Only a problem if you're trying to use them in North America. Since it appears (from your header information) that you're not, I don't see any problem. :-)
Not a good idea. Unless of course you're looking at a large public garage where you may wish to control the levels of CO through the use of exhaust and fresh air fans. In a normal two car garage, though, mounting a CO or Smoke Detector in it will only result in many irritating false alarms. Postion the CO detector near your bedroom and main living areas.
Yes. Don't use them in a garage. Smoke detectors of any sort should not be installed there. They need to be located in a controlled environment (temp / humidity) that is low on dust and does not routinely have smoke in it. A garage does not meet these requirements.
Also, while some people like them, I don't believe you should combine smoke and CO detection on a single output. The response to smoke detection is to alert the family, then check the place for signs of fire. If no fire is detected you can reset the alarm and go about your business. With CO it is not the same. The proper response (IMO) is to alert the family, exit the premises immediately and have the fire department check before re-entering.
Humans cannot see or smell CO gas. The only indication that there is lethal gas accumulating in the house will be that CO detector. If you decide everything is OK and go back to sleep when in fact there is a CO problem you may never wake up.
During the years I operated a small alarm company I had several real CO events where the homeowners did not believe there was a problem. In at least two of those the reason they survived is we called the fire department in spite of their protestations that all was well.
In an earlier thread I discussed one of these events. Another time a family I protected was having a New Year's party. They had numerous guests in their large, brand new (well sealed) home. The gas furnace was running because it was extremely cold outside. The home had two large fireplaces and each had a roaring fire going.
We had installed four CO detectors -- one in the furnace room, one on the main level and two on the bedroom level. All of them triggered, sounding the alarm and triggering the signal to the central station. We called the FD and then rang the house (our policy for CO alarms).
The homeowner insisted that all was well but that the detectors were just "going crazy" because everything seemed OK. When the FD arrived they brought in a tester and informed the residents that the house was full of CO gas. They put out the fireplace fires, opened several windows and doors and after a while the air was clear.
The problem, BTW, was the fireplaces. The flame in a furnace is a very small fire compared to a fireplace. The two fireplaces were drawing so much convective current up the chimney flues that it was being replaced *down* the furnace flue. This allowed CO to pour into the basement. The forced air heating system drew some of its air in through a basement return and distributed the CO throughout the house.
If the CO alarm had been combined with fire the homeowners might have thought it was a smoke alarm and, finding no fire, dismissed the matter with catastrophic results. It is because of several experiences like this that I don't recommend using combined smoke/CO detectors.
Hmmmm.... What about the fresh air source for the gas fired furnace/hot water tank? It would be far easier for the fireplaces to draw air through the fresh air source (usually piped directly to the furnace area and terminated about 2 feet off the floor). The reason for this being that the furnace flue is usually 4 inches in diameter and heated from below (hot air tends to rise, not fall). The fresh air inlet is usually
8 inches across. Unless of course some nimrod blocked it (which I've seen) but you haven't mentioned. In a house that's as well sealed as you imply that may be a possibility but I doubt it.
Robert's account sounds spot on. I have seen this happen twice -- once in my house and once in a friend's house. In my house, the fireplace drew so much updraft that the furnace chimney backpuffed. The newly installed windows and doors made the home so airtight that the most effective way for the fireplace to draw it's needed air was back through the fireplace chimney, causing a backpuff and the CO detector to go off. Cracking a window open during fireplace operation fixes this, of course. Secondly, I witnessed a friend's home backup CO and smoke from the furnace while having a birthday party. The attic fan was going full-tilt and all the windows and doors were closed. That fan, as powerful as it was, pulled enough air to choke the furnace and fill the basement with smoke and fumes, causing an alarm. My wife actually "smelled something funny" and thought that it "felt just like the problem we had," then waddya know, it happened to them minutes later.
I recommend a Rate-Of-Rise heat detector in the garage, not a smoke or CO detector.
I've talked to two mechanical contractors and an engineer this morning and asked them about this "phenomenon". They all agreed with me... The fresh air vent with a larger cross-section and no restrictions would draw the air required by both fireplaces (in Robert's fairy-tale). It's the equivalent of "cracking" a couple of windows open. A "back-puff" the likes of what you experienced would probably douse the pilot light in the furnace (which would tend to turn the furnace flue into another fresh air source). I've seen that happen in a wind storm. Robert got caught telling another fairy-tale (supported by a "total nerd"). Sort of like his "many years" installing... He'll argue with you that because he retained his CT L-6 until 2000, he can still count that as "installation experience" even though he'd been living in Florida for a year (or more). Sort of like the four years he spent in Ohio while he was "running his modestly successful central station alarm company" in CT... Sh-uuu-uurre...