There are some common guidelines that people follow when running PEX tubing, one being to keep loop length under 300 feet. While this is a good rule of thumb, it is not always true. If (after finding the Heat Loss Report for your home) you have a heat loss higher than 15 BTU's per square foot, you should keep your loop length under 225 feet. The higher the heat loss, the shorter your loops must be to accommodate for more rapid cooling. The difference in water temperature between the water leaving your pump and the water returning after it has circulated through the loop should be no more than 20 degrees.
PEX Tubing for radiant heat and other Radiant Heat supplies can be found at www.supplyhouse.com
