I'm not seeing a direct answer to a question I've had, which is:

If I've got a standby tank that is heated using geothermal (which will max at 130 degrees f, but works more efficiently below 120 degrees f, can I use an an electric tankless to boost the temp from about 110 to 120?

The tankless manufacturers always quote the rise in temp that a unit will supply (ie x gpm at input temp of y and output temp of z), but I'm interested in knowing if the input temperature of 110 would cause a problem for a tankless electric system? 

I've seen a max input temp listed on the Stiebel Eltron heaters of 107f, but I've not seen figures for other brands?  Would I need to be careful not to oversize the TWH if I'm only interested in a 10 degree jump at 3 - 4 gpm?

I'm interested to hear of any first hand experience in using a tankless system this way.

Thanks

I did that in our last house and it worked OK. The big problem we had was as flow changed the temp changed. Some of the newer ones are better at regulating temp, but they typically come on in 5kw or so increments. As you noted some won’t kick on if the water temp is above a certain temp. Ours was about 110F and that was all the way hot in the shower so it worked ok. If I had to do it again I would get a small 5, 10 or 20 gallon electric depending on usage and put that after the geo tank. It won't care if the incoming is 130F or 55F and be pretty close to the same efficiency if it's small and insulated well.