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re: seeking to quantify home heating systems in the usa
20 jan 2001
john becich  wrote:

>is a water heater rated in terms of btu/hr?

yes, for gas. electrical versions are rated in watts (or sometimes
watts per hour, by solar power consultants with upside-down brains.)

>what's typical for a 50 gallon tank?  50,000 btu/hr??

grainger lists a $200 "energy efficient gas water heater" with a
40k btu/h input (simply called "40k btu" by some hvac criminals.)
it says "40.4 recovery at 90 f rise, doe test method in gallons
per hour." this probably means the heat output is about 40.4x8x90
= 29.1k btu/h, for a 73% water heating efficiency. of course, it
also loses heat over time, even with no hot water usage.
>is a space heater rated in terms of btu/hr?


>what's typical for a forced air heater?  50,000 btu/hr?

maybe more like 100k btu/h for a house furnace.

>what kind of efficiency factor might i expect to realize with a solar water
>heating panel?  you can neglect losses in the piping that connects heating
>panels to tanks, etc.

they often come with a curve of efficiency vs (tc-ta)/i, ie the water-air 
temp difference divided by insolation. aaa solar in albuquerque sells used
250 pound 4'x10' panels for about $300 each plus crating and shipping ($500
for 5 to phila) with (tc-ta)/i = 0.4 with tc = 130 f and tw = 30 f and i
= 250 btu/h-ft^2, ie they are 40% efficient in full sun (am2) on a clear
30 f day. this drops to 15% if i = 167 btu/h-ft^2 over an average cloudy 
january day with 1000 btu over about 6 hours. where i live, nrel says 729
btu/h-ft^2 of beam sun falls on an average january day, so a 4'x10' aaa
panel that drains down between clouds can collect at least 0.4x4'x10'x729
= 11.7k btu/day on average. drain-down systems can be more efficient than
those that leave water in the panels all night...
>how can i use an r factor to calculate losses in pipes, water heater closet,
>attic insulation, windows, etc.?  what are the units of "r"?

us r-values are in ft^2-f-h/btu. the rate of heatflow through a surface with
r-value r and area a and a dt (f) temperature difference from one side to
the other is dtxa/r. for example, an 8'x20' r20 wall with 70 f air on one
side and 30 f air on the other lets (70f-30f)8'x20'/r20 = 320 btu/h flow
through. a 4'x6' r2 window lets (70-30)4'x6'/r2 = 480 through. 


it's a snap to save energy in this country. as soon as more people become
involved in the basic math of heat transfer and get a gut-level, as well as
intellectual, grasp on how a house works, solution after solution will appear.

                                          tom smith 

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