I see what looks like some sales pitches, which I am not interested in. I'd like to hear some first hand experience. Something like, "I installed XXX unit as a tankless hot water heater and as a radiant heat source and it works great. No issues, no call backs and doesn't wake the neighbours." Or "I don't think using one unit to do both is a good idea because..."
Having said that, it also appears there is a slant against tankless in general from some people. Don't understand that either. Tanks have their place, tankless too.
I'm not a specialist in this area for sure, so I'm not going to try and educate anyone. But the tankless heater/radiant heat source we have in our house in Europe lasted for many years, without issue and with no maintenance (20+ years). Had it been maintained it probably would still be working now but it would have failed the stricter emissions standards in Europe anyways so a replacement was required. The noise wasn't that bad neither. In fact it was quieter than my current PowerVent tank.
There are many that can help give you ideas and/or guidance here in the plumbing and HVAC sections. (Might want to post there also relating to your radiant system).
Do you recall the name of the product you used overseas?
Your info states that you are located in Florida. If that's correct, does radiant make sense there? See any others with it in your area?
Just thinking about some things that others might be able to help you with if they have these answers.
Actually this is for another house in Toronto so radiant heating is nice when it's below freezing for 4-5 months of the year. Even in FLA there are times when radiant would be nice but it's definitely only a couple of months when the heater is needed at all.
I need to get someone to check which unit I have in Europe. They just changed it last year. The previous one (the old battleaxe) was actually better I think. The current one has fancy controls and such but isn't as powerful.
Viessmann makes some excellent products.
Quietside has a questionable reputation, many were installed with combo units in the toronto area in their heyday and most are now being replaced prematurely due to various issues.