Determine the size and shape of your house. Calculate the square footage and how the space is used. For example, a lot of wide open space will cool off more quickly than smaller rooms. Consider the position of the house, if it is an end unit, standalone house or part of a complex. Buildings that have many walls open to the air lose more heat than a townhouse that has other units on each side.
Determine the type and quantity of your insulation. Look in the attic or basement where you have exposed insulation and examine it. Note down the color, structure and measure the depth. Wear gloves and long sleeves when working near insulation.
Take note of how many windows you have, the size and if they are double paned. Double-paned windows greatly reduce the amount of heat you lose in cold weather.
Calculate how much water you use per person in the house by comparing how many people live in the home to the monthly water usage on your water meter. If you use a lot of water per person, you'll need a larger boiler.
Input this data into computer software or an online program. This will calculate the size of boiler you need for your home. You can also talk to a licensed specialist who will use your information to determine your boiler needs.