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Radiant Floor Heating/electric tankless water heater for radiant floor


QUESTION: Hi.  I built a small strawbale home (30X40 footprint) in Central Washington state about 6 years ago that has a hydronic radiant heat floor in the slab.  The area gets a lot of snow with winter temperatures anywhere from 10-30 degrees F.  Given the size of the house and space considerations, our architect designed only one utility closet where we use a shorty electric water heater to heat the radiant floor (4 zones of approximately 300 sq. ft.), and for domestic hot water, we have a Hubbell 280-3 electric tankless water heater (3 element 27kW).  This model is supposedly well suited for whole house needs in areas where incoming water can drop below 45F.

What we're finding is that we're having a lot of problems with the electric tankless to supply whole house needs even though we're very conscious of using one major water use at a time.  If we could, we'd go to an electric tank heater for our domestic water use and give up on the tankless altogether.  However, we only have one utility closet so can't have two tanked heating sources.  We also live in hydropower country so gas is not available or very expensive where electricity is very cheap.  We're now thinking of putting the hydronic radiant floor on the tankless heater and installing a large tank electric heater for our domestic use (all in one closet).  The tankless heater is sized for residential domestic use so we're thinking that it should not have any problem heating water for the floor (which I assume will require lower amounts of flow in gpm, have more stable water temperature, and require heating to 90 degrees instead of say the 105 degrees required for domestic use).

We're wondering if there may be any issues with making this change?  Any feedback would be much appreciated.

Badger Boiler Electro boiler solution
Badger Boiler Electro  
ANSWER: Electric tankless water heaters should not be used for space heating.

We use storage/tank type(TTWH)gas-fired water heaters every day in our design and installation business. When electricity is found to be a better source of fuel--as in my own office here--we look to a purpose-built electric boiler e.g. Argo, Electro or Thermolec , preferably with outdoor reset and manufactured to handle the long sustained operating hours required of most space heating boilers. You will generally find the warranty on an electric boiler is much longer than that of a domestic hot water heater (DHW), more especially if the DWH is misapplied to space heating duty.

It is not unusual for the peak load of DHW to overwhelm a tankless water heater and where a TTWH is used in a combi-radiant floor/DHW system some special control is usually a must.

In your particular case, after performing a proper ACCA Manual'J' heat load analysis, I am quite sure I would specify and "physically" small electric boiler, which would hand on the wall next to or over a super-insulated Marathon electric water heater--also sized properly to your DHW loads.

This is what I did in my own office and have done in many parts of the country where electricity is a bargain...rare, but true.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your feedback.  That was very helpful.  I was hoping not to get rid of the electric tankless on demand heater right away (given cost concerns) but your recommendation makes sense.  One last question, can you tell me what the basic difference is between the electric boiler that you'd recommend for the hydronic heating and the tankless heater that I have now?  Would this electronic boiler utilize steam?  How is it different or better than the tankless heater?  As you can imagine, I'm a little suspicious of anything that says it can heat water quickly in an area the size of a suitcase which this boiler would also be doing.  Knowing how they differ might provide me with more confidence that this solution will allow me not to worry when winter sets in and the temperatures drop.

Badger Plumbing Marathon Water Heater MN
Badger Plumbing Marath  
An electric boiler can be similar in many ways to and electric tankless water heater. Both have electric resistance heating elements and various safety features. But an electric tankless or “on demand” water heater restricts the flow of potable water in order to transfer heat at a certain rate determined by the user setting, say 120°F, and ultimately by the output of the particular tankless water heater you select.
An electric boiler by contrast circulates water without restriction and naturally uses the recirculated water or other heat transfer fluid, much like the anti-freeze in the radiator of your car. My own electric boiler modulates supply water temperature according to the outdoor temperature and automatically sends the perfect temperature water to radiant floors and wall-hung panel radiators. This feature makes the floors run more and the cost of operation run less.
We rarely specify a tankless electric water heater, since domestic hot water peak loads can be high requiring large elements and again, restricting flow to the faucets. We prefer to use a super-insulated storage/tank type water heater with a much more modest electric heating element. So in my own case a Marathon 105 gallon super-insulated water heater (tapped for solar and wood backup) will fill any residential tub in a matter of minutes. This is important if your power is interrupted for rate savings or weather.
If you send plans, we can help you choose the right size electric boiler and storage water heater for a perfect fit.  

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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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