Boiler needs When we talk about using water we also have to mention hot/warm water, see article Current energy. There are all ideas to warm the water but at this moment in our opinion the cheapest way is of energy is a solar collector and transfer water over it. Then let it flow in a high isolated tank (boiler) for use later. In my experience if one of us takes a shower and another is doing the dishes, one of us has lukewarm water. In general the use is about 116 liter water a day. From the 116 liter we use warm water for: - dishes; - bath; - shower; - cooking; - cleaning windows etc. We also use warm water for the radiator but is a closed circuit. 48% of the water is also used for warm water. The critical point is when you use a certain amount of hot water in the same time. So the floor heat, the shower, the washing machine ( future option), the dishes will ask a big boiler which also have to heat before the use. You can arrange that the shower is superior to other use but for a washing machine it will be a problem. On a site I found there was a boiler with 120 liters and 90° C temperature. 90 % of it you could tap. 120 liter from 10°C to 90°C in 19,15 minutes. So if the transport of cold water is 10°C and the shower temperature is 40°C (mine is 38°C). The mixture of of the boiler and the cold water per liter to get 40°C is: 90 (°C) * Y (liter) + 10 (°C) * (1-Y) = 40 (°C) => Y is 0,375 liter/mixed liter warm water and then of course the cold water is 0,625 liter/mixed liter. So with a boiler of 120 liter hot water and a tap of 90%, gives 108 liter hot water to really use. You have effective by 40°C 120/0,375 liters before the boiler is empty and gives 288 liter. For a normal shower the use is 10 liter/ minute. With 288 liter you can shower 28,8 minutes in theory. In reality when you use water of the boiler, new cold water streams in the boiler and will be warmed from 10 °C to 90°C. In 1 minute the boiler warms 6,27 liter cold water to boiler temperature. We need for the shower in 0,375 x 10 liters mixed, 3,75 liter hot water of the 10 liter for the shower. If we use more then 6,27 liter hot water/minute the water is no longer 90°C and the 0,375 liter will increase to mix to 40 °C. In this figure I show where we use the hot water most of the time. Also by number I gave the priority of use. The bottle-neck is the same time of douche and washing machine (future warm water directly into the washing machine). If we look at the douche: It had 40°C and you shower 1 minutes of 10 liter/minute, you use 100 liter and by a boiler temperature of 90°C you use 37,50 liter of boiler water. In the boiler will flow in the mean time cold water and will be heated to 90°C. In the last line you read that the minutes heating time is 0,60. As long as it is beneath 1 the boiler has enough time to heat the cold water in the mean time. Watch the douche and the washing machine using at the same time. In the last line we read minutes heating time 0,60 + 0,75 = 1,35. This is > 1 and it means that the boiler is not capable to heat the cold water what will flow into the boiler. The washing machine will take 2 minutes Let us say we focus on the main problem. In the graph you see a boiler (70 liters) of 70°C and cold water of 0°C. 15 minutes shower 40°C and from minute 2, 2 minutes washing machine and 3 minutes dishes. The floor heat will work after the shower has finished (priority rule) and has no influence on the boiler temperature. The lowest temperature is 50,4°C in minute 5. What we see is that the 3 minutes use of the washing machine and the dishes, the first recovery takes a slowly 10 minutes. When we are finished showering after 15 minutes, the floor heat is going to start and takes another 3 minutes.