The National Fire Protection Association (NFPA)
talks about smoke detectors in terms of the sensitivity of a smoke detector. The smoke detector has to go into alarm if the smoke reaches a certain level of "Obscuration". Obscuration is how much smoke it takes to block, or obscure, enough light to set off the alarm.
If the "Smoke Detector" is actually a photo type smoke detector and an ion type smoked detector inside the same housing, the combination still has to go into alarm if there is enough "Obscuration". In essence, the ion smoke detector is best to detect small smoke particles, so the ion detector is tested with small smoke particles, the photo detector is best to detect large smoke particles, so the photo smoke detector is tested with large smoke particles.
The obscuration will be tested
by a testing laboratory like UL, ULC, CE, CCC, FM, etc. If a testing laboratory has put it on their list of "Listed" detectors, they have confirmed in their test that the smoke detector or combination detectors work as determined by the NFPA.
If the air temperature reaches a certain point (Fixed Temperature), a heat detector has to go into alarm. Or if the air temperature is rising fast enough that it is probably going to reach that temperature (Rate-of-Rise or ROR), the heat detector has to go into alarm.
For a conventional heat detector, mechanically, these are two different heat detectors
. For an addressable heat detector, even though there is only one heat sensor, electronically these are two different detectors.
If the two types of detectors (Fixed and ROR) are within the same enclosure, so the two look like a single detector, they still have to be tested one at a time by the testing laboratory, and they both have to be able to go into alarm by themselves.
Combination Heat Detector and Smoke Detector
Even if both types of heat detector and both types of smoke detector are in the same enclosure
, so there are actually four different detectors, each individual detector has to be tested on its own to make sure that it will detect smoke or heat.
The NFPA talks about the spacing - the maximum distance between heat detectors or smoke detectors. However, the various AHJs and the overriding occupancy requirements get more specific, including whether or not there should be detectors in the first place.
I haven't found any place the NFPA refers to the detectors as being too close together. That's because if they are close together, and one of them goes into alarm, the alarm doesn't prevent the other detectors from working.