The plan is to use a stiebel eltron electric tankless water heater for our radiant floor. The heater will be heating about 1600 sq. ft of concrete in a house with a modeled heating requirement of 5573 kWh/year. Any experience out there on the durability of these water heaters and suitability for use with radiant floor heating?

To size the system, you really need to know the heat gain requirement per hour...which is normally the building heat loss per hour for the proper climatic design condition for your location...or perhaps even significantly more if you will be using night time temp setback...but setback is NOT typically done for a high mass hydronic floor assembly...but more and more people seem to be using a light mass hydronic floor assembly such as Warmboard, so this might be a consideration...

5,573 KWH/year is about 19,015,856 BTUs/year and normalizing to days/hours (which is never a proper design approach because it ignores the climatic design condition and the fact that not all heating days are equal) is about 52,098 BTUs/day or about 2,170 BTUs/hour. So I think you will need better data to design with than this...

Tankless water heaters are becoming increasingly popular for small residential hydronic radiant floor heating systems. Assuming the heating system is properly designed and installed, tankless water heater durability and reliability likely depends more on the brand selected. We have not used the brand you are considering, however, I am sure you will get plenty or guidance and recommendations from other forum members with regard to this. I do think you would be better off with natural gas rather than electric (unless you have free hydroelectric at your site), and I fully trust that this will be a closed loop system and not an open loop system.

We have DIY hydronic radiant floor heating design software on our website if you want to get educated on this and run some numbers.

Exactly... Average heating usage is really not the way to engineer this, you need peak usage from the worst case scenario column.

What is the BTU per hour output of that tankless unit? If it is say 10,000 BTU/HR, but your house sheds 20,000 BTU per hour on a dark and stormy night, then it may not work out all that well for you... To get an accurate idea of what you need, you must first know how much you use under the worst case atmospheric conditions.

As a general rule for electric heaters 1 KW/hr = 3412 BTU/hr output. For gas, it is fuel burnt in BTU/hr x unit efficiency. The free calculators on Sailawayrb's website will give you an idea of loss, and requirements in terms of flow, temperature and BTU/hr thru your hydronic radiator(floor) to replace those losses to atmosphere in your home. You will then be able to answer some key questions. Read the instructions first, they explain very well exactly what you need to enter in the fields to get a usefull output. IF you havn't done this before, it will be a learning experience, but you will have a far better understanding of the problems when you are finished.

Will the planned unit output the BTU/HR required?
Is the planned unit capable of delivering the flow at temp required?
Will the flow/temp/duration demands exceed the duty cycle of the planned unit?
Will the planned unit be able to deal with low demand situations adequatly? IE: how will it react/survive short cycling during low heating demand?
And of course lastly, will it be cost effective to purchase and maintain? If after calculating, you only need say 12KBTU/hr peak, is it more cost effective to use a more expensive, more complex and prone to failure unit over a traditional domestic hot water heater? The domestic heater should last a decade or more in a closed loop system(less corrosion and mineralization) and which has built in redundancy with multiple elements.
Are replacement parts for the planned unit locally available like heating elements are for a traditional domestic water heater are at your local hardware store? Remember, you are without heat when this thing quits untill you get it fixed or replaced...

Definitely a place where you must measure twice(or more) before you cut once...

Good Luck

If you are heating with electricity, why not a ductless mini-split at 2-3 times the efficency of electric resistance heating?

Posted By rpatterman on 15 Apr 2013 01:43 PM
If you are heating with electricity, why not a ductless mini-split at 2-3 times the efficency of electric resistance heating?

And how does the heat get into the radiant floor?  I have had this same suggestion thrown at me a few times in my house planning. Don't get me wrong, the mini split ductless systems are great if heating the air is all you are after.  But if you want the comfort of a radiant floor, I have not found too many manufacturers offering hot water production options to use them as your radiant water heat source.

One of the cool things about the radiant floor with water as a medium, is that it easilly opens the door for alternate means of heat collection such as a wood fired boiler or solar, and storage such as a large mass insulated water tank.  Add heat source, pump and perhaps a heat exchanger and you are in buisness with very little energy required to collect the heat.

I guess another option for providing heat would be a heat recovery type hot water heater.  The small A/C unit used by these hot water tanks probably won't meet your peak heating demand and the traditional element backup heaters would run, but it may meet a large portion of the average demand at increased efficiency. I havn't really looked into their specs yet though...

Someday some manufacturer will take the compressor from a cold climate ductless unit, add a heat exchanger and sell it as a low cost air to water heat pump. Same idea as Daikin Altherma, just more competitive.