I am installing a radiant heating system and I am searching on the web for advice on what to use for the heat source. There are a lot of recommendations to use a high efficiency hot water heater (i.e. Polaris) for both household water heating (showers) and for the hydronic radiant heating. I found a this old house web page (http://www.thisoldhouse.com/toh/asktoh/question/0,,192905,00.html) where it was recommended to "never use the same water heater for heating the house and making hot water for faucets. Water left in heating pipes all summer and allowed to cool are breeding grounds for bacteria." Other sites such as Radiantec (http://www.radiantec.com/index.php) and radiant floor company (http://www.radiantcompany.com/) recommend open direct systems where the potable water does flow through the floor heating pipes. These systems constantly circulate the new fresh cold water through the floor pex on the way to the hot water heater so presumably this reduces or eliminates the potential for bacteria build up. The quoted statement above implies that cold water is left in the pipes all summer and I can't imagine how that could be true unless a manual bypass valve was used in summer to bypass the pex? In all systems I have seen the water always circulates through the PEX even in summer. What are your thoughts on these systems that constantly circulate new water through the PEX? Are there bacteria issues? Any special requirments on the PEX if they contain potable water?