
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 upsidedown 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?
sure.
>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 (tcta)/i, ie the waterair
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 (tcta)/i = 0.4 with tc = 130 f and tw = 30 f and i
= 250 btu/hft^2, ie they are 40% efficient in full sun (am2) on a clear
30 f day. this drops to 15% if i = 167 btu/hft^2 over an average cloudy
january day with 1000 btu over about 6 hours. where i live, nrel says 729
btu/hft^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. draindown 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 rvalues are in ft^2fh/btu. the rate of heatflow through a surface with
rvalue 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 (70f30f)8'x20'/r20 = 320 btu/h flow
through. a 4'x6' r2 window lets (7030)4'x6'/r2 = 480 through.
nick
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 gutlevel, as well as
intellectual, grasp on how a house works, solution after solution will appear.
tom smith

