Acceptable Duct Temps with OWB?

[boatboy63]

I have been bouncing around thoughts between installing a wood furnace or an OWB. After a lot of consideration, I have about decided to go with the OWB and plumb it into my furnace fan and electric water heater. After realizing the output water temps on these units (150-180 °F +), I started wondering about ducting issues. My house was built in the late 1800's and the ducts are 6" round galvanized pipe under the house. Some of these have developing rust, holes, and other leaks. At 1 time, the ducts were insulated, but wife's cats have seeked out heat from them and decided to paw away at some of the insulation. Because of this, we are currently getting a blast of cold air when the furnace first comes on. This air is about 15°F colder than room air when it first comes on.

The crawlspace under the back end of the house is a real challenge. We also recently built on an upstairs addition, which will make it impossible to duct into the attic area. Because of this, I decided to install custom fabbed galvalume ducting on the lower level ceiling and then installing a drop ceiling under this. I just ordered the galvalume the other day and will pick it up Friday. This is actually 29 gauge metal roofing in 43" widths and 10' long that is still flat and has not been bent and shaped into ridged roofing. I should be able to curl it into round duct, rivet this together and get it to flatten slightly, if necessary. My furnace is a downdraft model which can be mounted in any direction. I will be pulling it up, sealing the cutout to the plenum under the floor, flipping it over, and fabricating a custom plenum to mount on the top and a heat exchanger to connect to my OWB, when I can get it before next winter.

Because of the temps involved with OWB's, I was wondering if I should have any worry about 150°F+ duct temps making contact with any of the structures and causing a hazard. I know from what I have been reading recently, the slinky style insulated flex duct is only rated to 160°F. I had first thought about using it in short lengths just to tie it together. Because of the temps, I am leaning more toward the metallic accordian type. Have any of you had to make any modifications to your ducting to prevent damages from the excessive heat? TIA

 

[Gooserider]

Well, hopefully you will give some strong consideration to putting a gasification boiler in an outbuilding or look at one of the new Econoburn OWB style units, as opposed to a "traditional" OWB smoke factory...

That said, if you have designed your duct system to be code compliant with your conventional furnace, you should have no problems with a W/A HX in the same plenum - remember that the water going into the HX might be at 180°F or so, but not all of that heat will transfer to the air. The air coming out of your fossil furnace isn't going to be any cooler than what you'd get with the HX, so anything that meets code for one, will work on the other...

Gooserider

 

[boatboy63]

Thanks Gooserider. Being in the mountains of northeastern TN has it's advantages when it comes to "codes". My area is very rural and codes are pretty much considered as a guideline. When someone in this area does a room addition, the word "permit" is never used. Of course, if you build a new house, that is a little different. I am somewhat of a MacGyver by nature meaning that I come up with ideas that nobody else would ever dream of, but do them in a safe way. I rewired my house from a 100 amp box to a 200 amp box and connected it all myself. I am not afraid of voltage, but have the utmost respect for it as I deal with 3 phase power every day at work. I only take my cars to the shop for alignments and tire balancing. I repair and rebuild my own engines, both automotive and boat. I build my own room additions and could also build my own house if need be. I guess you have heard of an old Hank Williams Jr. song named "A Country Boy Can Survive". That is pretty much my motto.

As far as a gassifier, I would love to own one. I would even love to sell them, as I don't even know of any dealers within 100 miles of me. It has crossed my mind several times. There are plenty of trees around here along with wood burning homeowners. Many of these are complaining now over the highest electric/utility bills they have ever had. This would be a welcome idea to many people in my area. Before the end of December, I had no idea of what a gassifier or even a OWB was. I can honestly say that since then, I have been doing my homework and believe I know more about them than 99.9% of the people in my area. It is a simple idea that can and will be much improved in the upcoming years as people are looking toward alternative fuels.

As I said before, I would love to own a gassifier, but with the current economy and me being the only "bread winner" in my household, I will probably design and build my own. I have a mig and love to build stuff with it. I just wish I had known about this 6 months ago before it started getting cold around here. It has been said that for our area, this has been the coldest winter since WW2. My electric bill just went up from $220 last month to $330 this month. The bad thing is that we use a Buck stove for about 14 hours a day to heat the front 1/3 of the house where we live during the day. It isn't big enough to heat the whole house and with the layout of the house, you can't get the heated air to circulate back there. Also, with the condition of my ductwork now, it ends up cooling the house when you try to just use the furnace fan. Hopefully after this weekend, it will be better with the ductwork in the heated areas instead of under a cold house.

 

[Gooserider]

Well be safe if you build something on your own - seems like the Seton style boilers are probably among the best options in that direction, might be worth looking at. Also I'd try to do something that used thermal storage as that can help with both burning cleaner, and possibly boosting your efficiency a bit during the "shoulder season", which I understand is a lot longer in TN than it is up here in MA...

Gooserider

 

[boatboy63]

I have already been reading up on Seton's the past few days. I just can't see paying $500 for plans. I have seen some pics of them but would like to see more. I would also like to see some up close and personal pics of the Econoburns. I have already been running some plans thru my head about how to accomplish a downdraft situation. I remembered something like Bernouli's Law which relates to how a carburetor works by passing air thru a venturi and it causes a suction effect to draw fuel thru the nozzles from the float bowl. I studied all that about 15 years ago but still remember some of it. It actually made it up in the 40's today and felt like summer. Haven't seen the 40's in over a month. I have been thinking about something in the neighborhood of 150-300 gallons of storage for starters. I like my mig so well, I may be forced to build a 2nd one after I learn the tricks of the first one.

 

[pybyr]

If you have a knack for welding, look around for some of the folks here who've done "Garn-like" DIY models; I suspect that the proportions are still very important, but probably no where near as complex as a downdraft (where you not only need the metalwork, but refractories with internal passages, etc.).

EDIT- see here for one example (I've always thought it'd be interesting if someone tried a 1/3 scale concept on similar principles, maybe with a different form factor for the tank, but same concept of chamber/ exchangers...):

https://www.hearth.com/econtent/index.php/forums/viewthread/11262/

 

[boatboy63]

Now I can see what NASA does with those old rocket boosters. ;-) I think I saw something on ebay about refractory cement/mortar that will handle up to 3000 °F. You simply mix it up and apply it like cement/mortar. That would be an easy way of making custom refractroy forms or you could use it to run tubing thru, if needed. I will be doing something in the next couple of months for sure. Just want to learn all I can before I put the time and money into it.