Dave:

The carpet didn't expand due to temperature change; it stretched with use.

I installed all of the carpets in my 21 living rooms, except for one that I paid to have installed because I had sprained my ankle and couldn't do the work at the time. And, that carpet was the ONLY one to ever need restretching.

If the carpet is loose on the floor, you should be able to restretch it with just a knee kicker. Even though you're supposed to use a power stretcher on each installation, I wouldn't bother using a power stretcher for something like this. Also, I wouldn't bother renting a wall trimmer either because the amount you'll be cutting off probably won't be enough to even use a wall trimmer. You need the carpet to run up the wall a few inches at least to be able to use a wall trimmer. Otherwise the carpet just twists and binds in the wall trimmer.

You need to move the furniture out of the room before you stretch the carpet. It's best if the room is empty except for you and the carpet installation tools.

I'd just pull the carpet back along one wall and cut 1/2 to 3/4 of an inch or so off along it's entire length with a razor knife, straight edge and a piece of thin plywood so as not to damage whatever's under the carpet when you cut. You cut the carpet from the back, not from the pile side. Then just use the knee kicker to kick the carpet back in over the "tack strip". (See PS below) If the carpet is 20 feet long, you can easily stretch it 1/2 inch, and 3/4 inch with some effort using a knee kicker. You can stretch a 20 foot carpet a full inch easily with a power stretcher. So, if the dimensions of your room are 10 by 20, cut 1/4 to3/8 inch off the 10 foot dimension of the carpet and 3/8 to 3/4 inch off the 20 foot dimension of the carpet.

Now, if you're right handed, you'll be kicking with the right knee, and so that means you start on the left end of the wall and work your way to the right as you kick the carpet back into place. If you're left handed, start at the right and work your way to the left. That way, your body weight isn't on the part of the carpet that will be moving. When you stretch carpet, it behaves somewhat like a fishing net in that the area right in front of the knee kicker head will stretch the most and the stretch will diminish as you get further away from the head of the knee kicker. You want to kick the carpet every 12 inches or so. Don't try to stretch the carpet near the perpendicular walls because it'll be held down by the tack strips along those walls. You can start and stop stretching within a foot or two of the adjacent walls.

The knee kicker will have a dial on it's head which rotates to lower steel pins. You want those pins to just go into the backing of the carpet, but not deeper so that they jab into the underpad. There will also be a finger release on the underside of the knee kicker by which you can make it longer or shorter, depending on what feels comfortable for your body size. Place your hands on each side of the head of the knee kicker with your thumbs pressing downward on the head and the tips of your fingers about 2 inches away from the wall and pressing down on lightly on the carpet. When you kick with your knee, try to hold the head of the knee kicker down with your thumbs. The carpet will jerk forward and your finger tips will press the carpet down on the tackstrips as it snaps back. Do that until the cut edge of the carpet butts up against the wall or baseboard.

If the carpet is 20 feet long, you can easily stretch it 1/2 inch

Do one whole wall first, and then, pull back the carpet out along an adjacent wall (perpendicular to the first wall) and do an encore performance.

The carpet should be tight then.

PS: The correct term for "tack strip" is actually "tackless strip". That's because when the Robert's Company first came out with "Smoothedge" in the 1950's, it eliminated the need to use tacks to nail the carpet down around it's perimeter as had been the custom prior to then. So, "tackless strips" eliminated the need for carpet tacks. Nowadays, people don't even know what carpet tacks are, so they call those strips "tack strips". You know someone knows his history when he calls it a "tackless strip".

PS: In the first Blues Brothers Movie, Jake Blues (John Belushi) throws a box of carpet tacks out the window of the Bluesmobile to evade the dozen or so police cars that are chasing him and Elwood.