For a _gas_ heater, 1 would actually be unrealistically good. Conventional gas boilers tend to be about 70% efficient. Condenser boilers can do better, up to ab

For a _gas_ heater, 1 would actually be unrealistically good. Conventional gas boilers tend to be about 70% efficient. Condenser boilers can do better, up to about 90%, but in practice performance is highly dependent on temperature of returning water.