outdoor furnace(pipe insulation)

[winchman]

Hey Guys,

I'm going to be installing a used outdoor wood furnace at a distance of 125' from my house. I'm concerned with the waters heat loss in that distance. There will be two 3/4" lines and two 1.0" lines (pex tubing) with standard styrofoam pipe insulation running through 6" black flexible pipe with no holes, buried at a depth of 36 inches. I'm trying to come up with a inexpensive way to add extra insulation. Here are some things that I thought of and that were suggested to me.

1. Partially backfill ditch with sawdust=sawdust will rot eventually /forums/images/graemlins/frown.gif

2. Partially backfill with sand=is sand an insulator? /forums/images/graemlins/confused.gif

3. Build a box out of 1" thick pink styrofoam board to enclose the 6" black pipe? /forums/images/graemlins/confused.gif

4. Fill 8mil thick poly tubes(long plastic bags) that are 16" in diameter with sawdust and lay over the black pipe in the ditch, backfill partially with sand? /forums/images/graemlins/confused.gif

5. Ingenious ideas from you guys /forums/images/graemlins/laugh.gif

Thanks,
winchman

 

[Green_in_MI]

I suggest you get the advice of a real mechanical contractor. I have no doubt you can get your system to work, but to work effectively that is another matter. You have pipe sizing, pump sizing, heat loss, length-of-run, cavitation, safety, and other issues to deal with. A 250 foot run with 3/4 inch just to get to and from the house is already a long ways to pump no matter how well insulated.

 

[boustany]

It is a long way. I'd give up on the saw dust completely. Probably, keeping water away from the pipes is a good idea. Something like a foot or more of 3/4 gravel, a styrofoam box, more gravel might do it.

Of course, you could always ask the manufacturer for their advice. It probably wouldnt be the first time they've encountered the question.

We so far? Seems like a long way to walk to add a log of too...

 

[kcbarnes]

When I installed our outdoor boiler I was warned not to bury the lines to deep as they may get into the water table and get the heat sucked out by ground water. I think they said 12" or so.

 

[tc35dforme]

I dabble a bit in the mechanical trades....

The maximum generally accepted flow rates for those pipe sizes are 4 gallons per minute ( gpm ) for the 3/4" and 8 gpm for the 1". You can push it higher, but velocities increase as does pressure loss and the result is more pump = more money.

Now the key is moving the heat. A combined total of 12 gpm at a temperture drop of say 40 degrees ( 200 supply - 160 return ) will result in 240,000 BTU/hr of heat ! You could probably afford to loose a little. What's the BTU/HR rating on the wood boiler ?? Do you know what you need at the house ??

You may want to look into a product like IMCOA web page. Its a closed cell insulating product, water can't get into it, and it is glued section to section so it can be fully sealed. You can lay the tubing in a nice bed of rice stone or pea gravel 12" all around and not worry for a while. The only concren may be rodents munching, but the pea stone should help discourage that.

You may be worrying for nothing. Unless you have a high water table, you should have plenty of heat for the house, even if you give a bit to the earth. My experience with wood boilers is that you often end up dumping heat ( when you don't need it at the house).

 

[cmsedore]

I'll offer what I know as I have a very similar installation: mine is 150ft, with 2 1" PEX for heat and 2 3/4 for DHW. I left the DHW feed out of the insulation as I was burying below frost line and didn't see the need to insulate a pipe that would carry cold water anyway.

First, make sure that the pipes are isolated (ie, the styrofoam should insulate or at least separate the hot and the return from one another). You might be surprised how much heat exchange can happen if you press two 1" pipes together over that distance. Second, be sure you don't go under the water table, it probably won't matter how much insulation you put in if it is below the water line even for part of the season.

That said, I put mine in four layers of underground rated isocyanurate (sp?) foam (for about R-8), wrapped in a heavy plastic sheet (overlapped on the bottom to allow water to run out if the table ever came that high) and have trouble measuring much of a temp difference when the pump is running.

If I had it to do again, I'd insulate it better (as you're interested in doing), but hindsight is always 20/20. Actually, if I had it to do again, and cost and hassle were less of a concern, I've toyed with putting each pipe run in a separately insulated 4" corregated pipe. It is as easy as pie to work with, allows you to replace (or add to) the run later, and gives you something of a natural airspace for insulation. In my first stove installation (one I built myself), I used two 4" pipes. It was quite handy as I could add electrical/control lines (I instrumented the stove and used a computer to do blower control).

I would suggest using 4" pipe to penetrate your foundation. That way, you only have to dig up to 6 or 8 feet away from the foundation if you ever need to replace, add to, or reroute the lines.

One thing about styrofoam: most of it is not rated for exposure beyond 180 degrees, and, in my experience, it will deteriorate under those conditions (it shrinks 5-10%/hardens over time). I don't know if it affects the insulation value. In all likelihood, you will expose it to temps higher than 180 unless you keep the stove thermostat lower than usual (<170).

Make sure you know where the electrical lines are when you bury them--I dug up my 220v 60 amp barn feed when I put mine in /forums/images/graemlins/blush.gif. Plenty of sparks (and it did not pop the breaker to my surprise and scare).

-Chris

 

[F350Lawman]

My neighbor heats his house with an outdoor furnace. He must do it more for a hobby than for economics. Between the hours he spends collecting, wood, processing it and tending the burner he would be ahead of the game if he spent those hours working for minimum wage and paying for oil heat.

This doesn't take into account all equipment he has to have, or at least tie-up like a tractor, wood processing truck etc. Granted he has a very large house to heat but it seems like ALOT of work to save a fewl thousand dollars on heat.