Amazing! i called mcs and they arrived the same day great service and very friendly staff thank you so much! will recommend & will be using you in the future! So how do you know what size boiler you need? well, the most accurate way to size your home’s boiler is to. If you’re buying a new boiler, or just trying to decide if you need one, how efficient it is will be a big part of your decision. On time and charged me exactly.