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When is a Therm of Natural Gas not 100K BTU??? - Page 3

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Posted by daestrom on October 18, 2005, 1:09 am

Well, that answers that question. :-)

So your wife uses about 7 minutes * 1 1/16 water per day in the shower for
about 7.5 gal/day, while you manage about 4 gal/day.  But six loads of
laundry a week is about 120 gallons on 'warm'.  You might seriously consider
using 'cold' setting.

I don't know how much water a typical dishwasher uses, but at only 3x a
week, it can't be more than 50-60 gallons /week.

I agree, it doesn't sound like you use an inordinate amount of HW (now that
I know more about your actual usage).

Well, as I mentioned in one of my other posts, it is not unusual for a
natural draft gas HW heater to only get about 65% efficiency.  So 1 Therm
paid at the meter turns into only 0.65 therms of actually heated water.
Combine that with standby losses (but you mentioned you have an insulating
blanket I think), it can add up.

I'd check both the inlet and outlet of the HW heater when it has been idle
for several hours.  If either line is warm, you need to insulate the
vertical run.  I found, to my chagrine, that the 'cold' line leading into my
heater was just as much a 'radiator' as the warm line.  So I insulated both
(couldn't hoit ;-)

This discusses some testing of solar panels....

They include the statement, "think of solar panel ratings like the MPG
ratings of automobiles".  Yes, your exact performance will differ, the
ratings are best used in comparing between panels.

Even so, it looks like 3 days of sun would give you 64000 BTU, and two days
of 'mildly cloudy' another 44000, and finally two days of 'cloudy day'
another 24000 BTU (see AE-32 model on page 33).  So that's about 1.28 therms
per week, for 5.12 therms/month.  But in summer, isn't your weather better
than that??  How will it do in winter?

Agreed.  I work in simulations quite a bit and find that the best way to
improve a model is to compare with actual data and keep adjusting
'assumptions/simplifications' until the estimates match the data.  But it is
important to adjust the right parameters so that performance is more
accurately modeled.

And yes, one must be wary of salesmen claims and hype regardless.  As
always, "If it sounds too good to be true, it probably is."

I'd be interested if you monitor gas meter for a couple of days that the HW
heater doesn't run.  That would tell you all the 'other' loads such as gas
dryer, stove/oven, and pilot light.

It turns out that clothes dryer's use typically 1700 BTU per pound of water
removed from the clothing.  So depending on how wet the clothing is, 5-6
loads of laundry a week could use quite a bit of NG.  If a load has a total
of 10 lbm of water after spin-dry in the washer, that's 17000 BTU per load.
I have no idea how much water is actually in a typical dryer load, but
clothes always seem 'heavy' to me if I get stuck with the laundry ;-)

Dryer's made after '88 don't have pilots.


Posted by Bill Kreamer on October 19, 2005, 5:27 pm
I seem to remember that the rationale behind the adoption of the Therm as a
unit of convenience was to permit the comparing of gallons of home heating
oil to a similar quantity natural gas (140,000 Btu/gal of oil, burned @ 71%
eff., typical of older furnaces, = 100,000 Btu). Is that also your
If so, a "standard oil furnace efficiency" of 71% was assumed. Isn't an
implied "standard natural gas furnace efficiency" used to define the Therm?
Or is 100,000 Btu the value @ 100% eff.? Can you guide me to a reliable
source? - Bill



Posted by daestrom on October 19, 2005, 11:10 pm

Hadn't heard that one.  From what I understand, natural gas (and before that
'town gas') was sold by volume at one time.  The positive displacement gas
meter would measure the number of cubic feet of gas.  But there are some
difficulties with using the volume measurement of a gas.  Namely, gas is
denser in colder weather, and although the pressure is regulated, the
regulator uses atmospheric pressure as a reference.  And as we all know
atmospheric pressure varies with the weather.  So the volume measurements
aren't as precise as we (or the gas company) would hope.

But, not all 'natural gas' is created equal.  One cubic foot of pure methane
at STP will liberate 1027.55 BTU.  So, picking a 'round unit of volume' as
100 ft^3, that should equate to 102755 BTU.  But natural gas has a small
amount of other gasses in it, such as CO2 that do not add to the energy per
ft^3.  The gas company (at least in the northeast US) is required to sample
their gas periodically and track the actual energy content.  They then apply
a 'conversion factor' to convert the CCF of 'gas' to 'Therms'.  This month,
my 'conversion factor' on my bill is 1.00431.  So the gas coming into my
house last month had 1004 BTU per ft^3.

So the 'therm' they use is the number of btu in the gas as it enters your
house.  What your appliances do with it efficiency wise, is your business,
the gas company doesn't care.

I don't think it's just a coincidence that the 'therm' happens to work out
to the energy contained in 100 ft^3.  I think that's by design when gas
companies switched (forced?) from charging just for the volume of gas, to
the energy content in the gas.  I think it was 'forced' because some
unscrupulous companies could dilute their product and still charge the same
amount until the regulatory agencies got involved.

Interestingly, the 'Therm' is based on the lower heating value of the gas,
not the 'higher heating value'.  That is, one ft^3 of methane liberates
1027.55 BTU in burning to form CO2 and water vapor (i.e. steam).  But that
steam has the energy of vaporization in it.  If you condense it, you gain
some more energy and the 'higher heating value' of methane is based on this.
The HHV is 1140.29 BTU per ft^3.  In the old days, this was not significant
since it wasn't recoverable.  But as you probably know, modern furnaces can
condense some of the water vapor formed and thus recover some of that
energy, hence the much higher efficiency of such furnaces.


Posted by Steve Shantz on October 22, 2005, 2:54 am
 Thanks for the info.  This is exactly what I was looking for.
By the way, I have confirmed that the two pilots in my system (furnace
and hot water heater) really do use 0.6 ccf of gas per day.  No wonder
my 40 gal water heater stayed hot 5 days after I switched from ON to
Pilot.  I didn't save a bloomin nickel!   I am somewhat shocked.  So
the moral of the story here is...If you have a standard gas water
heater, and use only modest amounts of water, the pilot itself is doing
most of the heating.  The problem here is that the pilot is heating
whether you need it or not.  If a solar pre-heater is installed that
gets the water almost, but not quite hot enough, you still need your
gas water heater to finish it off, and with a standing pilot, the net
financial and greenhouse gas savings is sadly... essentially zero,
unless you use large amounts of hot water just occasionally for showers
and laundry.  This really sucks, big time.

But alas, good information leads to improvements.  I've started my
search for a good instant heater that can do DHW finishing and heat the
water for my radiant / baseboard system.


Posted by Bill Kreamer on October 22, 2005, 7:31 am
 Thanks Daestrom,

Reading between the lines of your informative response (as always - thank
you!), in calculating Btus delivered by burning a Therm of natural gas, one
always must factor in the device efficiency.

A potential confusion may exist stemming from the occasional use of the
phrase "burned at STP" in describing the potential energy of 100 cubic feet
of (conversion-factor-adjusted) NG. Some might assume that this rhetorical
device refers to the burning that their natural gas device performs, and
expect 100,000 Btus to be applied to the load.


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