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Radiant Floor Heating/tankless water heater problem radiant floors


I have a radiant in-floor system in my garage/shop, using a tankless heater.  The heater has always been set to 90F and the in-line temperature gauge has always agreed.  Recently the in-line temperature gauge started showing a lower temperature; I now have the heater set to 110 but the inline gauge shows just 85 and the system can't keep up.

Could this be as simple as air accumulating in the lines? The system uses glycol and hasn't been serviced since its installation about 5 years ago.

Badger Boiler Contender Radiant Floor
Badger Boiler Contende  
Hello Jim,

If you follow me here, or elsewhere, you will find me railing against the use of tankless (domestic hot water heaters) for space heating. And, you would get tired of me telling all who will listen never to use anti-freeze unless they absolutely have to, since it costs 20% more to heat it and 10% more to pump it. This is true if it is the proper anti-freeze at the correct concentration and can be worse if good practice is not followed. Furthermore, this is as good as it gets, so if you don't check the antifreeze for concentration, pH and inhibitor level, you will not only lose the one noble quality it offers, but it can turn into a corrosive medium that can damage hydronic heating components, save the PEX, which is essentially impervious.

As for the cure:

First, all gas-fired appliance manufacturers insist on annual maintenance service by a qualified technician.

Second, the heat exchanger is likely compromised by scale from cooking your antifreeze or corrosion from the antifreeze turning acidic and depositing materials in the fine filter screens designed to keep particulates out of the mechanisms that control flow in an appliance built to heat potable water in a single pass and never intended to circulate in a close system with antifreeze of any kind.

I would send a tech out to flush the system, dose it with cleaner, flush again and dose with the appropriate mix of propylene glycol to a level that would protect the PEX from bursting, since freezing and bursting are not the same. Then we would install a purpose-built condensing boiler with built-in outdoor reset  and promise to come back every year to properly service the race car of hydronic heating.

Here is a picture of what it would look like.  

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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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