Thanks for answering. But no. That just ain't it. Thank you. Myson and Thermar were produced by the same company in England - UK. When the EPA changed the rules about pilot lights on gas appliances, Myson just gave up and ceased production. For you youngsters out there, pilot lights on all appliances were common and legal. Now only a few appliances are allowed and have a pilot light. The Myson 325 (and some others) were unique in that they required no electricity. Myson is a specific brand and few are qualified and have knowledge to repair it. But thanks for responding.
The issue is UnLit Pilot Lights, are considered by the EPA, BOMBS waiting to go Off....... So, the EPA required Safety Valves, that are Manually ByPassed, to Light the Appliance, and then the Heat of the Pilot Light drives a Thermal Input to the Safety Valve that keeps it OPEN. This way, if the Pilot Light quits burning, No Heat is released and the Safety Valve CLOSES, so no unburnt Gas can be released into an Enclosed Space. it is True that UnBurnt Released Gas can build up in an Enclosed Space, and become "Explosive" This is the reason I do NOT have ANY Propane Appliances in my cabin. I Heat, and Cook, on an Open Fire, Pot Burning Oil Stove that also Makes our Domestic Hot Water. If I had Gas, in the cabin, and it leaked, there is ALWAYS an Ignition Source burning inside the Cabin.... No Gas, No Leaks, and No Problem.
The best information I can offer is: Myson & Thermar Tankless water heater parts from ASAP Heat I own and maintain a Myson 325DV and really, really like it. I have dealt with ASAP Heat for parts, but the real value is that they have a guy there that was willing to give technical assistance. He was very helpful to me in getting my unit working. I had a problem with the pilot light not staying lit. It turned out that I had over-tightened my ECO connections. I hope this information is not too long after the fact to be of use, but I imagine there are still a lot of these units in service as they are pretty robust and rather unique.
I understand the reason for your caution, but I grew up in a house where we had propane and a fireplace (including a run to the fireplace for lazy starting of a fire) and never had any issues, so I have grown quite comfortable with the propane I have for my cookstove and water heater despite heating my place exclusively with three wood stoves. Of course, the hazard is quite real, and one must use care, caution, and common sense, yes?