I took apart a cheap domestic vacuum cleaner because I am making a small vacuum-forming system, and I only need the motor/fan assembly, so I repackaged it to save space.

Inside the vacuum cleaner is a valve, which I gather is called a bypass valve and is present on most/all vacuum cleaners. In this case it's just a circular plate held against a gasket by a spring:

bypass valve

Based on my measurements, this valve will open and let air through if the pressure difference across the opening is greater than about 10 or 11 kPa. It sits inside the machine where the user can't get to it, between the fan intake and the outside atmosphere. In my hacked mini-vacuum, the gasket is on the outside of the box, where I can block it if I want to.

My question is, what is the point of this valve? For my purposes, the more vacuum I can get, the better. But I don't want to block the valve if it would permanently damage / set fire to the motor.

(it's a 700W 240V AC motor, and for vacuum forming I will be running it for 1 or 2 minutes at a time).


1 Answer 1


It’s purpose is to allow sufficient air to cool the motor when the filter / dirt bag is blocked - otherwise it will overheat and could catch fire.

  • 1
    $\begingroup$ To extend this: most vacuums cool the motor via the air sucked through the vacuum. If you have a vacuum which cools the motor via an additional fan (such as in Kirby vacuums), then this valve is not needed because the motor continues to receive cooling even if the vacuum gets clogged. $\endgroup$
    – Cort Ammon
    Nov 30, 2017 at 21:03

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