I have a radiant floor heat system up and running using a tankless water heater instead of a boiler.
Main reason is that I think they are one and the same thing, at least the German ones...

Boiler: Stiebel Eltron Hydro Shark SH3-10 9.6 kW - 32,764
Tankless Water heater: Stiebel Eltron DHC-E 8/10 7.2/9.6 kW

I couldn't find any difference between both except for the price. I got the DHC-E 8/10 for $250 while the SH3-10 is running at ~$600. What I looked out for was the capability of the DHC-E to keep constant temperature at the output, so a high inlet temp does not present a problem. Flow rates are also no issue, it still works even if I raise the primary pump speed.

Am I missing something? Or have I really bought a 32k electric boiler for $250?
So far it's working just fine but I want to know if I need to watch out for specific issues down the road.

Thanks,
Tim

I understand where you are coming from. Does saving money always save money in the long run? Most likely not. Honestly, I never liked the Stiebel Eltron brand. Obviously, if it works for you, and you're fine with it, keep using it. In the long run, the unit will probably burn out. The unit's manual specifically states it is only intended for domestic hot water. If we were to compare them side by side, what could be the differences? Possibly thicker materials, better welds which can stand up to the constant high temperatures. That's the problem ive personally seen first hand with using Tankless Water heaters in radiant systems. They fail much sooner. They were designed for heating a hand wash, a shower, a bubble bath. Not for constantly heating water throughout an entire house. If you intend to keep the Tankless Water heater installed, be sure to do your scheduled checks; check for leaks, be sure the pressure valves work. Check for warping, discoloration in the metal, cracking in the housing, etc. Most importantly, if the unit was not intended for radiant heat, I wonder if it would have any negative consequences on any seals or gaskets from exposer to longer burn times? Sure, you can put a bicycle tire on a motorcycle, and you may even be able to ride several feet, but ultimately, that bicycle tire will fail due to improper usage. Be sure the unit is properly grounded. I suppose my question is, was this a professional install or a DIY? I completely understand wanting to save a few dollars; but those expensive boilers which range from $600 to $5,000+ cost that much for a reason. If you find yourself having to keep the unit set to max temperature in order to meet your heating demands, you may want to investigate the purchase of an actual boiler. Its important to remember that the warranty is void seeing you are using the device for something other than its intended use.

I agree - boilers are built for more hours. And higher flow rates at reasonable pressures.

It's really the flow limitations when using tankless electrics for space heating more than anything.

Electric boilers are high flow, low to moderate delta-T devices.

Tankless water heaters are low flow, high delta-T devices.

That's not to say you can never get there with a water heater, but you have to respect it's limitations. Running it anywhere near it's max flow (even at a low delta-T) is going to wear the thing out prematurely.