In the jurisdiciton I work in, it requires MORE than just a single seated check valve. It requires (residentially) a double check with an intermediate vent, and if it is commercial project, minimum requirement here is a reduced pressure principle back flow preventer.
Best to check with the authority having jurisdiction. Also be aware of annual testing requirements.
It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
You will find that in the applicable plumbing code but it is also an EPA reg. that your local water board can and will enforce. Need isolation valves to test and service. As stated, double check with atmospheric vent. If you install a bypass around an automatic feed valve for fast refilling such as after equipment replacement, the bypass must also be backflow protected.
You need backflow and a check Valve below the waterline
Type of Backflow depends on size and whether you're using chemicals
If you can email me I will send you Latest codes that I have (2009) as well as contact info for boiler division
Boiler division Usually only inspect if it is mixed-use Or more than five families