would a well insulated hot water heater be an efficient way of heating a house using cast iron radiators?
Yes, that is called hot water heat, and a lot of houses use that. But you don't use the hot water heater, instead a separate burner heats the water, and a pump circulates it through the radiators and back. .
For space heating purposes, the device that is used is usually called a hot-water boiler (even though it doesn't boil water). The burner is sized large so that it can handle the heat load on a continuous basis. The tank (if there is one), is there so that the water that circulates through the heat exchanger can be at a higher temperature than the water in the heating loop in the house. The higher temperature in the heat exchanger cuts down on the formation of sulfuric acid in the flue gasses, but is hotter than you would want for use in the radiators. A water heater for domestic uses, by the way, has a tank for a different reason. The purpose of tank is to allow water to heat up over time (with a smaller, less-expensive burner heat exchanger) so that you have enough hot water available when everyone wants to shower, etc. So to answer your question, you could use a water heater, but it would need a really large burner. Although good insulation is important, an even more significant thing to evaluate is the efficiency of the burner. An 80% efficient burner will cost a lot more to operate than a 95% efficient burner. You won't get anywhere near that type of energy and cost savings just by insulating a tank well. By the way, the industry name for the device you mentioned is water heater, not hot water heater, in case you want to be hip to the terminology.
