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Radiant Floor Heating/legionella and open radiant floor heating systems


Can you clarify why legionella is a problem in the open system.   Since chlorinated water enters the hot water heater and the water remains in tubing, how does the water become contaminated?  Also, when it is heated to be delivered, doesn't it kill the legionella.    

Are you suggesting that 2 hot water heaters are better:  one at about 120 degrees to heat the radiant flooring and the potable water heater at least 120-144 or so degrees?

Thanks so much for your prompt response as I am holding my project up to resolve this dilemma.

Legionella is one of several risks associated with open combi water heating systems. Though it has been and continues to be common and accepted practice to use a residential water heater to perform both space and domestic (bathing) hot water, it is not common nor considered good practice to use potable (drinking) water to serve the same system.

The exception to the rule is a close-coupled fan coil such as those build by 1st Company Dallas, TX for decades. You can find 10's of thousand of these coiled piped directly to a tank-type water heater and potable water pumped directly to and from the coil making warm air and hot water for bathing. These systems are typically operated above 130栦or space heating comfort and have been safe for all of my 40 years in the plumbing/heating industry.

Radiant floors tied to open water heaters create a perfect environment for many thing to grow with typically low velocity and low (less than 100栴emperatures).

Even were it safe to operate and open system, I have yet to find a model building code that will allow it and if they did, I would not install them for the various problems that arise in their everyday operation. Hard or soft water, oxygenated will continual flow is bad for system components (more expensive NSF/potable water pumps and all accessories).

Here in Minneapolis we design and install many radiant floor, wall and ceiling heating systems using tank/storage type water heaters including Bradford White combi, Polaris, Vertex and HTP VersaHydro, but all are fit with a proper single-wall heat exchanger to protect our customer's health and the expensive system components found in modern radiant floor distribution systems. Even on our closed systems the radiant side is cleaned and treated for efficiency and long life.

The only valid argument for using an "open" system is that it is cheaper than doing it right.  

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Morgan M. Audetat


Radiant panel heating design, including floors and ceilings, European wall panels and snow melting for residential and light commercial buildings..............................................................-Master of Steam and Hot Water,City of Minneapolis....................................................--- Master Plumber, State of Minnesota...........


30 years..... Charter Board Member - Radiant Panel Association, former manufacturer of condensing boilers, former national distributor of radiant floor products, son and apprentice of mechanical & plumbing contractor. Current mechanical/plumbing contractor specializing in hydronic based, integrated HVAC systems. Radiant floor heating/cooling. Snow Melting. Condensing Boilers. Indirect water heaters. System design, consulting and technical training world-wide. Licensed contractor, designer, installer and consultant.

B.S. University WI 1981 CONTINUING EDUCATION: Viessmann Condensing boiler and Solar water heating certification 2010, N.D.S.U. Lead Worker RRP certification 2010, Knight condensing boiler certification 2009, Wrightsoft Manual J CAD certification 2009, RPA Designer & Installer Certification 2008, Nate Hydronic/Forced Air Certification 2008, Uponor/Wirsbo advanced design school certification 2007, Buderus Wall-hung certification 2007, Power Limited License (low voltage controls) 40 CE credits 2005, Basic Hydronic Certification IPEX & Northern Alberta Institute of Technology 1998, Charter Board Member - Radiant Panel Association 1994, Residential Off-Electric to Hydronic Conversion Heating School Canadian Hydronics Council 1994, B&G Little Red School House 1993, Rood Utilities (now Auburn Technical Institute) Oil Burner School 1993 ,Tekmar Controls residential and commercial 1993, Division Manager and Advanced Hydronic Seminar Instructor for the first Exclusively Hydronic Radiant Floor Distributor in the USA 1990, Hydronic Radiant Heating Association Workshop participant with Richard c. Bourne, PE spring 1988, Master Plumbing, Hydronic, Solar Course Red Rocks Community College 1987.

Awards and Honors
2009 System Showcase Award - Radiant Panel Association

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