What should I look for in buying a carbon monoxide detector?

I think the best is a multipurpose smoke detector and carbon monoxide detector wired in with battery back up. If you have old smoke detectors you might want to consider this.

Otherwise, you can get a plug in carbon monoxide with battery back up.

From what I have read it is best to keep carbon monoxide detectors higher up, so you might want to place it in a high outlet if you have one, I don’t think the gas rises very fast though, it is not much lighter than normal air. If you have a garage have it not too close and not too far from the garage. Also, put one near your bedroom door.

To go slightly against what I said about replacing your smoke detectors, if your garage is next to your kitchen you might not have a smoke detector anywhere nearby and still want a monoxide detector, so then the plug in is a nice addition in that case.

I like CO detectors that have a digital readout. That way it can be used as a tool – not just an alarm. First Alert has them for $35–40.
I personally prefer batteries. I know batteries can be a pain but the AC powered ones need to be plugged in and you don’t always have a convenient spot to do that.
Battery powered units will work all the time and can be moved where you need it.
For example, when there is a power outage, I run my generator. I’ll put the CO detector with readout in the same area as the generator just to see if I am getting any CO leakage.

Didn’t know they had digital read out. @JLeslie mentioned the multi purpose one. Does that come in digital readout too?

Yes, there are combination with digital display. I think there are portable ones too that are just battery and you can set it up anywhere.

I was just thinking you can probably ask your local fire department if they suggest a certain brand or model. They respond to all of the emergency calls and possibly have an opinion.

Something that doesn’t wake me up in the middle of the night with terrible sounds when the battery is running low, and that is easy to replace the battery, and that isn’t supposed to be mounted at an inaccessible height. And that isn’t so sensitive it will go off due to cooking or a bit of smoke from a wood stove or something.

Are Radon detectors still a thing?
Do they have combo units?

Yes, there are radon detectors. Radon detectors you would want to put in places you spend a lot of time and I would think the basement too. If you open the windows and doors a lot the radon is constantly being aired out.

When they do a radon “test” by an inspector they make sure no one visits the house for a day. They use a radon test kit, I think anyone can probably buy one.

I have no idea if there are combo units. I’ve never seen any.