It sounds like you are again in need of a circulator cartridge for the Taco model 006. Get one quick on ebay and you will no longer have overheating problems leading to tripping of the limit switch. As an experiment, I disconnected power to the circulator pump and turned on the HM30; it took less than a minute before the limit switch tripped and steam started rolling out of the top of the HM30.
I regret to announce that my HM30 started misbehaving in January - it was leaking water into the burner chamber causing a lot of noise and a lack of heat. I jury-rigged a siphon so that I could drain the chamber once a day and have heat for the next couple months while I investigated the options for a replacement.
Our HM30 was installed in Nov 1995 so we did get 12 fitful years of service from it before the replacement.
In March, I decided that I wanted to keep the AM30 unit and get a high recovery water heater that would also supply hot water to the AM30. I got one of the new A.O. Smith Vertex heaters and installed it myself. It hooked to the AM30 and I only had to change one control signal wire. The Vertex is a 50 gallon water heater with 75,000BTU capability. It is quieter than the HM30, takes up the space where it sat, and uses the 2" pcv exhaust pipe. One of my main objectives was that I get a water heater that acted like a conventional heater; I didn't want a high power exhaust fan with a burner on top requiring a circulator pump to keep the limit switch from tripping.
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