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When is a Therm of Natural Gas not 100K BTU??? - Page 4

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Posted by Bill Kreamer on October 25, 2005, 8:23 pm
 
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.

-Bill



Posted by Rob Dekker on October 17, 2005, 6:27 am
 
Hi Steve,

If your water usage is around 119 gallons/day (did I read that right?)  and
you
believe that 2/3 goes to water heating, then I think that the 30 therm/month
gas usage could pretty much be expected.

I'm using about 18 therms/month in summer, and 110 gallon/day water for
our home. I have an electric dryer, so all summer gas usage goes to the
water heater. So that is in the same ballpark.

Now, you mentioned that you just 'recently' installed the DHW system,
and it seems to be performing better than expected. That is good !
How did you conclude that it provides more BTUs than the old gas way ?
Could it be that you just got more sunshine than expected ?

Rob



Posted by Steve Shantz on October 18, 2005, 3:43 am
 Thanks for the replies Rob and Daestrom.

I live in North Central Indiana.  No risk of too much sun here!  One of
my objectives in building my system was to show that a well designed
system can be effective even in the cloudy bands around the Great
Lakes.  I don't expect any performance between mid December and early
to mid February due to almost perpetual cloud cover.

Additonally, one design flaw in my system is the PV panel for the large
(20 W) El Sid pump that drives the Glycol loop.  I only bought a 30 W
panel, and it doesn't provide enough current to run the pump with light
to moderate cloud cover, and doesn't start early enough or run late
enough.  Easy enough to fix with a few more $$.  I'm tempted to add 10
or 15 W more.  The other pump is a 3 W El Sid for the water side of the
heat exchanger, and that has an 8 W panel that makes the pump start
almost as soon as the sun hits the panel first thing in the morning.

Water pipes are insulated in both inlet and outlet, so no waste there.

I think I'm coming to several conclusions from all of this
discussion...

1.  Maybe I do use a bit more water than I expected.  I'm tempted to
buy a meter and put it up front of the heaters and see.  In any case,
if this is true, it does speak well for my design.

2.  Pilots may cost us more than we think.   This is another reason to
change to a pilotless instant heater.  I have a 21 year old gas fired
hot water furnace that might also benefit from an upgrade.  Does anyone
have recommendations of heaters that can finish off a solar preheater,
and heat a house?  Anyone with some extra cash???   :)

3.  Solar DHW systems deliver useful BTUs, while with gas heaters,
metered consumption must at best be derated by the efficiency of the
heater, assuming that storage and distribution losses remain the same.
This is an obvious issue that I initially had not considered.

More on pilots... I have a confession here... Last night I realized
that I left the furnace gas pilot on all summer.  That combined with
the water heater pilot may cost more than I realize.  The pilot in the
water heater is quite large, and it may be made that way for two
reasons... to keep the thermal-couple going, and to keep a tank up to
temperature without needless cycling of the main valve.  If one belives
in the virtues of gas heat, it seems like a very clever design.

I'm tempted to build a calorimeter and get an old gas water heater
burner and see if I can get some real data on pilot heat output.  I
think gas meters might be a bit pricy to buy, but if I can get my wife
to not use any gas for several days, maybe I can get a better sense on
pilot consumption.

Daestrom, I like your approach to compensation for variations from year
to year in weather when calculating effectiveness of projects.  Last
fall, I put in a staple-up radiant system under my Kitchen, Living
Room, Dining Room.  Wonderful heat!!!!!!!  And of course, I could lower
the temperature of the water in the gas furnace and pick up an
efficiency gain too.  My gas bill of course was lower... but then,  was
that due to my project, or a mild winter?  I'll have to compensate for
degree days.  Where do you find that data?  I've got 7 years of gas
consumption data, so I have a good baseline.

This fall I'm expanding to put the radiant heat in the family room
(where there is a crawl space under for easy access).  We are trying
out some Warm Window quilts too.  My long term objective is to reduce
head demands enough that I can consider a solar cistern project to heat
the house.

Finally... the main reason we have these filthy things called carpets
is because hot air heating naturally leaves us with cold floors!

Enough ranting for now.  Thank you all for your help!

Steve


Posted by daestrom on October 18, 2005, 10:07 pm
 

Yes, I agree that 7 years is an excellent baseline.  As far as daily
temperatures go, I use this page.
http://www.ncdc.noaa.gov/cgi-bin/res40.pl

On line version is free.  If you have a high speed connection, you can
download several year's worth of data pretty quick.

But it does take a bit of work after that.  First you 'unzip' the files and
'tar' to extract individual files.  You can find the 'station' closest to
you by looking up in the 'Station List Sorted By Country/State' (another
file to download).  The 'Data Description and Format' tell you all about
what's in the files.

I import each one, one at a time, into Excel as a comma-separated list.
Then I can easily cut/copy the columns of interest for the entire period and
paste it into a second spreadsheet.  Be sure to also get the dates copied
across as well.  Then you can line up dates with your monthly meter readings
and go from there.

At first I took my summertime gas consumption as a 'base load' and
subtracted it from my wintertime gas usage (in therms).  Then used an
estimated average household temperature (I have a daily set-back thermostat,
so house temperature isn't perfectly constant).  From that I subtract the
daily outdoor temperature from the weather data and then add up the
'degrees' for each 'day' in a monthly billing period.  Therms/'degree-day'
is then pretty simple calculation.  Then use Excel to plot an X-Y graph with
dates on the X axis, and Therms/dday on the Y axis.

Mind you, all this mixing/mashing of files and heating bills and such takes
a bit, so I only update my master spreadsheet a couple of times a year.  But
it only takes an hour or less to keep it up.  It took a bit more the first
time, to figure out what I wanted, and get the first year or two of data
entered.

As I mentioned before, it gets a bit 'ragged' in spring and fall when the
weather is warm enough to open windows some days, but other days we still
run the heat.  But the 'dead of winter' is a pretty nice line for four or
five months in a row.

Since my winter water supply temperature is different than summer, I went
back and 'tweaked' the calc a bit to vary the 'base load' number slightly
based on outdoor temps.  Helped to 'flatten' the Therms/dday a bit more.  I
then use Excel to 'Add a Trendline' (I just use 2nd order polynomial fit) to
the data and thus get a very nice curve/line.

So far, I've been able to keep the curve on a yearly downward trend by doing
one or more energy conservation projects each year.  This year's was to
remove the inadequate attic insulation, caulk and seal all the penetrations
up into the attic around all electrical/plumbing and other penetrations,
then put in new insulation.  I'm hopeful, but cautiously optimistic, between
the sealing of air leaks, and upgrading to more/better insulation.  Last
winter I was hovering right about 0.1 Therms/dday, so we'll see.

Of course, there have been a few periods that are 'way out there'.  Mostly
because of some unusual event.  On our 26th wedding anniversary two years
ago February, we had quite a crowd and the doors seemed to open/close all
night ;-).  So that month had a bit of a spike in Therms/dday.  But it was
worth it, we had a great time.

daestrom



Posted by Steve Shantz on October 19, 2005, 5:02 am
 Deastrom,

Thanks for the method.  I had previously downloaded monthly heating
dday data from NOAA and used that to calculate the therms/dday for each
heating month.  What I find is that the colder it gets (more ddays in a
month), the higher my gas consumption per dday.  This sort of makes
sense, as a house has background heat sources from all of the
electricity and the bit of passive solar we get.  This background
becomes a small fraction of the heat required to heat the house when it
gets really cold, thus driving up the therms / dday.

The data is still too noisy to see if the radiant heat really made a
difference.  There is a pattern where winters with lots of cold months
with high ddays tend to drive up the average seasonal cost / therm.
Last winter had no really super cold months, so that in itself would
tend to help lower my therms/dday, not neccessarily my radiant heat.
I'll need more time to really see a measurable improvement.

This is going to be an interesting way to measure things!

As an aside, tonight after work I looked at my gas meter really close.
The finest resolution is 1/4 cu ft/ rev, and that took almost exactly 6
minutes for one rev with only pilots running.  So one hour is 2.5 cu
ft.  One day is 60 cu ft, or 0.60 ccf. About $.00 worth of gas at
current prices... just to keep the darn pilots going for the water
heater and furnace.  No wonder my water heater was still hot after 5
days on pilot only.  More reasons for an upgrade!

Best regards,
Steve


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