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Author Topic: Calculations for BTU Loss in OWF Water Line  (Read 22361 times)

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Offline Dean186

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Calculations for BTU Loss in OWF Water Line
« on: January 19, 2011, 10:41:16 PM »
I have created this post to help those who have yet to install their Outdoor Wood Furnace and are contemplating their installation.  The calculations were something I ran though before I installed my furnace and it helped me realize just how important it is to have a well insulated water line from house to furnace.

I will be sharing photos of my installation and thought it would be good for others to share information about their water line installation pros and cons.  Photos are always appreciated.

BTUs lost in OWF water line


Facts:

        A BTU is defined as amount of heat required to raise the temperature of one pound of liquid water by one degree from 60 to 61 degrees Fahrenheit at a constant
          pressure of one atmosphere
        There are 15,200,000 BTUs per cord (4x4x8) of Ponderosa Pine
        Central Boilers TACO 007 pump is estimated at 8 gallons per minute for a typical OWB application (verified with manufacture and it will vary for each installation)
        One gallon of water weighs 8.181271 lbs at 150 degrees  Fahrenheit  (8.34 lbs at room temperature)

Calculations:

        8 gallons per minute = 65.45 lbs per minute = 3,927 lbs per hour = 94,248 lbs per day
        At a total of 1 degree Fahrenheit loss, 1/2 degree each way, in temperature running through the water line and 94,248 lbs per day of water moved; 
          then one would incur:  94,248 BTUs per day of loss
        (BTUs in cord of wood / BTUs per day) (15,200,000 / 94,248 = 161 days) or 5 1/2 months (approximately a heating season) to lose the BTUs in one cord of pine.

Conclusion in round numbers:    

        There is a heat loss, per season, of one cord of pine wood for every 1 degree Fahrenheit drop in water temperature from furnace to house and back.

Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #1 on: January 19, 2011, 10:41:43 PM »
Of course measuring the loss that occurs from the OWB supply side to where the water enters the house is always difficult.  This is because few installations have accurate thermometers installed in the OWB supply line and also have one installed in a manifold at water inlet.  In my opinion, relying on the reading obtained from the furnace control panel and comparing it to a reading inside the house is not a very accurate way of determining heat loss in the water line.

Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #2 on: January 19, 2011, 10:44:10 PM »
I have also heard some say, Whats a 5 degree loss when the water is 180 degrees, its still 175 degrees when it reaches the house.  Well yes the house still gets heated, but if it is really a 5 degrees loss one way, then that is 10 degrees total loss, which equates to 10 cords of wood per season just to heat the earth.

I have also heard, Its only a few sticks of wood per day .  If the water line is losing just degree each way, which is 1 cord per year (see above) then this would equate to 3 sticks per day.

Example:  I did a quick count of the number of sticks of wood in one of my cords, which are cut between 18 and 20 inches in length, and came up with 560 sticks per cord.  So if you figure 161 days to a heating season like in the above example, then 3 sticks a day will equal a cord of wood.  I know, too much time on my hands; but interesting numbers none the less.

Offline Tom_Averwater

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #3 on: January 20, 2011, 06:15:47 PM »
I like your numbers . I never thought of it that way .
He who dies with the most toys wins .

Offline doctorb

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #4 on: January 20, 2011, 06:44:33 PM »
Well done Dean.  Fuel for thought!
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #5 on: January 20, 2011, 11:46:07 PM »
My Thermopex Installation:  Part 1

In the early stages of research for this project I spent a considerable amount of time looking for the best method to transfer heat via water pipe and found, IMO, that Central Boiler offered the best product with their Thermopex dual water line.  This product is made to be installed underground and design to be a standalone product.  However, with 185 degree water circulating continuously through the pipe, there will be some BTUs lost even with the best pipe.

To minimize the BTU loss even more, I decided to surround the Thermopex with 6 inch schedule 40 pipe and insulate the PVC with blue board like it was the main pipe.  The Thermopex would basically be installed in an air plenum. The concern was, can the Thermopex be pulled though a 58 foot length of 6 inch PVC pipe with one large sweep 90 degree elbow and a large sweep 56 degree elbow.  I could not find anyone on the forums that had pulled this pipe though more than just one elbow at the pad.  My Central Boiler dealer and I thought it would be possible.  However, I had a contractor tell me it would not be possible. 

Here are some photos of the PVC pipe in a 30 inch deep trench:



Above is the PVC pipe rounding the house as it goes under the deck.  This elbow was cut from a 90 degree elbow to make a 56 degree bend.



Above is the PVC after blue board was cut to fit and placed on top.



Above is the PVC entering the basement.  One more short piece of blue board and I can cover the pipe.



Above is the PVC coming up through the 5 1/2 inch concrete pad that the furnace will set on.


Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #6 on: January 20, 2011, 11:50:59 PM »
My Thermopex Installation Part 2:

The day my dealer delivered the Thermopex, my heart sank when I realized just how inflexible and unruly this pipe is.  We could barely wrestle it into the garage to keep it warm while waiting our attempt at pulling the pipe.   I was only able to flex the pipe an inch when I sat on an eight foot span left in the sun during a test.

I knew it would not be possible to simply pull the pipe by hand.   We needed some mechanical advantage to succeed.  So I went to work building a support system in the basement to mount a winch with a mechanical advantage of 14.2 to 1 and a 1,500 lb capacity.  A half a day later and the winch is mounted to  a 4x6 support platform with a 60 foot strap that will need to be cut and reattached at about 30 feet because the spool is not big  enough to roll up 60 foot of strap.

The winch is rated at 1500 lbs and the strap at 3,300 lbs.  The winch support is made out of 4x6s and braced in three places against the wall and at the top and bottom, to prevent side to side movement.

At last the preparations are complete and I no longer could avoid the inevitable. Today we will attempt to pull the Thermopex through the PVC piping. I hope all my night and days of planning/anxiety/preparation/worry will pay off

Here are some photos of us wrestling with the Thermopex.



From the outside, I pushed the pipe into the hole with all of my weight while, in the basement, my wife turned the winch with both hands. While we pulled and pushed together, we communicated with a baby monitor (hands free for me outside) and walkie-talkie radio. Slowly, about one foot at a time, we finally succeed in pulling the pipe through and Im a happy guy!  Hardly able to comprehend that this part, that worried me so much is actually done, I found myself looking at it frequently the rest of the day to be sure we actually were successful.



Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #7 on: January 21, 2011, 12:12:33 AM »
My Thermopex Installation Part 3:

I dont know my heat loss, because I never installed thermometers at each end of the water line.  I wish I knew the loss to a 1/10 of degree, but I guess not bad enough for me to install calibrated thermometers.  

However, here is an interesting observation that I made.
 
Having just got my installation up and running in January of last year, I inserted a temperature probe from my Fluke meter 3 foot into the PVC pipe with the Thermopex installed, but prior to turning the furnace on and then I stuffed insulation around the opening.  I left it for a few days while I was doing other prep work.  I got a steady temperature reading of 45 degrees (the Fluke meter will indicate high, low and average temperature over a period of time).  

After turning the furnace on and letting it run for a week, I went back and inserted the thermometer probe from my Fluke meter again, being careful to not let the probe rest on the Thermopex or PVC wall.   The average and steady reading was 85 degrees.

My thoughts on this:  Insulation just slows the transfer of energy and does not stop it.  If the Thermopex was a perfect insulator the reading would have remained at 45 degrees inside the PVC, since it is not, the temperature went up.  Had the PVC pipe been insulated without loss, then the temperature inside the PVC would eventually rise to the average temperature of the supply and return line, about 175 degrees.  Since neither could be insulated enough to stop the transfer of heat, the temperature inside the pipe settle at 85 degrees.

What I effectively have is my Thermopex installed in an 85 degree, 55 foot long, air plenum that is buried 30 inches down.  

Offline Holmes

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #8 on: January 21, 2011, 07:03:58 AM »
  Dean186  That is an excellant installation. Now your pipes live in an 85* chase not a 45* ditch.  By far the best way to install the pipes.     Not trying to be funny but you might be saving a cord or more on heat loss. Great job.   Holmes
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Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #9 on: January 21, 2011, 10:46:34 AM »
First, Thanks Doctorb and Tom for your response.

Holmes,  My savings could be a cord per year.  Even though the installation took a lot of work and additional expense, I believe it was less work than it takes to collect and burn extra wood each year.   Thanks

Offline ken999

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #10 on: January 21, 2011, 07:14:44 PM »
Great thread Dean.

That's one heck of an installation too. VERY nicely done.

I added some blue board above my T-pex as well as I had very little depth of cover where I crossed my driveway. I also bedded the T-pex in sand and that was all on top of 4" perf pipe and #2 crushed stone wrapped in fabric. I've got alot of water in the ground around here as we are at the base of a decent sized hill with water constantly running at us down the hill on top of the legde rock. I really wanted to keep the T-pex drained and dry, reducing the heat loss from water contact.

So to add to your already great advice, I'll add...Be SURE to mitigate any water problems you have when you install.

Offline forest

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #11 on: January 21, 2011, 08:59:08 PM »
Nice work ... I am sure there is considerable savings. Insulation within reason is an excellent long term investment. I also really like the pad and brick work. It creates a very esthetic safety barrier. Does the hill in the back effect the draw, or does it matter as much with the fans pushing the air through the system?

Offline doctorb

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #12 on: January 22, 2011, 09:44:17 AM »
Dean-  let's make a couple of different assumptions.  I am going to round off these numbers.

One cord of oak = 25,000,000 btu's
loss of temp = 2.5 degrees each way (total 5 degrees of loss)

If the pumps push 94,000+ pounds of water per day (round off to 100,000)....
And you lose a total of 5 degrees in transit.....
That means your daily loss is about 500,000 btu's / day.

Therefore, at 25 mil btu's in a cord of oak, it would take 50 days to lose the energy of one cord of oak given an total of 5 degrees of heat loss in transit within the system.  So, I would lose about 3 cords of oak fuel to heat loss over a 150 day heating season.

Are my calculations right?  That's pretty impressive loss for something that doesn't melt the snow on the ground 30" above it.  doctorb
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #13 on: January 22, 2011, 11:46:11 AM »
Doctorb,

Your numbers are correct and I do like rounding them, much easier.
  
My comments on snow melt above pipe:  In the above example, the loss is 500,000 btu per day loss in a pipe that is stretched over 200 feet and buried 30 inches down.  This would calculate to 2,500 BTU per foot per day.   It would take a lot of BTUs to melt the snow on top of the ground from 30 inches down when competing against the outside temperatures acting on the above ground snow and factoring in the surrounding earth that will be absorbing much of that energy.

For thought:  The 8 gallons per minute "fact" that I used in my above calculations for the furnace's pump rating is an estimate and not a measure number.  I wish I had a measured reading to use.  I am sure someone at Central Boiler has real world readings of how much water this pumps move in a typical installation.  I asked Central Boiler, but did not get a response.  I emailed TACO and described my installation and stated my pump model number and they emailed me back the figure of 8 gallons per minute.  So, this number will vary from installation to installation.

Dean

Offline doctorb

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #14 on: January 22, 2011, 12:29:19 PM »
Dean -

Because of the increased distance (300 feet) and increased diameter of my thermopex (1 1/4), I am sure that I have a larger capacity pump than that normally installed.  I remember this was discussed at the time, but I don' t know the exact beefed up size that I have.  If that's the case, then my heat loss in transit is even higher because I move more water than you.
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #15 on: January 22, 2011, 12:29:40 PM »
Great thread Dean.

That's one heck of an installation too. VERY nicely done.

I added some blue board above my T-pex as well as I had very little depth of cover where I crossed my driveway. I also bedded the T-pex in sand and that was all on top of 4" perf pipe and #2 crushed stone wrapped in fabric. I've got alot of water in the ground around here as we are at the base of a decent sized hill with water constantly running at us down the hill on top of the legde rock. I really wanted to keep the T-pex drained and dry, reducing the heat loss from water contact.

So to add to your already great advice, I'll add...Be SURE to mitigate any water problems you have when you install.

Thanks Ken,  

I remember the photos from your installation and the large sheet of blue board over the top of your Thermopex.  It would compliment this post to insert that image showing how you managed any chance of water with all of the sand and the way you wrapped all of it with fabric.    Dean

Offline Dean186

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #16 on: January 22, 2011, 12:34:12 PM »
Dean -

Because of the increased distance (300 feet) and increased diameter of my thermopex (1 1/4), I am sure that I have a larger capacity pump than that normally installed.  I remember this was discussed at the time, but I don' t know the exact beefed up size that I have.  If that's the case, then my heat loss in transit is even higher because I move more water than you.

Doctorb,

This would be true.  However, don't forget my above comment about using the control panel thermometer reading to determine the temperature of the water existing the furnace.   In my opinion, it would not accurately reflect the water temperature exiting the furnace.  How did you arrive at your numbers?

Offline doctorb

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #17 on: January 22, 2011, 12:40:37 PM »
The numbers I used were hypothetical.  However, I have thermometers mounted on the inflow and outflow pipes at the back of the stove, and I have thermometers mounted in similar positions in my basement.  So I could get absolute readings.  With the distance of my furnace from my house, I'll bet the heat loss is more than my example illustrates.
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

Offline albirk

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #18 on: January 22, 2011, 06:47:06 PM »
Dean I like the way your install lookes and the way everything is around your place (in other pics ) the two large holes one the house the other shop? I like to read what you and doc post lot of good advise one thing on your return water you may not be loosing as much just because of the raidint heat you are getting in your house. One or two cords of is not that big of a deal to me because that is more peaceful time in the timber .

Offline albirk

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Re: Calculations for BTU Loss in OWF Water Line
« Reply #19 on: January 22, 2011, 06:57:00 PM »
doctorb
what type of wood do you usly burn? my stove and oak don't get along well i have a hard time keeping wood in it i get along better with soft maple,elm,ash and my fav locust


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