Calculations for BTU Loss in OWF Water Line

[forest]

Brucer is there a point where the thinkness of the insulation around the pipe would eliminate any heat loss completly, or make it so little as to not really to count. Certainly an R20 wall reduces more heat loss than an R10, but the largest factor is air penetration which is largely controlled by how air tight the vapor barrier is. In terms of the pipe, water would be the factor that could create the greatest loss. If there was no water penetration how many inches of foam would you suggest would eliminate most heat loss?  I suppose it would be different depending on the soil, so lets say sandy soil. J

[Brucer]

Brucer is there a point where the thickness of the insulation around the pipe would eliminate any heat loss completely ...

Nope. No amount of insulation will completely stop heat loss.

The closest you can come is to enclose the pipe in a vacuum. That eliminates all conduction and convection losses. You'd also have to coat both the pipe and the inside of the surrounding tube with reflective coating to prevent radiant heat loss. This is obviously impractical.

On flat surfaces doubling the thickness of the insulation will cut the heat loss in half. Notice that you get diminishing returns by adding insulation. Say you're losing 100 BTU per hour through 1" of insulation. Double that to 2" and you only lose 50 BTU/hr. You saved 50/hr BTU -- not bad. Now double it again to 4" and you only lose 25/hr BTU. But that means you only saved 25 BTU/hr by adding twice as much insulation as you did the last time.

It gets a whole lot more complicated with round surfaces. As you add more insulation, the surface area increases. Thicker insulation reduces heat loss, but greater area increases heat loss. In a concentric pipe, you'd have to increase the insulation by a factor of 4 to cut the heat loss in half.

As you already suggested, keeping the insulation dry is critical. Even foam insulation will absorb water slowly, so you have to be absolutely certain that surface water can't soak down to the insulation. You also have to be sure that ground water won't rise to the level of the pipe. Once the insulation gets wet, you may as well have not put it in.

Best of all is to reduce the length of the pipe. Not very useful advice if you've already installed your boiler -- unless you want to add an extension to your house in the direction of the furnace :D.

[Brucer]

There is a limit to how much you can gain by reducing heat loss through the ground. Here's 4 hypothetical examples of how to burn less wood.

Base Case: OWB efficiency = 50%, heat loss to ground = 40%
10.0   Cords burned
5.0   cords up the chimney
5.0   cords recovered
2.0   cords lost to ground
3.0   cords heat the house.

Case 1: Increase OWB efficiency to 67%, leave heat loss to ground at 40%.   
   
7.5   Cords burned
2.5   cords up the chimney
5.0   cords recovered
2.0   cords lost to ground
3.0   cords heat the house.

Case 2: Reduce ground losses to 20%, OWB efficiency stays at 50%.
7.5   Cords burned
3.8   cords up the chimney
3.8   cords recovered
0.8   cords lost to ground
3.0   cords heat the house.

Case 3: Leave OWB efficiency at 50% and ground losses at 40%, reduce heat loss in house by 25%.
7.5   Cords burned
3.8   cords up the chimney
3.8   cords recovered
1.5   cords lost to ground
2.3   cords heat the house to the same comfort level.

Case 4: Make all three improvements.
4.2   Cords burned
1.4   cords up the chimney
2.8   cords recovered
0.6   cords lost to ground
2.3   cords heat the house.

If you're burning green wood, you can improve your stove efficiency just the by seasoning the wood for a year.

Even if your house is well insulated, if doors and windows don't seal tightly you could lose a lot of heat through infiltration. And even triple glazed windows lose a lot of heat -- tight fitting insulated shutters can make a big difference.

Also, notice how reducing heat loss in the house will reduce heat loss up the chimney and to the ground, even though you don't change the OWB or add insulation to the pipes .

[Just Me]

Brucer

Interesting numbers.

In my last house I built a three sided concrete affair in the basement under the fireplace. It was built tough enough to hold up 30 feet of masonary, so it had a lot of mass. The floor upstairs was 14" thick concrete with a seperate flue running into the fireplace and up to the roof. It kept the fireplace warm at all times and worked very well. I had a 14x21 concrete room in the basement that was sealed to the inside, but wood could be loaded from the outside. I would load it with wood, close the door and throw in a couple of bug bombs. It was just a few steps from the door to the woodstove in the basement. The masonary chimney in the attic was insulated with 2"foam to the sheeting, and the chimney came out the peak just 2', so very little chimney exposed to the cold. It worked well with the exception of the house being 4700 Ft. with 9' ceilings and a lot of glass. Still, the heat bills when we were away from home were quite a bit larger, so I know it was effective. I never had problems with chimney buildup because it stayed warm till the very top.

I have the same options on the house I am building now, and insurance cost are my only real concern. I am checking that out now. The new house is only 1180 ft. Given the losses of a OWB I think I will probably seperate what I do for the shop and the house, and if I can come up with a plan that will not penalize me too much with insurance do a stove/wood furnace in the basement under a concrete floor section again. Most of the heat loss from the chimney can be reclaimed on the way out, there is no transportation losses, as well as no losses around the stove itself. As an added bonus, the wood will be inside. Wood stoves cost less and last longer.

I know many of you have OWB's, But when I look at the numbers and the downsides, I just don't see them as an ideal solution. Cool for sure, but with a small efficient house, there will never be a payoff before the system fails.

[Randy88]

First off I am impressed with the installation and how it went, I am also surprised that you got the line pulled through the pipe after it was buried, now with that said I have several questions that I can't see from the pictures and also something to ponder in thought a while and to tear apart my thinking here.   

The first question I have is did the line flow downhill somewhere that it eventually drained out without any low spots in the line anywhere and that's after settling is taken into account?   

The reason I'm asking is this, I'm a ag drainage tiling contractor as well as many other things and from what I read and have seen first hand, anywhere there's a low spot in the line water will settle out and sit, now unless my thinking is wrong anywhere hot and cold meet condensation occurs and water is an excellent conductor of heat transfer  Now unless there's a  pretty good slope with no low spots on the line where water can sit and pool water will condense around the line and between the outside line causing considerable heat loss, any low spots in the line are going to hold water and with the insulated line it will never get warm enough to evaporate out any excess water.    If theres even one simple low spot and water pools around the line even for a few feet there will be considerable heat loss in the line there and there is virtually no way to see it and it is a constant loss robbing btu's at a larger rate than if it were not inside the tube at all.   

The second question I have is if there is slope in the line how did you get the line elevated off the bottom of the second line to allow for excess moisture to drain out?    If there's nothing holding it up off the bottom of the outside line then how much heat loss is there with the line sitting in water constantly even if its only a fraction of an inch the entire length of the line seeing how water conducts heat loss at a considerably larger rate than just cold dry ground by itself?

I'm not being critical just asking some simple questions that nobody around me could answer or had ever thought of until I asked these same questions.

[Dean186]

Randy,  In response to your questions on PVC slope and condensation.

I designed a very slight slope towards the basement when I installed the PVC pipe.  My thinking was, if there was a leak in the Thermopex, PVC or condensation buildup, the water would not pool and have a way to exit.  I mostly wanted a way to drain the Thermopex if I ever needed to, since the other things were unlikely.  The 6 inch PVC is so rigid it is hard to have a low spot in this short of a run.  However, I put a level to it as I went, to create a very slight slope and no low spots.

The PVC pipe end cap in the basement is removable and I have checked the pipe on three occasions.  No water, no moisture, it is about as dry as it can get in there.  The Thermopex rests on the bottom of the PVC pipe (the best that stiff stuff can anyway).  In some places the Thermopex is up against the sides of the PVC.  With two end caps in place the air inside the PVC is trapped and very very dry.

[Randy88]

Thanks for the response, I was curious and nobody I knew had done it with a slope on the line and then they couldn't tell me if there was condensation or not.     All the insulated tube suppliers around here [thought] it would build up moisture and not to put it in another line, just use direct burial and keep the ground dry by tiling drain tile a short distance away from the line and slightly deeper than the insulated line.   

I don't know if it would make any difference with the state it would be done in or not, the more ground water there is and the yearly rainfall and soil type would matter or not.   As for pvc bending under ground over time, I've seen plenty of it that I've dug up, it will literally bow it into submission without cracking it at all, I've bug it up years later that it has a bow that exceeds a foot or more out of alignment  and it never cracked, just how it can do that is always a wonder to me. 

[Dean186]

I don't know if it would make any difference with the state it would be done in or not, the more ground water there is and the yearly rainfall and soil type would matter or not. 

I don't think it would matter, and except for heat loss, I believe the 6 inch PVC pipe could run through a pond and the condensation inside of the water impermeable pipe would be the same. 

[Randy88]

Maybe if the pipe were capped on both ends and no air movement was allowed, I'm still not sure that would be the case.  Around here they tried the air cooling and heating using ground tempering of the air to either heat it or cool it to 50 degrees, they'd run a line underground and put an intake and riser up away from the building so air could be pulled through and be tempered and the first units that went in were a disaster, they didn't lay the lines to grade and they would pool full of water from condensation and had to be pumped out in order for them to work, later units were installed to grade and those worked nicely but there was also a lot of water running out the lower end no matter what type of tile or pipe was used.   

The lines I've dug up over the years depending on the time of year most pvc lines will have condensation in them from one form or another, either from tempered air entering naturally or from differences in elevation of the line and trapped air can rise and cool or from frost just inside the start of the line as it enters the ground during the winter, there's a lot of reasons to cause condensation but most times even sealed lines will have some in it at some point during the year.

Another question I have is have you ever taken a temp probe from inside the pipe to see what temperature it is inside, or how much heat is given off from the insulated pipe?

I had to dig up one persons line that they tried to ran inside an 8 inch pvc line, the line was already there and had electrical lines running through so they thought they'd just run the new insulated line through but there was a tight bend that they couldn't get it to go through so I was called in to dig it up so the bend could be made more gradual, when we broke into the line we found a lot of water in it and as we put in the shallower bends we also put a tap in it so the excess water could be drained off and into a nearby tile line.   In order for all this to be done we had to pull the electrical lines out and taped it to the insulated line and pulled the entire thing back through before the area was again back filled.    The general consensus from all involved was that the excess water was due to condensation over time and its inability to ever get back out, the water was stale and stunk, so it had been there a long time, there was also frost just inside the start of the line as it entered the ground outside [it was winter out when we did it] even though the line ends were capped at both ends.

[Dean186]

Another question I have is have you ever taken a temp probe from inside the pipe to see what temperature it is inside, or how much heat is given off from the insulated pipe?

The Post has ran long, so you may have missed this from page 1:  

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.  

[Dean186]

Randy,  

It sounds like you have experience that I don't have with buried lines.  All I know is the inside of my PVC pipe it is very dry.  

From what I understand you to say, PVC pipe that has been buried for the purpose of running electrical wire and high tech data lines (like fiber optics for our phones and cable TV) could be filled with water in places?  

It would make more sense to me that the water penetrated, the pipes you discussed, from a broken seal or cracked pipe.  

[hockeyguy]

Sorry if this idea has already been mentioned, but I'm wondering if what little heat loss is given off by the thermopex is keeping the inside of the pipe dry.

In 25 years of dealing with underground pvc, I don't think I've ever seen a run without condensation in it. Maybe the t-pex combined with a a low water table is doing it for you.

This is a good thing.

[Dean186]

Sorry if this idea has already been mentioned, but I'm wondering if what little heat loss is given off by the thermopex is keeping the inside of the pipe dry.

The fact that the inside of the PVC pipe is warm and that the outside of the PVC pipe is insulated would make a difference.  Most buried PVC pipe would not have insulation surrounding it, nor would it be heated from the inside.

Upon starting the furnace this year, I checked the inside of the PVC pipe and it was dry.  The furnace had set idle for 5 months (water pump off).

A little condensation certainly would not cause a problem, but I'm glad I don't have to worry about it anyway.

[barkeatr]

Hello,

Im a former CB OWB user, im in the middle of installing a gasification unit to replace it.

Your install is impeccable.  With my CB i had high line loss so I want to tear it up and add more insulation and further decouple my pipe from the earth as you did.

I have gone through the thread a few times..have you been able to record your line loss? I assume it would be very low with your installation.

thanks for posting your work.

barkeater

[Dean186]

Hello,

I'm a former CB OWB user, I'm in the middle of installing a gasification unit to replace it.

Your install is impeccable.  With my CB i had high line loss so I want to tear it up and add more insulation and further decouple my pipe from the earth as you did.

I have gone through the thread a few times..have you been able to record your line loss? I assume it would be very low with your installation.

thanks for posting your work.

barkeater

Thanks and your welcome.   I was hoping that some new installers would benefit from my notes.  I do not know my line loss because it would take calibrated thermometer at each end of the pipe, which I do not have installed.  

I have read of several methods of reducing heat loss in water lines and PVC pipe is not the only way to help with loss.  I think one should start with the best pipe and in my opinion the Thermopex is very good.  I'm not a dealer for Thermopex or associated in any way with the product.  I think good drainage is the next most important step and then additional insulation.  Blue board works well, and I have heard of the heavy weight spray foam.  I personnel don't know how spray foam holds up over the long haul.  There are many more knowledgeable persons than I on this forum, who would know about spray foam for underground usage.  

Blue board is relatively inexpensive and easy to install around the thermopex.  The extra work and expense taken during installation will pay off later.

Best of luck with your installation and feel free to put up some photos.

[Holmes]

Dean186  You are right about the blue insulation, and the pink stuff is the same. Everyone should know that FOIL FACE  insulation board should not be used under ground. Is does not hold up , absorbs moisture and can become a nice ant farm. Holmes

[Dean186]

Thanks Holmes,  I knew the blue & pink board was the right stuff, but did not know about the foil face board breaking down.   It isn't rated for that application, so best not to use it.

Does the spray foam that some have used to cover their pipe, which gets covered by dirt, hold up over time?

[Holmes]

Dean  The 2 lb density r6.8 does hold up( urethane foam ) . Regular 1/2 lb density icynene  does not. I used the urethane on the inside of my stone foundation and see no signs of it breaking down. They used to spray it on the outside of foundations when they wanted to use the space inside as a living area.  Holmes