether you’re considering air conditioning installation or booking a repair, it helps to understand how air conditioners recirculate air to cool your home efficiently. A quality central AC system not only keeps temperatures down— it runs quietly, saves energy, and improves overall air quality. 

When it comes to routine maintenance and servicing, it helps to understand how your AC system works. Below is a guide on how your central air conditioner recirculates air into your home to provide cool, comfortable, and clean air. 

Does An Air Conditioner Recirculate Air?

Yes, they do—and understanding how air conditioners recirculate air helps explain why they’re so effective at keeping your home cool and comfortable. Air conditioners are designed to remove heat and moisture from indoor air, creating a comfortable and controlled environment. They achieve this by using a refrigeration cycle.

Here’s how it generally works:

• Air intake – The air conditioner draws warm indoor air into the unit through a vent or grille.
• Cooling process – Inside the air conditioner, the warm air passes over evaporator coils containing a cold refrigerant. The refrigerant absorbs heat from the air, causing the air temperature to drop.
• Dehumidification – As the warm air cools, moisture in the air condenses on the evaporator coils, resulting in dehumidification.
• Air recirculation – After cooling and dehumidification, the now cooler and drier air is then blown back into the room through another vent or grille. This process helps maintain a consistent and comfortable temperature.
• Exhaust – At the same time, the air conditioner releases the heat absorbed from the indoor air and some waste heat generated during the cooling process to the outside through its condenser coils.

While some air conditioners may have a fresh air intake option, most residential and commercial air conditioners primarily recirculate the indoor air for energy efficiency and to maintain the desired indoor temperature more effectively. However, it’s important to ensure proper ventilation and fresh air exchange, especially when indoor air quality becomes a concern. 

Why Recirculated Air Makes Cooling More Efficient

Using the air that’s already in your house makes everything easier. Your AC doesn’t have to work as hard cooling warm outdoor air from scratch. 

Less effort = less energy used = lower bills. 

Plus, it means your home cools down faster and stays more consistent throughout the day. 

What Happens When an Air Conditioner Stops Recirculating Air?

When air conditioners don’t recirculate air, several consequences can arise, affecting both comfort and energy efficiency. Let’s examine some of the potential issues that necessitate a professional air conditioning repair service.

• Reduced cooling efficiency – Air conditioners are designed to cool and dehumidify indoor air. If they don’t recirculate air, they won’t effectively extract heat and moisture from the room. As a result, the cooling efficiency will decrease, and the room may remain hot and humid.
• Uneven temperature distribution – Without proper air recirculation, the cooling effect may be concentrated near the air conditioner’s output, leading to uneven temperature distribution within the room. Some areas might feel colder, while others remain warm and uncomfortable.
• Increased energy consumption – Air conditioners work more efficiently when recirculating indoor air, as they only need to cool the air within the room. If they constantly draw in fresh air from the outside, they have to work harder to cool that incoming air, which increases energy consumption and raises electricity bills.
• Inefficient dehumidification – Air conditioners play a crucial role in dehumidifying indoor spaces. Without proper air recirculation, they won’t effectively remove moisture from the air, leading to higher humidity levels and a sticky, uncomfortable environment.
• Longer cooling time – When air conditioners don’t recirculate air, they have to cool down the incoming fresh air repeatedly. This prolongs the time it takes to achieve the desired indoor temperature, leading to less efficient cooling.
• Strain on the AC unit – Constantly bringing in hot outdoor air puts extra strain on the air conditioner’s compressor and other components. This can lead to premature wear and tear, reducing the lifespan of the unit and potentially causing breakdowns.
• Increased noise – Air conditioners may produce more noise when operating at higher capacities due to the increased demand when cooling fresh air instead of recirculating the indoor air.
• Difficulty maintaining set temperatures – Without proper air recirculation, the air conditioner may struggle to maintain the set temperature on the thermostat consistently.

To keep your air conditioner running at its best, it’s all about balance—circulating indoor air efficiently while still bringing in fresh air when needed. Some newer AC systems even adjust automatically based on your home’s air quality and temperature, making things more comfortable without you lifting a finger! It’s a smarter, more efficient way to stay cool and breathe easier. 

When to Call for Air Conditioner Repair or Maintenance

If your AC is blowing warm air, sounds like it’s struggling, or your power bill has mysteriously doubled—those are all red flags.  

Hot spots in the house, higher humidity, or weird smells? Time to call in a pro like Springs Servicing & Heating. Regular maintenance can help you avoid costly breakdowns and keep your system running smoothly when you need it most.

Enjoy Consistent & Even Cooling With a Central Air Conditioner

Recirculation Units Design

1. Introduction

Recirculation units (RCU) design is becoming increasingly common in the design of HVAC systems in pharmaceutical facilities due to their advantages over traditional systems. It is particularly convenient for use in off-site cleanroom manufacturing strategies. In this article, we will compare this system with another traditional system and examine the advantages and disadvantages in a specific example.

2. A typical Air Handling Unit Design

Let’s imagine an HVAC system that serves a suite with a series of rooms for an upstream biological production process. A traditional design, according to the ISPE HVAC guide, (you can find it here) would consist of the following components:

• Outdoor air treatment (make-up unit) to humidify, dehumidify, and ensure overpressure in the rooms.

• Air handling unit to supply the necessary airflow to achieve the required air changes per hour and design classification. The Air Handling Units cool down to the set point that the room with the highest heat gain requires.

• Reheaters to compensate for the supply air temperature required for the rest of the rooms (those with lower heat gains)

• Recirculation of room air to the air handling unit, through a suction fan located in the same air handling unit.

The diagram for this system would be as follows:

3. Recirculation Units (RCU) Design

The proposal with Recirculation Units (RCU) design would be as follows:

• Treatment of the outside air, in the same way as in the previous case. Nothing would change here.

• One or more RCU units for each served room. To determine the number of units, we will take into account factors such as the maximum flow rate to establish the most compact size of the unit possible.

• Common exhaust fan for all rooms.

This would be the sketch:

4. Advantages and disadvantages of RCU systems

RCU systems have a large number of advantages, let´s see a few of them:

• Drastically reduction of the probability of cross-contamination. Each RCU serves one room (or a set of rooms, PAL, MAL). From a GMP point of view, this approach is ideal.

• We will require less duct area. Since the RCU units will always be close to the spaces they serve, we will require less duct surface area. In some countries, regulations for duct sizing require a maximum velocity. For high flow rates, this is usually a problem that forces an increase in duct area. By using lower flow rate RCU units, we can eliminate this restriction.

• Similarly, the required static pressure will also be lower, and we can optimize the exhaust fans.

• As mentioned, the reduced flow rate allows us to use more compact units. Here is an example of a unit:

• Reheaters are not required. Each unit will adopt the supply temperature with respect to the heat gain of the served room.

• In parallel, we save on the additional pressure loss of these reheaters.

• The air from the room can be returned with the same supply fan, only an exhaust fan will be necessary, which can be common for all rooms, for pressure control.

• They can be installed on a walkable cleanroom void.

Disadvantages:

• Use of tempered chilled water. If the heat gains require it, it cannot be supplied at a temperature lower than 16°C.

• More maintenance. We have more fans, filters, and valves.

• More instrumentation.

• All of the above results in a higher cost.

• If installed on a walkable ceiling, precautions must be taken to avoid possible liquid leaks into the rooms.

5. Let´s take a look at an example

In the following example, we will present the same problem using the two proposed solutions for summer conditions. We will classify three areas corresponding to an upstream biological process: a formulation room, a cell culture room, and a buffer preparation room. All three rooms will have an ISO 7 classification, and according to the design ACH, we will need to supply the following filtered air flows: 16,875, 12,000, and 6,750 m3/h. Different heat gains and latent loads have been considered for each of them. The temperature and humidity conditions inside the rooms are identical.

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To achieve the required positive pressure and ventilation, we will need 3,913 m3/h of outdoor air in the Swiss city of Visp. The outdoor design conditions are the following:

5.1. Air handling unit with reheaters design

We will have a make-up unit that we will use for dehumidifying the outdoor air. To do this, we will cool it to 9 ºC and then reheat it to 16 ºC. This will bring the dew point from 20.3 ºC to 8.7 ºC. The energy exchange, including the fans, is as follows (click on the image to enlarge):

I usually use a rough calculation to estimate the ventilation motor power to determine the temperature increase due to it. However, in this case, since it is an important value in the comparison, I have decided to use the actual selection based on Ziehl-Abegg manufacturer’s curves, obtaining approximately 1.7 kW.

Next, we estimate some losses in the ducts from the makeup unit to the main unit. In the main unit, the pre-treated air will mix with the return air through a return fan. Then, the air will cool to overcome the heat gains of the rooms.

Thereafter the cooling will take into account the gains of the fan and the supply ducts. Since the air is sent to rooms with different heat gains, we will determine the supply temperature setpoint by the room that requires the lowest temperature. The rest of the rooms will need to reach the supply temperature using reheaters. All of this is explained in the post, Heat Gains. The air condition in the AHU is shown in the image below (click to enlarge):

We obtain that the return fan will be 12.5 kW and the supply fan will be 19.7 kW according to the following curves:

And finally, dimensioning the reheaters, we obtain (click on the image to enlarge):

The difference between supplied and recirculated air is explained in the post, Cleanroom Air Balance

And with this, we have already sized the first option. Next, we will see the sizing with RCU.

5.2. Recirculation Units design

In this example, the sizing of the makeup unit will be exactly the same, nothing changes since we need the same treated outdoor airflow to supply it at the same conditions, i.e. 16°C and a dew point of 8.7°C.

This air will supply to the three RCUs, which will mix with the return air from each of the rooms. We will have an exhaust fan to regulate the pressure of the rooms. Click on the image to enlarge it.

The power of each of the supply fans is 8.6, 6.3, and 3.5 kW as we can see in the following performance curves:

Note that using RCU we do not need return fans.

6. Comparison between the two systems

In the table below, we will compare the two systems in terms of installed fan power, cooling, and reheat power.

7. Conclusions

As we can conclude from the table above, there are substantial improvements in the selection of Recirculation Units (RCU) design.

There is no cross-contamination between rooms or suites. We don’t mix recirculated air from a room with air coming from other areas.

On the one hand, with this system, we can reduce the ventilation power since we will not need a return fan to collect all the recirculated air from the rooms. Likewise, we can bring the supply airflow as close as possible to the place of the terminal HEPA filters, thus reducing the pressure drop.

We reduce the cooling power since it is not necessary to over-cool the air in rooms that do not have as much heat gain as others.

As a consequence of the previous point, we determine that it is not necessary to reheat.

Finally, we will say that this case is significantly favorable for an RCU system, in terms of energy efficiency and compliance with GMP. We have not discussed in this post about the cost, but the initial investment will be higher for an RCU system. We should study each case individually based on the criticality of the process, the available space for air handling units, energy efficiency, and other factors.

Are you interested in learning more about Recirculated Air Heating Unit? Contact us today to secure an expert consultation!