[Greenbuilding] Radiant heat boiler

[David Seth Melchert]

We have started a large residential remodel in Orinda, CA. The 2,800 sf 
house will be heated with radiant floor heat. We were planning to use a 
Baxi Luna 330 instant heater for combined radiant and domestic hot 
water. I really love the Baxi Luna for its compact design and simplicity 
of installation, not to mention its general high efficiency.

However we are concerned about its capacity to serve the demands of a 3 
bath house. The homeowner consulted with a supplier who suggested that 5 
gpm, delta 70 degrees is the minimum required, more than the 144,000 
BTU's the Baxi Luna provides. Baxi has a new Modula supplemental tank 
that holds 21 gallons and serves as a kind of buffer for temporary high 
demands. My homeowner is concerned this will not cut it. They are edging 
towards having separate heat sources, such as a Noritz or an efficient 
large tank for DHW.

Here are my questions: how many gpm should we figure for hot water needs 
at any one time, and how often might a family of four run into 
insufficient hot water using the Baxi? Would you agree that providing 
separate heat sources for radiant and DHW would be the best path?

Thanks

Seth Melchert
Oakland CA