Insulating buried outdoor run

[Scarecrow57]

I am currently considering the installation of an indoor wood boiler in an outbuilding. The run from the building is about 100 feet. I have been looking at the various manufactured insulated piping systems that come in a piece of 4 inch plastic corrugated pipe. I'm not really sold on these as they seem pricey.

I also looked at the system on these pages were liquid foam was placed around the pipes buried in the trench. Again, the liquid foam seems pricey.

As an alternative I am considering constructing a U shaped trench with 4inch foam board insulation. The trench would be 6 inches wide and 6 inches tall. I would then pour about 3 inches of cement in the bottom of the trench, lay my pex on top of the cement, then cover the pipe with cement. Finally I would put on top layer of insulation to give a complete box of insulation. This will address any issues with water, insulate the pipe, and be relatively inexpensive. Any thoughts or concerns on this approach?

 

[Deere10]

Think the concrete will absorb heat from the pex. Just what I am thinking. I am sure others will post what they have done. I used the 4in. corrigated pipe w the insulated pex on my install.About 150 ft total.

 

[btuser]

Pex in direct contact with the concrete? That sounds like one heck of an outdoor radiator. What your supposing is how radiant heating works. Concrete is a great conductor, what you want is insulation around that pipe. I agree that the underground piping is expensive for what you get but the fact that no one has come up with a way around the cost has me thinking it might be the only way.

 

[Tarmsolo60]

I would't do that. As mentioned above you want an insulator around your pipe, not a conductor. Even with the foam around the outside, If groundwater gets in there you will be heating the ground.

Make sure you don't skimp on the line size. Look at the pump curves and your heat load before installing the lines.

 

[juddspaintballs]

The concrete will pull heat from the lines into the ground. The foam board will let water in no matter how well you think you sealed it. Besides, after half a cubic yard of concrete and 4" foam board in a U shape for 100', you'll be right up there with the cost of spray foaming the lines in place.

 

[rkusek]

I agree with others. Your talking a few hundred dollars with your approach. Pink foam board and concrete are not cheap. Find an insulation contractor that sprays urethane in your area. Sealrite was a local company I found that sprayed a 350 ft. trench for me for $675. I would think you can find someone to do 100 ft. for about $350. I wish I had my propane tank at the time since they would have done that too for not much more. I know it's not cheap but it only took them about 2-3 hrs. I think you will find you would really not save any money doing it the way you proposed even without the concrete which would suck away the heat. If water infiltrates anywhere the whole thing would become useless.

 

[benjamin]

remove concrete, the consensus here is that concrete has no place in this application.

IF you are dead set on doin it on the cheap, use the foam board, but spray foam the tubing to the board. Or use a 12" wide base of foam board of chosen thickness, add a 4" wide (by thickness to accommodate your pex) strip on each side, and foam the pex in place and the boards together and the top on with expanding foam in a can, $2-2.50 on sale.

Remember heat loss equals temp difference x time x surface area / insulation value. Foam right up to the pipe dramatically reduces the surface area compared to having a conductive or convective core that is surrounded by equal insulation.

 

[btuser]

How good is foam in place compared to the pre-made lines? Do you still head below the frost line?

 

[boatboy63]

You can get a cousin to spray foam that is called pour foam. It comes in a part A and part B. When you mix the 2 together, within minutes, it will expand to 25-30 times it's original volume. It is 2 pound per cubic foot density polyurethane liquid foam and a 10 gallon mix (5 each of A & B) will give you 40 cubic feet of foam.

What I had planned on doing was to dig a trench 8-10" wide, lay heavy plastic or rubber membrane in the trench and hold both sides of the plastic/rubber on the "banks" of the trench. Put about a 4" rock on the plastic/rubber about every 5'. Lay your pex on top of this so it is held above the plastic. Mix this pourable foam about 1 gallon at a time and pour it in the trench, working about 8-10' at a time. Work your way on down the ditch to the end. When finished, place the rubber/plastic that was on the banks back over the pex/foam assembly. From the way I figure it, 40 cubic feet on a 100' run would make the foam about 9-10" in diameter when you figure in the volume of the pex. The cost of 10 gallons of pourable foam is around $260. You can read more about it at Pourable foam. I have used this stuff in the marine industry and I can tell you it is strong.

I hope you can follow what I am trying to say. I had a 2 yr old granddaughter who was standing here the whole time I was trying to post and was talking to me the whole time. It is hard to get your mind on track when she is talking and then her grandma is telling me to pay some attention to her.

 

[Scarecrow57]

Thanks all for the input. Can the concrete. I am now thinking of doing the Blue construction grade foam in the U shape and then pouring in that poured foam that was mentioned. I will use about a 6 inch inside dimension U .

My initial thought with the concrete was that it would be heated as well however with the insulation around the outside it would only heat a small piece up to temperature. Taking little heat to keep it there, in essence it would become a heat store. However, As I think about this I would also be heating the return lines as well. My plan is to run two one inch lines in each direction. Which will give the equivalent of a 1.4 inch pipe (ignoring the friction loss).

 

[juddspaintballs]

If by 1.4, you mean 1-1/4", then you are correct that dual 1" lines have nearly the same nominal cross sectional area. I was going to go with dual 1" lines, but after figuring out that I'd need double the fittings and I'd have to either use 2 circulators or bring the two lines into one circulator and back out into two again. After a certain number of fittings, the cost savings are hardly enough to justify wrestling 4 lines into place and foaming them in instead of 2.

 

[btuser]

Depending on where your storage will be (if there is any) I can see a benefit to two 1" lines by taking advantage of the stratification that will happen in the tanks. You could draw warmer water on the top for some zones (like DHW) and cooler water for other zones. Others with actual experience may want to chime in. My after-thought would be that the longer the run the less this would be a reality. Also, if your storage is inside (where it should be) you'd only be using one temp anyway.

If you're tricky with the manifold you could get by with one circulator and just parallel the two lines back to 1 1/4". You can get a 500' roll of pex in 1" cheaper than two 100' rolls of 1 1/4". The tool for crimp/expansion is also another factor.

 

[ewdudley]

Thanks all for the input. Can the concrete. I am now thinking of doing the Blue construction grade foam in the U shape and then pouring in that poured foam that was mentioned. I will use about a 6 inch inside dimension U .

...

My plan is to run two one inch lines in each direction. Which will give the equivalent of a 1.4 inch pipe (ignoring the friction loss).

What's equivalent to what else depends on flow rate.

Here's a datasheet that gives pressure drop per foot at various gpm flow rates for the various pex sizes.

http://www.viega.net/cps/rde/xbcr/en-us/TD-PFVPU-0509-1_(ViegaPEX_Ultra_UL).pdf

[Of course the usual bain-dramaged hearth.com URL pasting vagaries apply, probably you'll need to screen-scrape the whole line and paste into your address bar.]

 

[DaveBP]

Excellent reference, Eliot.

I've sermonized a couple times here about two 1" lines not being as easy to push water through as a single 1-1/4" line. The chart you linked to puts real numbers to it.

Looking side-by-side at 1" and 1-1/4" at any flow rate and you can see that by cutting the 1" resistance in half (using two lines) the resulting resistance-to-flow is still considerably higher than a single 1-1/4" line; more than 25% higher.

And the two 1" lines have a much greater surface area to lose heat from.

I've never compared the price of the fittings but the 1-1/4" tubing is less than twice the price of 1". On the internet, anyway.

 

[ewdudley]

Excellent reference, Eliot.

I've sermonized a couple times here about two 1" lines not being as easy to push water through as a single 1-1/4" line. The chart you linked to puts real numbers to it.

Looking side-by-side at 1" and 1-1/4" at any flow rate and you can see that by cutting the 1" resistance in half (using two lines) the resulting resistance-to-flow is still considerably higher than a single 1-1/4" line; more than 25% higher.

And the two 1" lines have a much greater surface area to lose heat from.

I've never compared the price of the fittings but the 1-1/4" tubing is less than twice the price of 1". On the internet, anyway.

I often wonder if maybe many of the 1-1/4" and 2X1" runs could have met their requirements with a single 1" run.

For instance a Taco 008 with no IFC should push at least 5gpm through 100f-one-way/200ft-round-trip of nominal 1" pex with 80 watts or so.

A properly configured system should be able to achieve 50 degF deltaT, which is 125,000 btu / hour, which would be plenty for the vast majority of domestic loads out there I would think.

I'm about to find out this winter, I'm going with the smallest pex the charts will allow for a 1/25hp pump and 100,000 btu / hour.

--ewd

 

[DaveBP]

Being able to use a deltaT of 50F make a lot of things easier.

I think a lot of folks on this forum are trying to get best use of existing baseboard designed for steady 180F (or even 200F) water in big poorly insulated houses,and more than one building many times. They are stuck driving a lot of GPMs across long distances underground.

These are the folks who get sold the commonly stocked and lower priced 1" pipe on what seemed at the time to be knowledgeable advice. Finding out too late that 1" just isn't always enough pipe.

Before I started haunting this forum I would have done the same thing. And I, too would have felt poorly used by the salesman.

 

[Chris Hoskin]

absolutely right DaveBP, a 50 degree delta T is a huge advantage and many folks have to work with a higher temp system and a 20 degree delta T. Perhaps there is an argument for adding some flat panel radiation to the home in order to increase the delta T?

 

[Tennman]

FYI.... Coefficient of Thermal Conductivity (rough numbers): Polyurethane Foam ~ .02; Water ~.58; Concrete ~1.73; Gold ~310. These numbers (CTC) are an indication of how good materials transfer heat. If you don't want to transfer heat look for a little number. Bottom line, water is a better "insulator" than concrete. I know you've moved on from the concrete but in rough numbers the polyurethane is almost 100 times better at keeping heat in (insulating). Due to my experience detailed in the "Underground Sticky", I can't help chiming in on underground discussions. I used Tiger Foam in my first disasterous attempt but this was not the fault of the foam, seems like a pretty good product. My method failed to prevent water contact with the PEX which doomed my DIY "cost saving" attempt. It's disapointing to me there seems to be a shortage of affordable foam contractors in some areas where you guys live.

 

[Scarecrow57]

I am revisiting this. As I research this I find that storage is beneficial. As such I have designed the trench as shown in the image attached. The dark green represents construction grade foam board. The red circle is the hot feed, the blue circle is the cold return.

My thoughts are that I could encapsulate the hot feed and cold return lines in concrete (quickrete) and encapsulate them in 4 inches of foam board. The concrete in this case would act as a storage medium. A 100 foot trench would give 44 cubic feet; this represents an equivalent volume of 332 gallons of water.

Thoughts on this????

 

[Tarmsolo60]

I am revisiting this. As I research this I find that storage is beneficial. As such I have designed the trench as shown in the image attached. The dark green represents construction grade foam board. The red circle is the hot feed, the blue circle is the cold return.

My thoughts are that I could encapsulate the hot feed and cold return lines in concrete (quickrete) and encapsulate them in 4 inches of foam board. The concrete in this case would act as a storage medium. A 100 foot trench would give 44 cubic feet; this represents an equivalent volume of 332 gallons of water.

Thoughts on this????

Wouldn't the concrete crack due to the high pex temps?

 

[juddspaintballs]

Just quickly thinking about it, I don't see why that couldn't work. It sounds like a heck of a lot of work, and fairly expensive. I think my only concern with this idea right now is that you'd have to be very sure you seal up all of the foam board to keep water out, and make sure that the 44 cu. ft. of concrete doesn't crack the foam board. If water gets in to touch the concrete, you're in for a huge heat loss. Maybe go the extra step and wrap the foam in 4' wide plastic sheet of one continuous length and heat seal the two sides of the plastic together like a bag of potato chips?

 

[in hot water]

The goal is to get the heat energy from A to B as quickely, and with as little loss as possible.

I think adding mass in the ground is going the wrong way?? The more conducter )(concrete) the more surface are you need to insulate away from the ground temperature.

Even if your ground stays at 55 °F putting a tube with 180 °F presents a delta T of 130 °F. Heat travels to cold always and no amount of insulation can stop the transfer 100%. The rate of heat transfer increases with the delta T. If the ground gets down to 32 °F the delta T increases and so does the heat transfer and heat loss.

I'd rather have the pex surrounded by air than concrete. Insulation is your "air"

hr

 

[Tennman]

You want to transfer to your house as much of the energy created by your wood as possible. This means loosing as little water temp as possible from the boiler exit to the HX inlet. Efficiently dump as little energy as necessary into the house and return as much residual energy as possible. Excess energy created can be redirected to thermal storage to be used later if your boiler generates more energy than your home requires. Continuously charging your concrete thermal battery before dumping maximum water temp at the HX will really hurt your efficiency. Other reasons related to the required thermal mass and thermal conductivity makes concrete a poor thermal battery. There's many technical reasons all the systems here basically look much alike. Review the schematics available at all the wood boiler manufacturers/distributors and you'll see they're all basically the same for good technical reasons. Hope I didn't sound too teachy.

 

[Tony H]

Using the foamboard is an old time proven way provide some insulation , used to insulate water and septic lines in this area for years. There are other and better ways one is using the insulated 4' duct and pulling PEX thru that another is using the foamboard as a container in a U shape and spraying foam in and over it .
Concrete is NOT an insulator and should not be used . This is one area not to skimp or experiment as a large heat loss can result in your system being useless and expensive to dig up and start over.
I used an insulated pipe by logstor that worked well for me and I would use a similar product plus sprayfoam or foamboard and sprayfoam if I did it again.

 

[Scarecrow57]

If I understand this right we have several objectives within the system.

1. Burn hot
2. Provide Storage - More is better?
3. Get the heat from the storage to the living area efficiently.

Questions:

What is the R value typically used on the Storage system?
Where is the storage typically placed? Close to the boiler or near the living area?

Thought

By constructing a water tight insulated cavity in the ground and then filling it with a medium I provide a means to get the heat from point A to point B as well as establish some heat storage.