Air temp leaving wood furnace?

[bicylindrico]

What would you guys say is the approx. max temp leaving your wood furnaces? Do you recommend a certain length of rigid metal ductwork before utilizing an insulated flexible insulated duct? I think most of the insulated ducts are rated at a max temp of around 200 degrees or so. I have seen installs here on the site that are running the insulated duct pretty close to the furnace.

What do you think about the surface temp of the exterior of the unit when running full of wood? I see regular unbacked fiberglass is good for 1000 degrees. Has anyone insulated the exterior of the unit in an outdoor install?

I am ready to start my outdoor install and will post some pics as I go along. If there have been any lessons learned I would love to hear them.

 

[blis]

IMO flexible ducts are big no-no on flues becouse they have rough surface on the inside which means tons of creasote and other crap will stick there, just stick with rigid metal ductwork...

 

[laynes69]

Hes talking about flexible insulated ductwork. No flexible ducting is not allowed due to the potential of high heat in the ductwork.

 

[david78]

My OWB typically runs about 120F coming out of the vent. All of my ductwork is the flex pipe. Works fine. Are you talking about an OWB or a regular wood furnace?

 

[CrappieKeith]

What would you guys say is the approx. max temp leaving your wood furnaces? Do you recommend a certain length of rigid metal ductwork before utilizing an insulated flexible insulated duct? I think most of the insulated ducts are rated at a max temp of around 200 degrees or so. I have seen installs here on the site that are running the insulated duct pretty close to the furnace.

What do you think about the surface temp of the exterior of the unit when running full of wood? I see regular unbacked fiberglass is good for 1000 degrees. Has anyone insulated the exterior of the unit in an outdoor install?

I am ready to start my outdoor install and will post some pics as I go along. If there have been any lessons learned I would love to hear them.

max.temp in the flue or ducting?

A flue exhaust can vary depending on how fast(water column) your draft is set. If you have a barometric draft regulator you can adjust your flue temp by changing the speed of the draft.
These temps should run at .03-.06 inches depending on the make of your wood burner.
Some folks have flues that draft really well and can afford a .02" speed.
the idea here is to stay above the point that flues gases will condense which is 250 degrees.
At .03" a flue gas temp will be at or around 400 degrees.

When it come to ducting ....there's a potential a guy could have 500-600 degrees if there were no blower to cycle.
In the case of a gravity set up this may occur normally.
With a fan and limit switch operating a blowing the setting on most furnaces cycle in the 150 degrees (on) and 125 degrees (off) with the limit side of the switch triggering the blower to run non stop in the event of a over heat situation at 250 degrees.

Your ducting should ALL be made of metal.
The 1st 6 feet needs to have 18 inches of clearence unless it was tested to be closer as code allows.
After 6 feet there typically ...due to physics...is a heat drop.
So in the code it allows you to go too 1 inch of clearence until the run makes a 90 degree turn which at that point your clearences dissapear.
Let me reiterate.
All of your ducting needs to be made of metal.
Not some or most...ALL.

 

[bicylindrico]

Sorry, I have not been able to sign in the last 2 days to respond.

Yes, I was talking about the heated air supply ducts on a forced air wood furnace. I have decided to definetly use hard duct for the heated portion throughout. I had planned to use the flex on the cold air side but now that I think about it I dont think there will be much benefit other than $$. If i find there is substantial temp transfer i can always wrap the duct as well with the unfaced fiberglass insulation. Again it should be good up to 1000 degrees. I will be using 10 inch duct which has been hard to find.

I dont really understand to heavy emphasis on the hard duct for the cold air return. Do the manufacturers not supply a filter box with some models that house a flammable filter? What about the wiring to the blowers? If a down blower can cause the system to gravity feed, wont there be cool air entering the return side of the furnace? Thus not being heated to the potential 500 degrees on the output side? If this condition was to occur would it make sense to remove the filter to allow as much gravity feed as possible?

What do you guys think about insulating the exterior of the furnace to maximize efficiency?

 

[CrappieKeith]

Go to this link for all of your ducting needs.
It has the 10" listed but if you look to the left you'll see all sorts of boots,tees,elbows.....everything you'll need.
http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=duct-kdpiping


your a bit mixed up about gravity heat.All of your ducting on the hot air side can not have any downward turns.

 

[bicylindrico]

I guess what I meant to say is that while high temps can radiate into the ducts from a down blower and that is why you stated ALL ducts have to be metal, then why is it allowable to have flammable materials located right at the furnace ducting? Sounds like it shouldnt. Though this is a kit from the manufacturer. What you say makes total sense to me and I can see how that could be dangerous.

 

[CrappieKeith]

As I stated in a recent post today.....
Your ducting should ALL be made of metal.
The 1st 6 feet needs to have 18 inches of clearence unless it was tested to be closer as code allows.
After 6 feet there typically ...due to physics...is a heat drop.
So in the code it allows you to go too 1 inch of clearence until the run makes a 90 degree turn which at that point your clearences dissapear.

 

[bicylindrico]

I read your post the first time CrappyKeith. Doesnt really answer the question. I am not looking for code just real world experience. I think most installs on this forum would have a code violation or two and I am sure mine likely will too. I am more concerned with safety and function than code compliance as I dont believe what a code book states is the only way something can be safe. I was just looking for some insight as to how others have installed their furnaces outdoors and how satisfied they are with their particular setup.

 

[laynes69]

If your talking about the return air kits for some furnace, they are allowed because you have a blower sucking air through the filters. Your supply side is the side that will get hot. When most furnaces lose power, it is recommended to remove those filters and I know the old furnace to lower the damper in an outage. Even with no blower and gravity heat, you can achieve some high temps quickly, especially if you have restrictions in ducting. Having combustibles (Furnace Filter) by the furnace isn't dangerous when a furnace is installed correctly and is operating according to the instructions. Its when power fails, they try to heat their homes without a blower and rev up the fire. I have seen melted wiring inside juction boxes because of people trying this, along with melted limit/controls. If its not meant to operate in gravity, then it should be dampered back or let the fire die out. What type of furnace are you installing outside? I guess its using common sense. You could install a heat dump at the furnace outside in case of an outage. This would help remove excess heat from the furnace.

 

[Straightgrain]

The residential code (in most states) allows up to 250 degrees for metal, 125 for gypsum ducts-flame spread index of no more than 200 degrees.

 

[CrappieKeith]

I read your post the first time CrappyKeith. Doesnt really answer the question. I am not looking for code just real world experience. I think most installs on this forum would have a code violation or two and I am sure mine likely will too. I am more concerned with safety and function than code compliance as I dont believe what a code book states is the only way something can be safe. I was just looking for some insight as to how others have installed their furnaces outdoors and how satisfied they are with their particular setup.

Sure it did or does.....do not have anything combustible touching your ducting within 6 feet of the furnace.
Once your run makes a 90 degree turn ....touch your ducting all you want with whatever.
If you were looking for another answer I'm sorry......oh make sure the wiring in the blower is class B rated....high temp.Motors can and do start on fire.

You had made mention of insulating your furnace. My advice is to get a shed....insulate the heck out of it. Major heat loss will occur otherwise.

Please understand many homes have burned down due to people installing furnaces unsafely....ergo the code. I have no investment in you other than I am trying be be a source of good intel. Good luck Sir!