Instant Water Heater to make preheated water hotter

[Komrade]

Is there a recommended instant water heater that won't burn up if we give it 100F+ temps on input when the primary system don't keep up?
Preferably propane, as we'd like the system to run without power on house batteries w/o having to start up the generator.
Grey box center-left, after a stratification tank powered by Sanden Sanco2.

 

[Komrade]

Takagi

On-demand Water Heaters | | DIY Radiant Floor Heating | Radiant Floor Company

Radiant Floor Company pioneered the use of on- demand, tank-less water heaters for radiant floor heating nearly 20 years ago when many in the heating industry believed that “on-demand water heaters would never work for radiant heat”...we disagreed! We've been doing this longer than anyone, with...

www.radiantcompany.com

from brochure it supports heating spaces.. although not exactly in way I was thinking..

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Takagi-T-H3J-DV-P-brochure.pdf

 

[hogback]

My Rheem seems to do fine when receiving hot water in the summer. Out inlet line is a 1.5" poly pipe that is exposed to the sun for 100' or so. I believe it gets to 100degF.

 

[RCinFLA]

Cheaper solution is to use just a normal hot water heater for post heating.

You need to modify the cold side input feed pipe into normal hot water heater to block off end and drill outlet holes on sides near bottom of feed pipe insert. This is to spread and shoot incoming water sideways to avoid incoming hot water from temp shocking enamel paint at cool bottom of tank cracking enamel paint resulting in tank rusting out in a year or two.

 

[Nobodybusiness]

Takagi

Definitely a good unit.

I had one of their NG units and it was absolutely bullet proof.

You had to descale it once a year for hard water but nothing else..