First of all boilers use water to distribute heat. Anyone who has been a homeowner for any length of time can tell you how destructive water can be. This is also true in a builder system. The water in the lines can ruin the boiler in short order. If you live in a rural area and the boiler water is supplied by a well you will want to pay close attention. The heat in a boiler tends to bring the worst impurities out of the water. These impurities in turn tend to destroy the boiler over time. The best approach is to add rust inhibitors into the water if you have a closed loop system. If you’re heating the slab in the garage I strongly recommend that you add glycol as well to prevent freezing. If you are in a city or town your system may not be a closed loop and my suggestions will not apply.