Hi Dennis,
I had a similar issue with my Aquahot and did a bypass on the float connections and all was fine which indicated a faulty float in the tank. A replacement solved the problem and when I removed the old one I noted the float had fluid in it so of course wasn't making a connection and incorrectly causing a "low coolant" shut down. Pretty easy to fix on an Aquahot, not sure what is involved with the Hydrohot though. You do need to remove enough of the fluid to be able to remove the float without a mess. I just picked up a cheap manual syphon pump at a local auto parts store and it worked great. I needed to suck out about a gallon to get it below the float opening.
Good luck,
Dwight