Yes, there are check valves. From the maintenance manual, page 20:
"Check Valve:
Check valves are installed into the zone outlet ports for each
heating loop to ensure that the antifreeze and water heating
solution only flows in one direction. If the heating solution
attempts to backflow into the boiler tank, the check valve
closes to prevent that from happening.
"Troubleshoot the check valves if the following condition has
occurred:
There is a lack of interior heat in a particular zone.
"NOTE: Before Troubleshooting the zone check valve it is
important to remember that if the
zone circulation pump is
air locked, or if there is a kink in a zone heating loop plumbing
line, the customer will experience the same symptoms as
a stuck check valve."
http://www.aquahot.com/files/service_manual/AHE-100-02S%20Service%20Manual%20Rev.%20B%209-27-2011.pdfDownload the file and go to page 20. There is more - about 3 pages - on the subject.