Many custom-designed setups require more ventilation than a single fan can deliver, so this can prompt the question whether two (or pairs of two) fans should be placed in series or parallel operation. In general, combining
The first fan law relates the airflow rate to the fan rotational speed: Volume flow rate (CFM) is directly proportional to the fan rotational speed (RPM). If the fan RPM is increased, the fan will discharge a greater volume of air in exact proportion to the change in speed.
So, to increase the CFM (working from logic), you could increase the motor speed (but if it is a already a 3450 rpm motor there isn't much you can do). Or you could (maybe) modify the fan assembly to have a slightly bigger or more efficient fan.
Airflow Efficiency
On high speed: Good CFM ranges from 4,000 to 5,000. Better ranges from 5,000 to 6,000. Best is over 6,000.
1:2 Ratio: In some cases, having more exhaust fans (twice as many as intake) can be beneficial. This configuration can help create a negative pressure environment, which can improve cooling performance and reduce the buildup of dust inside the case.
When fans are operating in parallel, the combined performance shows an increased volume of airflow. In theory, the total airflow volume will be doubled compared to a single fan. By contrast, total static pressure at zero airflow (stall) will not increase compared to a single fan.
A fan that is too big can cause a bathroom to be cold and noisy. This is why calculating the correct CFM is important.
On the other hand, if your CFM is too high for your room size, it can cause ventilation problems. Too much air results in too little ventilation, which in turn causes high humidity. The excess moisture in the air can then lead to mold and mildew, as well as other problems.
Air movement is typically measured in cubic feet per minute (CFM). Ventilation rates based upon eight air changes per hour are generally suggested. For most bathrooms this works out to one CFM per square foot of bathroom area. For example, a 7' x 10' bathroom would require a 70 CFM fan.
A typical whole house fan will have an airflow capacity of around 1,500 to 7,000 CFM.
It is also observed that in the parallel setup, the airflow rate does double, up to 50 CFM when compared to the single fan setup, which has an airflow of 25 CFM.
Dust and debris accumulation on fan blades can impede airflow and reduce efficiency. How to make a fan work better? Regularly clean your fan blades using a damp cloth or a vacuum cleaner with a brush attachment to remove dirt and dust. This will help your fan work more efficiently.
CFM measures the volume of air a fan moves in cubic feet. The bigger the fan, the more CFM.
In certain environments, speeding a fan up can also result in humidity issues. Your fan speed may be too high if your home feels cool but also feels clammy. During a normal cooling cycle, air conditioners simultaneously cool down the indoor air and remove excess humidity.
To summarize these 3 fan laws, flow changes proportionately to speed. Static pressure changes as a function of the change in speed squared. And brake horsepower changes as a function of the change in speed cubed.
Often, a ventilation power varying between 350 CFM and 500 CFM is sufficient, depending on your cooking habits. For example, if you are used to using many rings at the same time, you will require a higher ventilation power.
For bathrooms 50 square feet and smaller, it's recommended you purchase a bath fan designed for small rooms. In other words, choose under 79 CFM bath fans. For medium-sized bathrooms, use 80 to 99 CFM bath fans.
If you cook every day or frequently cook for larger groups of people, it's best to choose more CFM.
Higher CFM means the fan is more efficient, moving more air per watt. Good CFM values start at 4,000, but the best CFM values are above 6,000. An efficient ceiling fan must have at least 75 cfm/w.
Yes, provided both fans are fairly equally powered and will operate at the same time. You obviously don't want one fan significantly more powerful than t'other as it could - theoretically - send its contents out the other fan.
It is okay to oversize the fan a little. When we talk about a fan being able to handle a room X square feet – that is a minimum, not the maximum size. Replace the existing one. Replacing an exhaust fan is a somewhat easy DIY project as long as you can access it from the attic.
Don't leave fans on for an extended period of time!
Leaving the fan on for hours at a time can really put the unit at risk for trouble. Consider installing a timer-based switch that will turn the fan off automatically. Turn off all fans as soon as they have done the job.
To adjust the CFM, you can change the speed of the blower or alter the ductwork size. Increasing the blower speed won't have much effect if a room's duct is too narrow. A 12' x 12' foot room is 144 square feet. You generally need at least 1 CFM per square foot, meaning that room's duct should have a 144 CFM capacity.