Why did you install an outdoor air kit?

[womaus]

Learned something new again today:

Tramp air - Any air that enters (infiltrates) the furnace through leaks. This air may be measured by the O2 analyzer and often contributes to the burning of the fuel.

 

[skibumm100]

Learned something new again today:

Tramp air - Any air that enters (infiltrates) the furnace through leaks. This air may be measured by the O2 analyzer and often contributes to the burning of the fuel.

Or, in most cases (in my experience on balanced-draft industrial coal stoker-fired boilers), does NOT contribute to the burning of the fuel. Most of the leaks I've found are well after the combustion zone in the other passes. A lot of times that is why you can't run anywhere close to your "target" excess O2 level. You see a high O2 reading but when you try to cut back on the forced draft fan, the fuel bed doesn't have enough air for proper combustion and it clinkers up. The induced draft fan on the back end of the boiler is pulling a slight negative pressure on the boiler casing and air leaks in all over the place. The O2 meter picks this up and that's why you see high O2 readings, not because you have too much combustion air. The O2 meter is usually located closer to the induced draft fan than the combustion zone on the boilers I'm familiar with. So the air-wash on a pellet stove is after the combustion zone on pellet stoves (which is in and just above the burn pot) so the combustion fan (induced draft fan) has to handle all of the tramp air, which is over and above the air needed by the fuel to burn completely.

 

[Monica in France]

3KW is a pretty small heat input. That's only 10,200 BTU/hr. Is this for our theoretical 10,000 ft^3 house?

Thanks for checking out my arithmetic.
I wonder if it the fact that the entry air flow is so small that the polemic over the OAK exists. (only applicable to a well regulated stove obviously).
Can we actually feel a draught so small ? My feet say Yes but my head says No.

For the 3 KW , I just took the figure corresponding to my usage this year . In general It has been mild and wet over here !
My house is probably about 100 m3 x 2.5m = 250m3 ~9000 ft^3.
But if we double the heat it is still under 6 cu ft/min - a figure still intuitively low.

But is it linear ? I get the feeling that the lower the power the more 'excess' air you need.
Up to now I have put this down to the quantitative variation in the pellet feed at low power : you have to cater for the maximum not the average.

 

[skibumm100]

Thanks for checking out my arithmetic.
I wonder if it the fact that the entry air flow is so small that the polemic over the OAK exists. (only applicable to a well regulated stove obviously).
Can we actually feel a draught so small ? My feet say Yes but my head says No.

For the 3 KW , I just took the figure corresponding to my usage this year . In general It has been mild and wet over here !
My house is probably about 100 m3 x 2.5m = 250m3 ~9000 ft^3.
But if we double the heat it is still under 6 cu ft/min - a figure still intuitively low.

But is it linear ? I get the feeling that the lower the power the more 'excess' air you need.
Up to now I have put this down to the quantitative variation in the pellet feed at low power : you have to cater for the maximum not the average.

I would agree that the amount of excess air varies with firing rate in most applications. There are physical limitations in the design of the stove and burn pot and everything is a compromise. From what I have read, the excess air is generally less at higher firing rates, meaning it's closer to "ideal". It pretty much has to be that way because you have to make sure the burn doesn't go "lean" at high firing rates or bad things can happen when dealing with things on an industrial scale. I would think emissions rates would have something to do with it also. I agree that the numbers say one thing but the physical feel of the cold draft say something else.....

 

[Pete Zahria]

I agree that the numbers say one thing
but the physical feel of the cold draft say something else.....

So which do you go by?
I know what my choice is.........
All of these calculations are interesting, and complex. but it all boils down to the above.
And so much more simple.

Dan

 

[skibumm100]

So which do you go by?
I know what my choice is.........
All of these calculations are interesting, and complex. but it all boils down to the above.
And so much more simple.

Dan

I agree Dan. I just find the results of the exercise interesting. I've been around long enough to know not to argue with success. I wish I could read the airflow signal from the sensor on my stove. That would be better than cranking out theoretical calculations.

 

[checkthisout]

OK Monica. Let me work on this. The make-up air flow still sounds low but I'll do some digging to see where it takes me. You are right that CFM in exhaust does not equal CFM in the OAK. This ratio will vary significantly because the heat transfer efficiency varies a lot between stoves and firing rates. I suspect that there is a pretty high fouling factor in most stoves under actual running conditions. The conversion factor of m^3/hr to CFM at 35 is correct. I cheated as I use both metric and imperial systems at work so I have ConvertPad on my phone. When I get to a conversion that's not on ConvertPad, I do it the old fashioned way....by hand.

Your stoich ratio number for wood is pretty close to what I have found on the net. It ranged from 4.58 for generic biomass to 5.75-6.3 for wood. My combustion book says 6.4 so I think that number is good. That's for air, not O2.

According to my Fossil Power Combustion book, "theoretical air" (A), or what's referred to as "stoich", for dry wood ranges from about 710-720 pounds of air per MMBTU of wood fuel. That does not include any excess air.

HHV of various species of wood, dry (in BTU/lb) of 8200-9800 BTU/lb.

Using pellets at 9000 BTU/lb you would need about 6.4 lb of air to combust 1 lb of pellets. At 30 deg F that would be about 79 ft^3 of air. Now add the excess air (15% excess O2) and you get 91 ft^3 per pound of pellets.

Just for comparison, the amount of air needed at 70 deg F (heated room air, not OAK) is 85 ft^3 w/o excess air and 98 ft^3 with 15% excess O2.

So that's just for 1 lb of pellets. Now multiply by the amount of pellets you burn per hour.

If we convert this to your example:

0.8 kg/hr = 1.76 lb/hr

1.76 X 91 ft^3 OAK air = 160 ft^3/hr

160 ft^3/hr = 2.7 CFM

Your math seems to check out. Surprising isn't it?

BTW, if it's cold, I burn twice that much. I also wouldn't be surprised if the efficiency isn't quite a bit lower than 80% in real life.

Regarding flue gas volume vs OAK air volume:

at 30 deg F (-1 deg C) air density is 0.081 lb/ft^3
at 70 deg F (21 deg C) air density is 0.075 lb/ft^3
at 300 deg F (149 deg C) air density is 0.052 lb/ft^3

......so on a mass flow basis this would make 2.7 ft^3/min equal to 0.22 lb/min at 30 deg F
and 0.22 lb/min at 300 deg F would be 4.23 Ft^3/min (CFM), so that's an increase of 56% in volume as the air heats up.

Now, something is rotten in Denmark. If most combustion fans are running at something like 20-30 CFM, per the experts, what does that tell us? Maybe there's a lot of tramp air coming in through the air wash for the glass? Maybe sneaking in somewhere else? It's hard to believe you could lose that much. I suspect most pellet stoves run at a much higher excess air number. For a well regulated industrial burner you would need 10-20% excess air to run efficiently. Most pellet stoves are pretty crude.

My apologies to the Danes for bringing them into this as this is pretty much a Franco-American thing.......

Anybody see any mistakes in my analysis?

Nope, looks good.

In my opinion, you get the best performance if you use exhaust pipe that was the fresh air inlet concentric to the exhaust I.E. Selkirk Directemp. This preheats the incoming air which means less fuel burned for the same net output into the house. It also helps in climates where temps dip into minuses and apparently causes issues if you are using outside air.

 

[Edsson]

Question, The air inlet opening on my Quad E2 is 2". most all oak kits that I've seen are 3" or 4". I am considering installing an OAK in the off season, if they have a 2" inlet opening then that should be all that's necessary right. My plan is to run a flexible hose from the back of the stove, across the brick hearth through a hole in the carpeted floor. flex over and out through the sill plate to the outside. Any issues with that?

 

[Pete Zahria]

Question, The air inlet opening on my Quad E2 is 2". most all oak kits that I've seen are 3" or 4". I am considering installing an OAK in the off season, if they have a 2" inlet opening then that should be all that's necessary right. My plan is to run a flexible hose from the back of the stove, across the brick hearth through a hole in the carpeted floor. flex over and out through the sill plate to the outside. Any issues with that?

I am not an expert, so you can take this for what it's worth..
If your stove usually takes a 2" BUT, you have to travel a little distance...
AND you have to have some turns in it..
I would put an adapter on the 2", and go with 3"..
But that's me. YMMV..

Dan

 

[Don2222]

I just hope it is not cooling the exhaust gases to the point at creosote forms in the exhaust pipe.

Eric

That is only a problem for wood stoves Eric. DT works well for most pellet stoves except AES magnums.

I just put in my 3rd DT system last Sunday. The warm dry air keeps the ash dry and fluffy and keeps the stove from rusting too!

 

[Lake Girl]

Dan has a point ... EVL works the same for an OAK. 90 = 5; 45 = 2.5; horizontal 1' = 1; vertical 1' = 1/2 Edsson, do the math before you start cutting holes!

 

[womaus]

I have an XXV in the front room and a Qudra Fire Isle Royal in the dining room. NO OAKs.

Eric

Let's stir up this old chestnut thread...I was going through it looking for a good referral point for current questions, and ran across this reply from the OP of this thread.

Looks like kinsmanstoves has 2 pellet stoves in his house, both without OAK.

I'll bet 10 bucks if he installs an OAK on one of them he won't need to run the other.

Heh heh heh....

 

[BrotherBart]

Let's not.