If I start my Izusu 42 from cold with cold water in the calorifier and drive away immediately it takes at least 20 minutes, maybe longer, for the temperature gauge to reach normal operating temperature of about 85 deg. If however I close the water valves from the engine to the calorifier, the engine is up to temperature in less than half the time, about 8 or 10 minutes. If I then open these valves the engine temperature falls back to about 75 deg and slowly returns to normal as the calorifier warms up which takes about a further 15 minutes. So on a completely cold boat the engine warm up period can be reduced by approx 50% by isolating the domestic hot water system until the engine is hot. Is that a good practice to adopt???