[Greenbuilding] [BULK] Radiant heat boiler

[Lawrence Lile]

Well, a shower head is about 3 GPM, you can figure your peak flow from
there.  For the most part, people don't use the shower and the sink at
the same time in one bathroom, and the shower generally can't be used at
the same time as the tub.  You can also figure peak flows from the
number of occupants - are there enough people in the house to take 3
showers at a time?  People may also use the washer and dishwasher at the
same time as a bath.  In my current house, they won't do this more than
once, since we have a wimpy tankless that can't keep up with more than
one load at a go.  Brrr. 

In my new house, a Tagaki jr, model is keeping up with two showers at
once.  


If these bathrooms are not grouped, or if the kitchen is not nearby,
you'll want two sources of hot water.  A typical tankless heater has
several seconds of delay (My tagaki waits 6 seconds before firing) plus
the long run, can mean minutes of wait to get hot water.  The buffer
tank sounds like a good idea actually - I was considering putting
something like that in a kitchen that has the long run problem.  Add
extra insulation to whatever the manufacturer thought was enough,
because the buffer tank defeats some of the savings associated with the
tankless.  

I can't imagine 3 tankless heaters, seems pretty excessive.  Do the math
and see what you come up with.  

 
 
Lawrence Lile, P.E., LEED AP

-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of David Seth
Melchert
Sent: Tuesday, February 06, 2007 6:04 PM
To: Greenbuilder list
Subject: [BULK] [Greenbuilding] Radiant heat boiler
Importance: Low

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
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