Radiant Floor Heating/Combi boiler water heater


We are building a new home in Sherburne County, MN, and are planning on using a Combi-Tankless system. 1344 sqft slab on-grade, one level home. There are a few that we have looked at, and with anything good and bad reviews. What brand due you prefer to use. Have looked at Navien, Quietside, and Bodorus.
Thanks so much for you time.

Badger Boiler Service Bosch Combi-Boiler
Badger Boiler Service  
Combi is becoming a nearly meaningless term, just as tank-less is today.

First, I most do not have a plan, since the first step in any HVAC plan for new construction or old is to secure an ACCA room-by-room heat load analysis. Once you know how much heating and cooling you need you can begin to consider the type, style and output of said unit and the possible integration of loads.

In your case I assume it is the integration or combination of space heating and domestic hot water heating (DHW). There are many variations on this theme but they are by no means new. The confusion comes from the marketing people.

It is very rare for a tank-less water heater to be properly pressed into duty as a DHW heater and space heater for reasons often sited in my previous post herein.

I will assume from your naming of names then that you refer to a low-mass, condensing boiler with a built-in plate heat exchanger rated for DHW and designed for automatic switching from one mode to the other. We get a lot of interest in the Combi-boilers available today, including Navien, Bosch and even Quietside offered by one of the Big Box stores.

Unfortunately it is very hard to match up a Combi boiler to a heat load and DHW load. Sounds good but works bad in most applications since the "instant" DHW demand almost always exceeds the space heat loads by a factor of 2 or more. This is especially true where ground water temperature are below 50F as the temperature rise requires such a large output. In new construction with relatively low heat loads and ever increasing DHW loads, such as filling a two-person tub, laundry, dish washer, etc. the asymetry just makes a Combi-boiler a pour choice.

We more often fit a condensing boiler (sometimes mistakenly referred to and a "tankless" boiler), to the the space heating load and size an indirect water heater to the domestic load  with no allowance or adjustment to the boiler output. This strategy has worked for us for a couple of decades now.

Of course here in Minnesota you have to fight the extreme ignorance of the State Pluming Board when it comes to indirect-fired domestic hot water heating, which would include all Combi-boilers and nearly every indirect-fired water heaters available in the world. Minnesota is one of the few states that require the energy-wasting double-wall heat exchanger for residential indirect water heaters giving the Amtrol BoilerMate an effective monopoly for this fantastic technology, as practiced by their many competitors, and stifling an entire market sector.

Needless to say when installing indirect water heaters outside of the cities cowed by the MN State Plumbing Board, all the licensed plumbers I know insist on a single-wall heat exchanger be it combi boiler or indirect-fired water heater for their improved output and efficiency.  

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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 owner---www.BadgerRadiantDesigns.com---Minneapolis 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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