Comcast has brought a beta version of its Xfinity TV app to Roku streaming devices, but customers who use the app in addition to a Comcast TV box after the beta trial ends will have to pay an extra fee.

Using the app is free during the beta trial, but for now the app can only be used if you're also paying for a Comcast set-top box. That limitation will eventually go away, but initially that means the Roku app won't be able to completely replace a traditional TV box.

Roku announced the Xfinity TV beta app yesterday, and Comcast posted an FAQ with more information. The Comcast FAQ explains that "additional outlet" charges are being waived during the beta trial, but that won't be the case after the beta ends: "Customers will not pay equipment charges with respect to their use of Roku devices," Comcast wrote. "All other fees associated with a customer's service will apply, except that, during the Beta trial, additional outlet charges for services to outlets connected to Roku devices are being waived. On conclusion of the trial, you will be informed of the charges that will apply for connecting this device with your Xfinity TV service and will have the opportunity to opt in."

Former Federal Communications Commission Chairman Tom Wheeler tried to push through rules that would require pay-TV companies to provide free apps that can replace rented set-top boxes. But he wasn't able to get enough votes, and current FCC Chairman Ajit Pai has taken the proposal off the table.

Variety points out that "Comcast's additional outlet service fees per set-top box are $9.95 per month; customers using their own substitute equipment, such as a TiVo, receive a $2.50 credit (resulting in a net charge of $7.45 per month)." That might give an indication of what Comcast will charge for Roku use, though Variety quoted a Comcast rep as saying that the company is "evaluating our additional outlet policies."

Comcast is calling this a beta because the app "is still in its development stages," and the company wants customer feedback. One big limitation during the beta is that the service can't replace a Comcast set-top box: "During this Beta phase, due to technical limitations, customers using the Xfinity TV Beta app on Roku (other than Stream customers) can only use that app on secondary TV outlets and will need to have at least one Comcast-provided TV box in their home. You do not have to connect the Comcast-provided TV box and Roku to the same TV."

The "Stream customers" Comcast referred to are those signed up for Stream TV, a basic TV service for Comcast Internet customers that does not require a TV box. The beta app is available on all Roku players released in the last couple of years (full list in the FAQ), and support for additional Roku models is in the works.

We asked Comcast several questions about why Roku outlet charges are necessary and how much they will cost, the need for a Comcast TV box during the beta phase, and expected availability date for a non-beta version of the app. We'll provide an update if we get one.

(UPDATE: Comcast responded to us and confirmed that Roku devices will be treated similarly to CableCard devices in terms of pricing. Customers who use a Roku as a "primary outlet" after the beta is over will get a $2.50 credit on their bill; the "primary outlet" isn't an itemized fee but is included as part of paying for a TV subscription. Using a Roku as an "additional outlet" will cost $9.95, but the $2.50 credit will lower the price to $7.45. Xfinity TV app access on phones and tablets does not cost extra.)

The Comcast Roku app lets customers stream live TV from their entire channel lineup, access the Xfinity video on-demand library, and provides recordings to customers who also have the X1 Cloud DVR. To use the beta app, customers must subscribe to Comcast TV and Internet service "and have a compatible IP gateway." Though it requires Internet service, TV on Roku is still considered a "cable" service and doesn't count against Comcast data caps. "The Xfinity TV service delivered through the Xfinity TV Beta app is not an Internet service and does not touch or use the Internet," Comcast's FAQ said. "Rather, it is a Title VI cable service delivered solely over Comcast's private, managed cable network, so it will not count toward your Xfinity Internet Data Usage Plan."

The Comcast Internet gateway can also be used as the cable service gateway, Comcast said. In the future, it might be possible to watch Comcast TV on a Roku without a Comcast Internet subscription, as Comcast says it is "working hard to make cable-only gateway equipment available soon."

The beta app has some other limitations. It "will not include the ability to rent or buy programming or play back previously purchased programs," and "SAP may experience intermittent availability." It also won't work with DVR recordings stored on a local hard drive, though as previously mentioned, it can use Comcast's cloud-based DVR. While not all X1 features are available in the beta, Comcast said it is "working diligently" to add new functionality in future releases.