For water heater, how could we calculate the heat required for a certain water consumption ?

For a central heating source, firstly estimate the demand for the entire facility/building in gph (US Gallons per hour). Next based on the type of facility (whether Commercial/Hotel/Hospital and many more) get the actual demand multiplying with the DEMAND FACTOR. Then use a steady state equation (Ref. ASPE Vol2, Chpt6) to estimate the heating required in KW. Also keep in mind about the system; if it is a heat recovery system using HWR (hot water return) piping, then there is a separate guideline for estimating heating KW. Spreadsheet attached.

calculate the heating load by hap or block load or any other way.....the heater type dont ,meater,electrical or oil or boiler

Dear Sherief

I'm not an expert in plumbing but I have an idea about how to size and I hope I could be of help, find below a guide on how to size your water heater

To size water heater you need to identify the following as a1st step:-

1- type of water heater (tankless/storage/ solar water heater)

2- demand ( amount of water flow rate in case of tankless and bulk mass in case of storage)

3- Temperature rise ( maximum - minimum temperature)

Once you identified those3 basis you can move on to the calculations, I found some sources that can be beneficial for you , below are the links but for more details I suggest you take a look at the IPC and Plumbing design Handbook,

http://energy.gov/energysaver/articles/sizing-new-water-heater

bfa.sdsu.edu/ehs/pdf/HotWater.doc

best Regards

Ahmed Gamal