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Radiant Floor Heating/Tankless water heater for radiant floor and DHW


I am designing a 1500 sqft cottage for my family that will be an insulated slab floor. I am intending to use a tankless water heater for the floor and DHW. I do know of a contractor that I trust enough to have the knowledge to correctly size and configure what I require. It is not a complex project by any means but everyone is a self appointed professional with different recommendations. I am looking for a known credible resource to direct my decisions. Can you help?


(Striving for perfection as well)

Badger Boiler Bosch Combi-boiler
Badger Boiler Bosch Co  

There is much confusion in terminology as is pertains to "tank-less water heaters". It does not help that most of the English speaking world know water heaters of all kinds as "boilers". So to sort it out one must say what one intends to do with the appliance he has.

In your case a gas-fired combi-boiler, leading with boilers because I love them best, and there are many choices so described by their respective manufacturers.

The trick to specifying one gas-fired appliance that provides both space heating for comfort and domestic hot water (DHW) for bathing is knowing the heating load (Manual 'J') and the DHW load (number of showers, tubs to overflow and experience). Once you have these you can choose the right unit. Note that I did not give any tankless water heater as an option since it is not good practice, however common, to use a tankless DHW heater for space heating. I use many storage tank-type DHW heaters for space heating and for combi space and DHW heating due the beneficial high mass.

We have installed combi-boilers by Bosch, Navien, Triangle Tube to name a few and they all work flawlessly. However, I rarely use combi-boilers because the disparate loads in the typical residence, new or retrofit make sizing difficult.

The more likely candidate for modest loads such as I assume you will have, including micro-zones such as bathrooms on their own zone, would be a properly sized DHW heater, high or low efficiency with a sub-system for space heating. Properly isolating spacing heating heat transfer fluid from potable water, assures safety, reliability, performance and longevity, all of which will suffer if you decide on an "open" system running potable water for space and DHW.

If you send a plan we will design and specify a system that you can let to bid to any or all competent mechanics in your area.

Remember, DIY radiant floor design is always the first mistake.  

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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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