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When is a Therm of Natural Gas not 100K BTU???

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Posted by Steve Shantz on October 14, 2005, 4:01 am
I recently completed my domestic solar water heater, and it is working
wonderful.  Too good to be true, to be honest.  I have a 120 gallon
tank, and 4 x 55 gallon barrels with oversized home-made heat
exchangers.  Performance is exceptional, with my glycol only running
about 15 degrees F hotter than the temperature at the top of the 120
gallon tank.

What I am observing is that the BTUs that my system collects seem to be
very different than the BTUs that the gas company sells me.  It seems
that when my gas utility sells me a therm of gas, I sure don't get
100,000 useful BTU's from it.  Not even close.  Lets do the math...

My average summer usage is about 30 therms per month, before I
installed my system.  This is based on 7 years of trend data of our
consumption.  We have a gas water heater, a gas stove, and a gas drier.
 The stove and the drier don't get used that much in the summer.  Lets
assume that 2/3 of the gas is to heat the water.  (Based on our usage,
I think this is a conservative assumption.)

2/3 x 30 Therms = 20 Therms
20 Therms x 100,000 BTU/ Therm =2 Million BTUs
2 Million BTUs / 65 Degree F Temperature rise / 8 lb per gallon = 3846
gallons of hot water heated from 55 degrees to 120 degrees F per month.
3846 gallons / 30 days = 128 gallons per day

OK, now there is no way we use anywhere close to 119 gallons per day,
just my wife and I.  Even if one assumes 60% efficiency for the water
heater, this still is 77 gallons per day.  Additionally, when one draws
small to moderate amounts of hot water, the water comes into the gas
heater at 70 degress, not 55.  That makes the discrepancy even worse

So the question is... what is wrong with my calculations?  Does the
pilot use that much gas?  Regarding the pilot in the gas water heater,
I was suprised to learn how much heat is given off.  Two Sundays ago,
my solar tank temperature was 120 degrees, so I bypassed the gas heater
and turned the gas valve to pilot.  No sun all week and by Friday, the
temperature of the water coming out of my solar tank was down to 95,
and my wife was complaining about the icicles coming out of the shower,
so I turned on the water heater and switched the valves back to normal
mode.  I was suprised to find that the water in the gas heater was
still hot, with only the pilot running for 5 days!

I have heard that a gas heater pilot uses about 500 BTU / hr.  Is this
gas utility BTUs, or useful energy BTUs?  Maybe leaving the pilot on is
a bad thing, especially when the sun can meet all of my needs.  Makes
me wonder if there is a pilot in the drier too.  Just checked...  It
has an electric glow plug that comes on to light the flame, and then
turns off once the flame is lit.  Seems like a good design.

What I seem to be concluding is that if one uses 30 Therms ( = 3
Million 'gas company' BTUs) of gas to heat your hot water per month,
then one may only need a solar pre-heater to deliver about  500,000 -
700,000 BTU's per month to do the equivalent amount of heating.  This
may be one of the main reasons solar water heaters are such a good
deal.. they deliver useful BTUs.

Specifically, it seems that a 'solar therm' is much larger and more
useful than my gas company's therm, by a factor of about 4 to 6.

Any help or ideas here would be much appreciated.

Posted by Solar Flare on October 14, 2005, 4:06 am
More modern, more efficient water heaters do not use a pilot flame but rather
spark or heating element ignition.

Posted by Ecnerwal on October 14, 2005, 4:29 am

To begin with, you've assumed that the insulation on the system is
"perfect". Merely keeping stored water hot takes energy. Also, a rather
small leak  (if you have any) in any hot water appliance (or hidden run
of piping) will slurp a good deal over the course of a month. Hot water
pipes add considerably to the area available for heat to leave the hot
water heater. Thermosyponing in the pipes can add greatly to that.

How old is your hot water heater? Gas appliances tend to last a long
time - in terms of greater efficiency, sometimes too long.

Cats, coffee, chocolate...vices to live by

Posted by Christian Kaiser on October 14, 2005, 1:36 pm

Did you mean "reduce" instead of "add to"?


Posted by Ecnerwal on October 14, 2005, 1:51 pm

No. Given stored hot water and a set of pipes, thermosyponing in the
pipes can add greatly to the heat loss from the storage - water in the
pipe cools, sinks back into the tank, and is replaced by hot water from
the storage tank. More modern tank setups often include either a check
valve or a loop/dip in the pipe to try and stop or reduce this.

Thermosyponing does not imply a solar collector in this case. It's
simply the tendency of warm water to rise and cool water to sink, which
results in heat moving upwards. While it can be used in a collector loop
from solar panels or fuel stoves if they are located below the storage
tank, in this case I'm discussing it as an effect which increases the
cooling rate (or heat loss, or required heat input to maintain an
elevated temperature) of a stored tank of hot water to the general

Cats, coffee, chocolate...vices to live by

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