: I have a 75 gallon reliant 501 water heater that needs to be replaced. Home depot sells a 75 gallon residential model for $487. I can buy two 50 gallon of the same model for half the price. Is there any one who knows the proper way to install two water heaters side by side and the right way to do it?There are two ways, in series or in parallel.If I was to say one way is the right way I would offend 50% of the plumbers out there. I can only give my opinion. Do it in series.