2 XE90s converted to propane were installed in my home in 1998, one in the attic the other in the basement. They have never failed or been serviced. While replacing the filter this season, I noticed that each of them has a red light flashing once per second in the side panel sight glass. I called the service department of the company that installed them and was told that: "We think that is normal". I'm uncomfortable with the word "think". Any advice?