Two East Bay residents are suing Comcast for plugging their home’s wireless router into what they call a power-wasting, Internet-clogging, privacy threatening network of public Wi-Fi hotspots.

The class-action suit, filed last week in U.S. District Court in San Francisco on behalf of Toyer Grear and daughter Joycelyn Harris, claims Comcast is “exploiting them for profit” by using their Pittsburg home’s router as part of a nationwide network of public hotspots.

Comcast is trying to compete with major cell phone carriers by creating a public Xfinity WiFi Hotspot network in 19 of the country’s largest cities. The company is activating a second high-speed Internet channel broadcast from newer-model wireless gateway modems that residential customers lease from the company. It plans to spread to 8 million hotspots by the end of the year.

The secondary signal is supposed to be separate from the private Wi-Fi channel customers use, and it is intended for houseguests or Comcast subscribers who happen to be in range and using mobile devices.

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The Philadelphia company, which owns the nation’s largest cable TV service and entertainment giant NBCUniversal, began activating the secondary channel in the Bay Area this summer. Customers who own cable modems aren’t affected.

Although Comcast has said its subscribers have the right to disable the secondary signal, the suit claims the company turns the service on without permission and places “the costs of its national Wi-Fi network onto its customers.”

“Comcast’s contract with its customers is so vague that it is unclear as to whether Comcast even addresses this practice at all, much less adequately enough to be said to have obtained its customers’ authorization of this practice,” the suit claims.

The suit quotes a test conducted by Philadelphia networking technology company Speedify that concluded the secondary Internet channel will eventually push “tens of millions of dollars per month of the electricity bills needed to run their nationwide public Wi-Fi network onto consumers.”

Tests showed that under heavy use, the secondary channel adds 30 to 40 percent more costs to a customer’s electricity bill than the modem itself, the suit said.

The suit also said “the data and information on a Comcast customer’s network is at greater risk” because the hotspot network “allows strangers to connect to the Internet through the same wireless router used by Comcast customers.”

Comcast did not have any immediate comment on the suit. But the company has previously said the modems have two antennas — separating the signals and offer a layer of privacy protection. Comcast has also said the energy costs will depend on how much guests use a resident’s Wi-Fi.

Although Comcast has said it has enough bandwidth to handle the extra traffic, the suit claims Grear and Harris have suffered from “decreased, inadequate speeds on their home Wi-Fi network.”

The suit asks for unspecified damages and an injunction preventing Comcast from using home wireless routers for its hotspot network.

Benny Evangelista is a San Francisco Chronicle staff writer. E-mail: bevangelista@sfchronicle.com Twitter: @ChronicleBenny