Heat duct temps

[pistonslap]

I am heating my whole house with a woodburning furnace tapped into my existing gas furnace ductwork. What are safe temps for the ductwork? I just stuck my fluepipe surface temp gauge on my main duct. Right now they are fairly warm and the gauge says 120. I have had them a good bit hotter but it's late and the night isn't that cold, so I don't want to really want to crank it up. Also, sometimes my gauge is in the low temp range that is supposed to be creating creasote. But when I go outside and look at my chimney there is no smoke. Does it have to smoke to create creasote?

 

[begreen]

When you say main duct, about how far from the wood furnace is this? If directly above, that would be pretty normal. Typical forced air limit switches are set at about 200 degrees. Above this temp, they turn the burner off. This switch is usually right above the heat exchanger, in the main plenum. It has a probe that is directly in the airstream. I would suspect that a stove surface thermometer might not give an accurate reading of interior duct temperature, especially in the 100-200 degree range. A probe thermometer will work better. But a reading of 120 degrees on the surface of the main plenum above the stove would seem ok to me.

 

[elkimmeg]

Begreen I differ with you: 200 degrees for hot water is normal, but most hot air sysytems never approach 140. The 120 is normal At 200 degrees the plastic iner parts of flex duct would have
melted away, they are rated up to 160 degrees

 

[pistonslap]

I put the gauge about 3 feet away from the woodburner. I noticed that the ductwork gets pretty hot from forced air, although not as hot as from the woodburner and not for long periods of time. My ductwork is all sheetmetal, my concern is how hot it can get for long periods of time before it becomes a fire risk.

 

[begreen]

Begreen I differ with you: 200 degrees for hot water is normal, but most hot air sysytems never approach 140. The 120 is normal At 200 degrees the plastic iner parts of flex duct would have
melted away, they are rated up to 160 degrees

That's why the high-limit sits right above the heat exchanger. I have to get to work, but I'll dig up the Honeywell specs. Many are adjustable and can be set lower, but 200 from the factory is what I remember. I would be surprised if 200 degree air would melt the plastic, but I will test empirically. I have some flex duct and will put it in boiling water (212F) to see if it melts.

 

[Roospike]

I always understood the normal for a heat Pentium for a fan per a wood furnace is "ON" at 160°-180° and fan "OFF" at around 80°-90° . Thats how i used to run my wood furnace as well.
Now what the temps are going to be in a floor vent will depend on how far it is from the furnace and if your checking right as it came on vs just about to go off.

New wood furnaces have come out over the years so the heat exchanger temps could also have change over time and per brand.

I run mine at 160° "ON" - 80° "OFF in the Fall & Spring / 180° "ON" 95° "OFF" in the Winter.

 

[begreen]

I checked a few hot air furnaces and found high limit switch ranges from 180 to 210F. John at Charmaster (great, quick response) wrote me back that their switches are set to 110 - 170 - 200.

Anco FlexDuct is UL Rated Operating Temperature: -0°F up to 200°F, -18°C up to 93°C.

 

[laynes69]

I have my wood furnace set at 140 on and 85 off. It runs constantly with no problems. I seem to get more heat extracted from the furnace this way. With your temps, 200 is the normal limit for a wood furnace. With possible warmer temps, this is why its important to follow the clearances for the ducts to the floor joists. Also making sure that none of the cold air returns are panned inbetween the joists. If there ever was a power outage and the furnace overheated its important to have the air space to help protect the ductwork from the joists. Each furnace manufacturer has clearances for the ducts.

 

[elkimmeg]

ATCO
Now if the centers are flexible metal yes some will have a 200 degree maximum Atco direct cut and paste from their product sheet this is the economy class dust most frequently used


UPC #086
25' Insulated
UL 181
Class 1 Air Duct


Product Data
Maximum Operating Temperatures:-20°F to 140°F Continuous (@maximum pressure)

Note we are talking about flexible ducts that have poly interiors the ones commonly used in residential construction. I knew I have read the product sheets in the past and knew there was temps limit around 140 degrees

Don't believe me check it out yourself remember the poly core #76

(broken link removed to http://www.atcoflex.com/specs.html)

 

[begreen]

Anco, 4625 or 5625, which is what I installed (Not a typo) is not metal lined. The ATCO product specs are listed at 140 degrees "at maximum pressure". This is rare, and I wouldn't design a system at max. Otherwise, their #086 temp is right up there with Anco. Here's the spec:
Maximum Operating Temperatures:
-20°F to 140°F Continuous (@ maximum pressure)
-20°F to 180°F Continuous (@ 2" pos.w.g. max.)
-20°F to 250°F Intermittent (@ ½" pos.w.g. max.)

http://www.ancoproductsinc.com/flex/flex_4625.php
(broken link removed to http://www.atcoflex.com/specs.html)

I'm pretty confident that, under correct operation with properly operating limit switches, the flex duct would not be at risk.

 

[elkimmeg]

International Mechanical Code

603,3.5.1

Floor pan joist bay and gypsum ducts,
shall be limited to return air systems the air temperatures do not exceed 125 degrees
and the gypsum or other combustibles surface temperatures is maintained above the airstream dew-point
temperatures.

Again s most homes have return systems, that incorporate floor joist pan off bays, that are constructed with combustibles.

Another application where potential code violations exist using a return system for move heated s air from the stove area

In my area there are two common duct manufactures. The 140 limit on flexible ducts is the standard. most used duck work, I find in all inspections

the two major duct manufactures found in my area and ATCO ah Hart& Cooley

Again there pdoducts page linits the interior duct temps to 140 degrees when using a poly iner liner
(broken link removed to http://www.hartandcooley.com/flex/all_flex.htm)

you may have gotten luckey and had a better range duct installed but my inspections experience this is the products I see they are also the products sold at home crapo
Another reason moving air Hot air from above the stove is not a good idea when your system consist of flexible duct work

 

[begreen]

Elk - we're talking about - supply- side here, not return air. The high-limit switch is on the - supply - side plenum or on the furnace itself directly above the heat exchanger. Why bring up the return air side at all? If a house is over 125 degrees in the return air ductwork there are more issues to be worrying about than flexduct.

As noted you are only quoting the spec at *Maximum* pressure. No system runs at maximum pressure. At 2" sp, the ATCO flex ducts are rated at 180F @ 2" sp, not 140. Even at 2" sp, the system would be blowing a gale out of the vents. Our system is running at a generous .9 sp. Now it's Hart/Cooley. Why not stick with the original examples? Also, all furnace systems run intermittently, that is the true spec. These are not continuous ventilation systems.

By this argument, many gas/oil furnaces inspected that have flex ductwork is illegal. Because the high-limit switch rating on these oil and gas furnaces is the spec I listed, 180-210 industry average. Now a normally operating system shouldn't reach that temp, but it is not at all unusual for the temp to be greater than 140 measured above the heat exchanger. Stick a thermometer in one after it's been running for awhile and check it out. On some systems 140F is when the blower fan is set to turn on, though 125 is more typical.

edit - There are not many true furnace installation manuals posted on the web, but ThermoPride has some excellent ones. Go to pages 49-50 under - Setting temperature rise. Normal operation (at .5 sp) calls for a 55-85 degree rise. If the interior of the house is at 70 and the rise is say 75 degree, that = 145 degrees in the supply plenum.

(broken link removed to http://www.thermopride.com/manuals/mg-993.pdf)

some typical high-limit switches -
(broken link removed)
(broken link removed)