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If one motor is good, two must be better? Not necessarily.
There are two ways to increase performance in a dual motor system (see Figure 1):
Air Series: In a Series application, one vacuum motor feeds the heated discharge air into the intake of the second motor. This effectively increases vacuum suction (inches of waterlift) by 60-70%. This design is not recommended because higher temperatures result in lower motor life expectancy. In addition, extra care, particularly with Flow-Thru type and Peripheral bypass motors must be taken to ensure that the discharge air and motor cooling air are separated. The best dual motor systems utilize two Tangential bypass motors which ensure heated discharge air and cooling air are separate.
Air Parallel: While operating vacuum motors in air parallel, two motors draw air from a single plenum chamber. This has the effect of nearly doubling the Airflow (CFM) of the system while the vacuum (inches of waterlift) level remains comparable to if a single motor was used. "When applying motors in this manner, care must be taken in the design to accommodate the increased airflow. "If restrictive tools and hoses are used in a cleaner, the potential air flow advantages gained by the parallel arrangement may be negated." Ametek/Lamb Product Review.
This is evident in (Figures 2 & 3), where it would appear that the Air Parallel system (blue line) has good Airwatt performance when unrestricted at the motor. It is quite different however when put into a central vacuum cleaner with a 30 foot hose (Figure 3). As you can see, the Air Parallel System (blue line) has a lot less end of hose performance. This is because it lacks the Suction to overpower Airflow robbing restrictions in a central vacuum installation and hence any gains from adding a second motor to increase Airflow are dramatically reduced. The performance results of the systems with higher suction however, have higher true cleaning performance where it matters, at the hose!
Graph Conclusion:
Using a single high performance motor (Green Line) with high Suction versus two systems that used idential motors in tandum, one producing high Suction the other high Airflow. We can conclude that Suction appears to be the dominant requirement for the best cleaning performance where it matters. At the end of your hose!
So Why Dual Motor Central Vacuums?
Well it depends once again on the motors used. Some systems use two smaller less powerful motors which even though working in tandem don't necessarily out perform the performance of one high performance single motor system as demontrated in (Figure 3).
Beware of the product brochure performance specifications. If you are using Airwatts as a gauge of cleaning performance and it happens to be a dual motor system, then be careful. The Airwatts performance of a dual motor system you are looking at, is almost always the result of simply adding the Airwatts of the first motor to the Airwatts of the second motor. This results in a 100% improvement? Impossible!
 You can't simply add the Airwatts of one motor to the Airwatts of the second motor. We learned that in a dual motor application either the overall Suction of the system went up or the Airflow of the system went up, not both! Because of this, you can not simply add Airwatts together.
If your home is large and you feel that you would like added cleaning performance. Installing two separate central vacuum systems into separate zones is ideal whether or not it is a dual motor central vacuum or single motor central vacuum. Both achieve better end of hose cleaning when installed into separate zones.
Generally speaking, if a dual motor system operates on a single 110V of power then it is using two small lower performance motors together to boost their brochure "Airwatts". Two lower performance motors can be used together on a 110V-120V circuit but give a marketing advantage of showing higher Airwatts, however as you know, you can't add the Airwatts of the two motors together.
If the system requires 220V of power then the system is using two larger mid grade performance motors (probably 5.7 inch diameter). These central vacuums are not a bad choice and should offer some added cleaning power but remember the Airwatts in the brochure is not double the single motor Airwatts ratings for those motors!
It would be rare to find a dual motor system using dual ultra high performance motors such as 6.6", 7.2" or 8.4" diameter motors (see previous page about motors).
Since motors in tandem generally don't last as long or use lower grade motors (unless 220V) we would suggest choosing the manufacturer's single motor high end unit (if it uses a single ultra high performance motor) over a dual motor system which may show higher Airwatts in the brochure but use to lower performance. Again, if your home is large and you want added cleaning performance, it is always better to install to independent systems with a smaller plumbing installation.
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