Let me do this by showing you an example. If the boiler is an ear spitting 120 decibels at 10 meters feet away, it will be 6 decibels lower at 20 meters away or 114 decibels.
if you double that to 40 meters away the sound drops to 108 decibels, that is still very loud
If you double that to 80 away, the noise drops to 102 decibels.
If you double that to 160 meters that the noise is 96 decibels (still very loud and in the damaging range)
If you double that to 320 meters the noise is 90 decibels.
If you double that to 620 meters the noise is 86 decibels. Noisy but not particularly damaging.
If you double the distance to 1240 meters (a bit over a kilometer) the noise is 80 decibels... that is still relatively loud... it would be close to ear damaging levels at 1000 meters.
IF THE SOURCE NOISE AT THE BOILER WERE 120 DB. The actual noise at your place could be higher or lower.
This is about noise level exposure hazards at various decibel sound ratings.
Notice that at 85 decibels, the NIOSH allows workers only 8 hours a day of such exposure...
I am guessing at the boiler noise levels, and figuring 120 decibels, that would put you at 82 or 83 db, annoying levels but not physically dangerous. (however it is my view that 'annoying' is also dangerous, maybe not to hearing but surely to a persons mental health for sure.)
Note however, that I was guessing about the source boiler noise. The boiler could easily measure louder than 120 db at 10 meters away... if it measured 140 DB that would put the noise at your place one km away at actually damaging levels.
You can rent a decibel meter, and arrange a tour of the plant, and stand 10 meters from the boiler and measure its noise level. You should practice with the meter first on traffic, and TV noises etc to get it calibrated so that you know how to use it.
If you choose to rate my work, so that the company pays me for my time, I will be able to continue with you for an extended period of a day or so.
That is a noise calculating program.