OK, quite a bit to unpack here, so please take that with a HUGE grain of salt (also consider that it’s already past midnight here, so I may not be thinking clearly, LOL)
A general rule of thumb is to use a water heater with a capacity of 30-40 gallons per 1000 square feet of living space. However, this is only a rough estimate and the actual capacity needed will depend on the specific circumstances of your home. The climate and the type of insulation used in the building would also play a role in the sizing of the water heater
A more accurate way to determine the size of the water heater is to calculate the heat loss of the space. There are different ways to calculate heat loss, but a common formula used is:
Heat Loss (BTU/hr) = 1.1 x Area (sq ft) x (Temperature Difference (F°) / R-value (insulation) )
Where:
Area (sq ft) is the total square footage of the space
Temperature Difference (F°) is the difference between the desired indoor temperature and the outdoor temperature
R-value (insulation) is the resistance to heat flow of the insulation used in the building
By applying the values you provided:
Heat Loss (BTU/hr) = 1.1 x 1100 sq ft x (35F / R10 insulation)
First, we have to convert the R-value of the insulation from R10 to hrft²°F/BTU, which is the unit used in the formula. To do that, we divide 1/R-value.
R-value of 2 inches of XPS insulation is R10, so the hrft²°F/BTU = 1/10 = 0.1
So the heat loss calculation becomes:
Heat Loss (BTU/hr) = 1.1 x 1100 sq ft x (35F / 0.1)
Heat Loss (BTU/hr) = 1.1 x 1100 sq ft x 350F
Heat Loss (BTU/hr) = 42,350 BTU/hr
This formula is a simplified version for heat loss calculation and does not take into account all the factors that can affect heat loss such as type of window, orientation, wind exposure, etc. Therefore, this is a rough estimate, and a professional will be able to provide you with a more detailed and accurate calculation taking into account all these factors.
Keep in mind that *this calculation is only for heat loss, you will also have to consider the hot water usage, and peak usage of the radiant floor heat*. You can use this value as a starting point to determine the size of the water heater needed, but it is always best to consult with a professional to ensure that the system is designed to meet the specific needs of your home and comply with local building codes and regulations.