How to monitor burn for a wood furnace

[Chuck the Canuck]

Hi. I have a newmac wb100 wood furnace up in my shop. There is approx 2' of horizontal flue coming out of the back of the furnace, then a 45 degree elbow which takes the flue straight up and out through the attic and roof. I don't have any problem with draft (no smoke ever coming out the door or anything), but I'd like to know if there is a proper way to monitor the burn in a furnace like this in order to achieve optimal heat and burn times. I have a thermocouple probe mounted in hole that I drilled into the vertical section of my flue about 16" above the 45 degree elbow, wired to a digital readout, that gives me some idea about how hot the flue gasses are running. On startup I generally run the forced draft fan for about 15 minutes (with the combustion air draft slide fully open) so that I can get the flue temps up to 600 or 700 degrees Fahrenheit, but I notice that as soon as I shut down the forced draft fan the flue temps start dropping (pretty fast too) and when I start closing the combustion air draft slide (after around 30 minutes) towards the minimum setting the temps really drop (I'm usually looking at between 200 - 300 degrees Fahrenheit). The only way to keep the flue temps up and burning nice and hot is to leave the combustion air draft slide pretty much open, but then the furnace just gobbles the wood like some kind of glutton Is there some other way that I can monitor the burn on this furnace? For instance how can I use my IR gun to monitor surface temps on the door or on the plenum to figure out that I've got a good burn going? Or maybe I need to be putting an Outdoor Air Kit on the furnace to help with combustion air??? (my shop is a pretty leaky old building, but the furnace is located in a kind of alcove nook enclosed by cinder block walls...... Any help or advice is certainly appreciated.

Cheers,

 

[TCaldwell]

no knowledge of this stove but, 6-700deg flue temps are way too high for a efficient burn. The higher the flue temp does not mean higher effiency! With the fan on high you are burning the wood too fast, you have gone beyond the heat exchange capability, that extra heat up the flue is wasted energy. I would suspect 350-450deg would be a more reasonable target to lower the fan speed to maintain. By doing this your fires will last longer and burn hot enough to keep your stove clean.

 

[sloeffle]

I have a PSG Caddy and I rarely see temps over 200F on my chimney mounted thermometer. It is on my single wall pipe after the 90 degree turn. The Caddy also does a secondary burn of the flue gasses so we tend to see lower temps.

IMHO it sounds like you have too much draft. Have you put a menometer on your pipe to see what your draft is ? I put a barometric damper on my furnace too slow my draft down and that has helped even more with burn times.

http://vogelzang.com/index.php?route=product/product&filter_name=Barometric&product_id=152

Scott

 

[laynes69]

Flue temps don't have to be in a burn zone on a thermometer, if the unit is burning clean. What you want to monitor is the chimney, if it's smoking, then increase the air. If it's not smoking and burning well, reduce the air and then check again. You want to find that line between burntimes and a clean burn. Not knowing anything about your furnace, it's hard to say if your flue temps are normal or not. Some units run higher, while more efficient models run lower. Scott has the right idea, you may just have too much draft.

 

[TCaldwell]

the manual says the primary air shudder should be about 50% for wood and about 100% for coal. Have you tried starting the fire at 50% and monitor the flue temps, I suspect the 100% is too much primary, two possibilities, one that by the time you lower the fan speed the wood load is too small to sustain a high flue temp or your wood is wet and requires more primary air to get a good fire and when you reduce the fan speed or primary air shudder the flue temp decreases because of wet wood. If you havent made any adjustments yet, burn a load and when the flue drops check to see how much wood is left. If there is more than you would think than the wood is probably wet, if there is virtually coals left then your wood is good, try reducing fan speed or air shudder.

 

[Chuck the Canuck]

Hi. Thanks for all the thoughtful replies. The wood I'm using in the furnace is the same that I use in my wood stove for the house, and it's nice and dry (16-18% on fresh split face). I've just been learning how to use the stove this winter (from the good people of hearth.com) and that's what got me thinking about the furnace. Although I've been running the furnace for the past year and a half, I was never really taught how to properly operate it except to be told that I needed to have a nice hot flue to avoid creosote.

I don't have any forced air fan to help me start up a fire in the stove down at the house, and I never have a problem starting a fire in the stove, so maybe I don't need to use it to run the furnace either.... I will try running the furnace without turning on the fan and turn the combustion air draft slide (is that what you mean by the "air shudder?) to 50% as soon as the fire gets going well, and as for monitoring the burn I'll keep my eye on the flue temp (look for 200-400 degrees F???) and see what the smoke looks like coming out of the chimney.... How's that sound for a plan?

Cheers

 

[Chuck the Canuck]

IMHO it sounds like you have too much draft. Have you put a menometer on your pipe to see what your draft is ? I put a barometric damper on my furnace too slow my draft down and that has helped even more with burn times.

When the installer (a heating contractor) replaced an old electric/wood combo furnace (the electric was never connected) with the newmac he installed a barometric damper. However, back then the flue and chimney was 6" (the newmac is 8"), not to mention it was also ancient, rusting out, and had probably seen a good many chimney fires... so when I had a stove company come out and replace the entire chimney with 8" all the way from the furnace up and out, they took the damper out. They said I didn't need one.... What do I know??? Maybe they should have left it in......

PS, I don't have a menometer.....

 

[Chuck the Canuck]

I have a PSG Caddy and I rarely see temps over 200F on my chimney mounted thermometer.

Are you monitoring the outside surface of your flue with a magnetic therm, or are you measuring the actual gas temps inside the flue?

 

[sloeffle]

I am using a magnetic thermometer on the single wall pipe before it transitions to double wall. In the two plus years that I have owned the furnace I have never seen it reach the "burn zone". If your furnace does not have secondary burn then I would expect to see higher flue temps than what you see with the Caddy.

Scott