Instantaneous W/H driving radiant floor system?

Rob10667

Would appreciate any/all input on experiences using instantaneous water heaters (Rannai, etc) for radiant floor applications.  I have two jobs that I've used these on, one O.K. And one an awful mess. Both jobs used the units for dual purpose (domestic and comfort heating).  All problems have stemmed from breakdown or malfunction within the heaters themselves.

Thanks for the input.

Rob

Ironman

Very Bad Idea

I know, the Internet is riddled with peddlers who pass themselves off as experts selling these systems, but it's a fundamentally- flawed design.



First, a water heater is just that: a water heater. It's not a boiler and is not approved for space heating. It does not have an ASME "H" stamp which is required by code for a hydronic space heating appliance.



An on-demand water heater is not designed, constructed or controlled for space heating. That's why your warranty will go down to three years or less if it's used in that manner.



Second, you're certainly going to have comfort issues depending on how the system is piped and controlled and what type of emitters it has. The heat exchanger in the on-demand has a high head (flow resistance) and is designed for high Delta T (70*). A properly designed hydronic system has a relatively low head and low Delta T (10 - 30*).



Third, if your hydronic loop(s) is connected directly to the water heater and is not isolated through a heat exchanger, then you've created a severe and potentially deadly health hazard: legionnella. An open loop that stagnates, even for a few hours, is breeding ground for the legionnella bacteria. Not to mention the other issues that will arise with time in an open loop system.



This type of system comes up in threads around here at least a couple of times a week and it's almost always after it's been done and the O.P has suffered the consequences and is looking for relief. There a numerous threads on it here which you can find through searching. Here is one:



http://www.heatinghelp.com/forum-thread/139955/Noisy-tankless-heater#p1255267



Here's more: http://www.heatinghelp.com/forum-thread/139962/Bad-Radiant#p1255263



http://www.heatinghelp.com/forum-thread/139639/Want-a-laugh#p1253928

kcopp

Ditto...

Wrong tool for the application....  You are better off w/ a combi boiler. Embassy Axia, Smith gc160, Triangle tube excellence, Biasi Riva and the the like. In the long run the gas usage will be down and the headaches less.

Jack

My experience with them

Mimics yours! Some systems would hold together, BUT, I don't think I ever saw one operate efficiently. Some had huge gas bills. Rinnai has not allowed the use of their equipment on closed loop systems for several years now. Takagi seems to be the leader in the water heater as boiler applications today, at least from what I hear of.

The thingis, the tankless are designed to see a high delta T and variable flows. Boiler apps will have fixed flows and low delta T's. As a result you will have 16 burners in the rack and the unit will operate on 3 of them. Not even a little bit good! Buy real boilers for real boiler applications.