The robot vacuum that moved in under Grandpa's recliner

The first time it happened, Grandpa thought his chair was talking to him.

He was deep in the recliner with the ballgame on when a cheerful little voice came up from directly beneath him and announced that it was stuck. He looked at the armrest. He looked at the remote. He looked at the ceiling, briefly, in case it was a bigger announcement than he'd planned for.

"I've had this chair since 1989," he said to nobody, "and it has never once asked me for anything."

It was Herman. Herman is the robot vacuum his granddaughter Katie gave him for his birthday, because his knees have opinions about bending over these days. Herman runs every morning at ten. And every morning, somewhere around ten-forty, Herman drives under the recliner, wedges himself into the steel linkage like a man who's found the one good parking spot at the hardware store, and calls for a tow.

What it says it does

A robot vacuum is a battery-powered disc that drives itself around your floors on a schedule, sweeping debris toward a small suction opening and into an onboard bin. Spinning side brushes flick dirt out of edges and corners. A main roller brush underneath does most of the actual work.

It finds its way with sensors. Bump sensors tell it when it's hit something. Cliff sensors on the bottom, usually infrared, look down so it doesn't drive off the top step. Higher-end models build a map of the house using a laser sensor (often called lidar) or a camera, which lets you draw no-go zones in a phone app. When the battery runs low, it drives back to its dock to charge. Some docks also empty the robot's bin into a bag, so you're not dumping it by hand every day.

What none of them can do is know your furniture. They know how tall they are. You have to know how tall the gap is.

What Grandpa thinks it does

Grandpa's position is that Herman is a hockey puck with a union contract. Works a short shift, takes a long break, and won't move until somebody comes to get him.

He also believes Herman is looking for something under that recliner. "Forty years of dropped peanuts," he says. "He's on an archaeological dig."

He has documented that Herman eats one sock per week, which Grandpa now considers a toll. He pays it in the hallway.

And Herman won't cross the dark blue runner in the hall. Approaches it, stops, turns around, like it's the edge of the Grand Canyon. This one's real, by the way. On some models the downward cliff sensors read a very dark rug as a drop-off and refuse to go over it. Herman isn't being dramatic. Herman thinks he's about to die.

"Smartest thing in the house," Grandpa says, "and it's scared of a rug your grandmother bought at a yard sale."

Where he has a point

First, the recliner. Before you reach under any recliner to pull a robot out, put the footrest all the way down and stay out of the chair. If it's a power recliner, unplug it. The linkage underneath can close hard enough to crush fingers, and it will do that whether a robot is stuck in it or a hand is. Keep kids and pets clear when anyone's working the footrest, too.

Now the rest of his complaints, which are fair:

• It's never done buying things. Filters, side brushes, roller brushes and, if you have a self-emptying dock, bags. Those are ongoing costs. Find out what they run before you buy the robot, not after.
• The battery wears out. Every lithium-ion battery loses capacity over time. Some makers sell replacement batteries you can swap with a screwdriver. Some make it hard. Check before you buy.
• Hair is its natural enemy. Long hair and pet hair wrap around the roller brush. Plan on cutting it free with scissors every so often. Some brushes are designed to tangle less, so look for that if you've got a dog or a long-haired grandkid.
• It knows your floor plan. A mapping robot stores a map of your house, and a camera model can capture images. Read the privacy settings, turn off what you don't need, and put a strong, unique password on the account. Lock down every account you own in one evening covers the rest.
• It depends on the app. Some robots do a good job from the button on top. Others are half a machine without the app and a Wi-Fi connection. If the maker stops supporting the app someday, you want the first kind.

And yes, his old upright from the seventies still runs. So does the broom. Neither one has ever needed a firmware update, and neither one has ever hidden under his chair.

Where he is wrong

Here's the part Grandpa says slower and quieter.

The floors are cleaner. Not deep-cleaned, but clean every single day, which is the difference. Dust and crumbs never pile up because Herman picks them up before they get the chance. Grandpa used to vacuum once a week when his back allowed it. Now the kitchen looks like somebody cares about it on a Tuesday.

His knees are happier. That was the whole point, and Katie was right about it.

And the recliner problem took Katie about four minutes to fix. She measured the gap under the chair with a tape measure, opened the app, and drew a no-go box around it. Herman hasn't been under there since. On models without an app map, magnetic boundary strips or a low physical barrier do the same job. Grandpa had been planning to screw a length of 1x2 lumber across the front of the chair. He admits now that would also have worked, but it would've been uglier and he'd have tripped on it by Thursday.

"I was mad at the robot," he says, "when I should've been mad at my tape measure for sitting in a drawer."

If you're adding a few of these gadgets at once, Smart home kit that earns its place, and kit that just sits there will help you sort out which ones earn the outlet.

The verdict

Worth it, with conditions.

It's for people with pets, bad knees or bad backs, mostly hard floors and low rugs, and a house that doesn't look like a laundry hamper exploded. It's not for homes full of stairs (it does one floor at a time), thick shag carpet, or floors covered in cords and toys. Somebody has to pick up before it runs. That's not a flaw. That's the deal.

Three things to check before you buy:

1. Your clearance versus its height. Find the robot's height on the maker's spec sheet, then measure under your couch, bed and recliner. If the gap is barely bigger than the robot, expect it to wedge, and plan on a no-go zone or a barrier.
2. Whether it works without the app, and what the privacy settings let you turn off. Read them before you set it up, not after.
3. What the parts cost and whether you can get them. Brushes, filters, bags and the battery. Check that the maker sells them, then check the current price of the robot and those parts together.

Grandpa still talks to Herman, for the record. Says good morning. Says thank you. Herman hasn't said anything back since he stopped getting stuck, which Grandpa considers the best review any machine in his house has ever given him.