Quote:
Originally Posted by wifi
BTU = heat required to raise one lb of water 1 degree F
Water weights about 8 lbs/gallon.
Lets guestimate we start at 50 degrees and want to get to 120 degrees. This is a 120-50=70 degree rise times 8 lbs/gallon = 560 BTUs to raise one gallon of water from 50 to 120.
So, 3 gals/min = 1680 BTUs/minute times 60 minutes = 100,800 BTUs per hour. Efficiency is ~50%?
Hmm... where am I wrong ?
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Its 8.34BTUs for 1 gallon of water, but that is only skewing a little.
The 200K number I believe is arbitrary. Linked is one of the smaller units Rinnai makes and we use this for discussion
http://www.rinnai.us/documentation/d.../V53e_SP-1.pdf It has a rating of .82 so, 82% efficient with a max output of 120K BTU with max flow of 5.3GPM. If you look at the graph, for 3GPM requires 98400K BTU (actual output being transfered into the water) to achieve approx. 60 degree rise in temp with 21600K BTU being lost. Obviously, this unit would not be sized properly, but it works for the point.
The 200K BTU model would not be using the full max output to achieve that rise in temp it would only be using 121K BTUs with a loss of 21K BTUs.
These units use a ambiant temperature sensor to determine output BTUs needed based on call demand.