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Radiant Floor Heating/small home radiant heat tankless mix


Mr Audetat, is it possible to tie both radiators and an in floor system under tile to a tankless water heater for a one level tiny house?

Badger Radiant Designs Combi WH
Badger Radiant Designs  
It is certainly possible to use a tankless gas-fired water heater to drive a radiant floor. It is done all the time. The reason so many look to a nice tank-less domestic hot water heater (DHW) for for "space heating" is the relatively low price. Relative, that is, to a purpose-built space heating boiler.

I have long been an advocate of gas-fired condensing boilers, knowing that the long term performance, efficiency and reliability, will out over short-term initial savings and poor performance.

To start, you will likely end up with a tank-less water heater in the 190,000 Buth input range. The minimum fire rate for this typical water heater, say a Takagi TK-3, will be 11,000, let's say 10k output.

Our first task is to get rid of the 10k without shutting down the burner during a call for heat. Ideally the WH will run the whole heating season at low-fire (10k) or above. Unfortunately I see far too many bumping off the bottom of the low-fire, i.e. short-cycling. This will shorten the life of any gas-fired appliance and worse, kill fuel savings.

As 10k is also about the lowest firing rate for conventional modulating/condensing boilers (ModCons) a body has to think twice before investing in high technology for modest heating loads e.g. 30,000 Btuh and below. For these modest loads, and micro-loads such as a dedicated bathroom or sun-porch, I prefer to store the hot water in a buffer tank or storage-type DHW.

The choice in conventional storage water heaters is nearly endless if a conventional chimney (gravity) is to be used. Less variety is available for direct-vent (PVC) venting out the sidewall is required, and very limited if you want sealed-combustion, direct-vent WH such as the condensing tank-less or ModCon.

In fact there are only two that we use here in Minneapolis, the Polaris--in limited supply and support and A.O.Smiths "Vertex".

The Vertex will allows the smallest of zones to draw without necessarily calling the burner on. It also fires to 100,000 Btuh covering a lot of real estate if necessary and does not require a chimney or dedicated combustion air since PVC is used to bring air to the unit and exhausts to the outside. When pressed into combi service providing DHW and space heating the small 22" diameter footprint is worth the space.

Naturally you still need to separate the DHW from space heating water with a plate heat exchanger and sub-system, but it's worth it. See Taco XPB.  

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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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