The new batteries are fine. What you are hearing is the 7 year end of life chirp. You can read about it here.

Here is a link about photoelectric vs ionization smoke detectors. Given you have 5 smoke detectors I don't think you have to worry about it.

There was no chirp until I replaced the batteries.

The previous owner may have installed the batteries incorrectly or the batteries were completely dead. You can try another set of batteries. If it continues to chirp you will need a new smoke/carbon monoxide detector.

Replacing the batteries didn't help. I happen to have another new detector, but it's not a CO detector. I'll replace them all when most are 10 years old, in three years. How important is it to have a CO detector in a room with no devices which burn anything? This is my utility room and only has an electrically powered boiler and DHW heater.

"it was suggested to me that my array should include two different types; some photocell based ones and some ionization ones"

Correct. Types of Smoke Detectors.

"How important is it to have a CO detector in a room with no devices which burn anything?"

Maybe just get a combo model.

"How important is it to have a CO detector in a room with no devices?"

VERY. Need one in my home on second floor outside BRs... detects CO from generator outside my garage.

"Carbon monoxide detectors should be installed on every level of your home — specifically outside of sleeping areas, in the kitchen, in living/dining areas, and in the garage. Also install one near your gas furnace, if you have one, as well as in the laundry room if you have a gas clothes dryer."

Smoke detectors have a finite life expectancy. They require annual maintenance while includes replacing batteries and vacuuming out the detectors--at least the photoelectric type. You'll be well served to replace all of them with new detectors and install one or more combo detectors as suggested above to include CO detection.

The only combustion I have within 500 feet of my house is my propane stove in the kitchen and, potentially, a campfire 200 feet away (other than motor vehicles, which aren't left running). However, I'll replace all of them with combo units.

The Battery Life info above may be out of date - or not correct in many states.

My batteries, both Smoke Detectors, CO, and Combo have ten year batteries and here in my State the Smoke Detector batteries must be 10 year. YMMV.

"Maryland law now requires all Maryland residents to have 10-year lithium batteries on their smoke detectors that are tamper-resistant with the silence/hush feature. There must be one on every level of a home."

My Smoke Detector/CO Alarm COMBO - TEN YEAR BATTERY

The old batteries may be legal in Cady, TX. Wait for a post from there to see. lol

"The only combustion I have within 500 feet of my house is my propane stove in the kitchen... "

Here in Maryland this propane stove would require CO detectors.

Also, if you have an attached garage. Likely no one intentionally leaves a car running so as to kill their family members or themselves!

I buy only ten year sealed battery alarms now. Even if they cost a bit more, the peace of mind and no hassle of chirps and replacing batteries makes them a good investment.

Are they interconnected?

In my case no, had the house been built in 2009 or later they would have to be interconnected.

"Any new home in Maryland constructed after January 1, 1989 required at least one hardwired electric smoke alarm on every level of the home, including the basement and required the units to be interconnected in order that activation of any one of the required smoke alarms resulted in the sounding all of the required ..."

Perusing the Big Box stores, it appears that the 10-year battery ones are promoted to not require hard wiring or interconnection. My existing ones are powered and interconnected, and I would like to replace them with ones which the batteries don't have to be replaced.

• First Alert BRK 7010B Hardwired Smoke Detector with Photoelectric Sensor and Battery Backup
• FIRST ALERT BRK 9120LBL Hardwired Smoke Detector (ionization sensor)
• FIRST ALERT BRK SC9120B-3 Hardwired Smoke and Carbon Monoxide (CO) Detector with Battery Backup. 3-Pack (ionization sensor)

They're all hardwired, can be interconnected, and have a backup battery.

In 2015 I bought detectors that didn't have batteries but they're not manufactured anymore.

The Home Depots in my area carry hardwired smoke detectors. I'm not sure if all can be interconnected or not. All have onboard battery backups.

When I did an addition, then prevailing code required a hardwired smoke detector. It was placed near the top beam in a room with an open beam ceiling. I got frustrated when the battery started chirping some years later because I, moi, am not someone ready to climb up a ladder to reach something 15+ feet off the ground. I got an electrician to disconnect it and then put a battery operated smoke detector in the room on the wall, at a level I can reach using a stepstool. It may be less useful at that level but it was either that or nothing.

I'm not sure what the applicable building code in the OP's state requires, but the 2018 IRC (R313.4.4) requires physical interconnection (i.e., hardwiring) of smoke detectors. However, interconnection is not required when listed wireless alarms are installed and all alarms sound upon activation of one alarm.