What are effects of installing smaller flue?

What size if the flue outlet of the stove?

Are both stoves hooked up? And to separate chimneys?
I have 27' of insulated liner and no problems. Draft is strong, but controllable.
Have you checked the usual stuff..... door gaskets, ash pan gaskets etc for leaks?
As far as I have learned, smaller flue size = stronger draft in many cases.
How bout an inline damper?

It seems unlikely that both stoves would have air leaks (ash pan, door gasket, etc.) but check each out to rule this issue out. Maybe it's the 28' of chimney at 3000' that creates that nice draft. Let's see.. I am at 935' elevation so you have a chimney that is roughly 2100' taller than my 21' stack!! I'm surprised that the dealer is stumped for an answer. Try the manual stove pipe damper. Anyone know how well they can be installed in double wall?? Mine old one was in 8" single wall.

Amazingly enough, some of this may relate to the design of the stoves and the wood you are burning. From the location, I assume softwood? This has a high resin (gases) content, and is extremely difficult to burn low. Some woods have more coals than gases, and therefore are easier to slow. Consider this point.


Before you go too far, try two things:
1. Is there any way you can rake coals so that the wood burns from one end to the other? If the stove is east-west (VC, etc.) in loading, try raking coals to BOTH ends before loading. This might make the stove burn the load from the ends toward the middle. Also try larger splits if you have them.

2. Can you install a barometric damper on the stovepipe? This will allow you to mimic most any chimney, and although it does send room air up the stack, in your climate that is not as big of a deal.

I agree with Craig.

Also, if the same amount of air goes up a 6" as does an 8" then that air has to move at a much greater velocity. Is this what you want?

We have a problem with the stove going nuts if we burn, say, all soft maple or anything else that burns hot. We learned to put both maple and ash in at the same time. On warmer days like we had this week we can just put a couple small pieces in instead of loading it full. That thing would cook us out in no time at all if we filled it now. Also, nights getting warmer too (20's) we can not put in a full load else we really get cooked.

We have to start with the obvious - that a stove is not a toaster - meaning results will vary in every situation.

Consider some wood burning inside a metal box. The wood burns at temps which vary throughout the load from 1200 to 1800....in the bed and flames. When these temps are considered, then how can the outside of a finned metal plate sitting above said fire be kept at 400F ?

The answer is that it probably cannot.

Perhaps a heavy soapstone stove can be kept at such a temp, but a "batch burning" wood stove with nothing (baffles, etc.) in between the hot fire and the top griddle will have a difficult time being cut down to this temp.

It may be that a fuller load will LOWER the top temp, as the radiation from the wood burning on the bottom will be somewhat stymied by the wetter and cooler wood on top of that.

To answer the question, neither a 6" or an 8' pulls harder for a given height in general - but my guess is that given a closed stove, a 6" (smaller) flue will draw harder if everything else remains the same. That is because it would contain hotter gases which rise faster. But the 8" would be CAPABLE of venting a larger load of wood (BTU's), although the output of your stove does not even approach the full capacity of a 6".

In other words, anything larger than a 5 or 6" in most space heating wood stoves is ONLY for venting the smoke when the stove is loaded or doors opened.

Marc, you are right.

http://www.engineeringtoolbox.com/chimney-sizing-d_175.html

Taking this back to the real world, for a fuel burning appliance with an input of 30,000 or less BTU - which is the heat output range you are trying to achieve - my opinion is that a 6" will draft stronger. In other words, since a 25 foot 6" chimney can accept 200,000 or more BTU's....and you want to operate at 1/10 that (to get 400 degree top temps).....then I would throw all those calcs right out the window!

Meaning that I think that it will make no difference at all to reduce such a stack to 6".

So, is the real question:

Given a closed combustion appliance burning wood with a "x" combustion efficiency and "x" heat transfer efficiency....etc. etc. ???

This sounds to me like trying to figure out if the amount of water coming out of my 1/2" hose bib would be different if the house feed was 1" or 1 1/4". Since the feed is already many times larger than what is needed, I don't think it would make much difference!

Unscientific, to say the least, but see "the stove is not a toaster" for that one. Common sense will dictate that if heat rises and you have a 1500 degree fire a few inches underneath a cast iron plate, that keeping that plate at 400 F is going to be a tough one. Ashes insulate the bottom of the firebox, double wall plates insulate the sides and rear.....so where the heck is all the heat going to radiate?

From the same toobox:
It is a general rule to provide 1100 mm2 chimney area per 1 kW boiler rating.

1100 mm2 is about 1.75 sq inches
1KW is about 4000 BTU.

so using 10KW as an example (less than 40K BTU), a chimney of 18 square inches would be enough to vent this. Anything larger serves no purpose in many cases.

The only reason VC has a 8" flue is because of the large door loading opening. That makes it a "fireplace" in effect and then it needs to use the "rule" about the flue being 1/10 or so the area of the front door opening.

Marc, I have experience with both of those stoves and here is what I think. The Everburn stove is a piece of work. But, if you have the right draft conditions, it can burn like crazy. The Defiant CAT stove is a good burner and you are actually getting good performance from it. The problem is you are loading it in a fashion that causes it to burn hot. The CAT stove wants to burn at 500 and loves 700 if the fuel is there to support that. With the air shut down all the way, the stove is still going to like 500 stovetop.
The VC Defiant is a monster heater. The materials they are using for its construction aren't acceptable to me personally, but when it works as designed it is a HOT running stove that will heat you completely and that's what its doing for you.
As Craig points out, the soapstone will keep a lower stovetop temp and radiate it for a long time. I moved to the soapstone after deciding that VC was just not doing it for me personally.
If you install the 6 inch pipe you will most likely slow your unit down, but it is still going to want to operate hot because that is what it does. 400 for that stove is almost smoldering.
And, the problem you get is that you want to load a full load to get a long burn and the stove burns at 500-700 (sometimes hotter). If you load a smaller load, it won't burn as long, but it will burn closer to what you desire more of the time; it will get up to hot for a while even with the small load.
So, bottom line, you bought a hottie, you got a hottie. Most folks would die for those conditions.
I actually liked my CAT version very much and would still have it except that I went from it to the Everburn and when I realized what a mistake that was, I decided to go to burn tubes and also found that the soapstone was very nice given what I personally wanted.
There you go, my 2 cents.....

Marc said:

I have had two new stoves in our home and both, one CAT and one non-CAT w/secondary burn, have proven incapable of a slow burn. My dealer and I have worked on this for two seasons. My system begins with 12 feet of double wall 8" flue pipe. Overall stack is approximately 28 feet. We are at 3K foot elevation. It has been impossible to load either stove with more than two splits without stove top temps rising to 700 or beyond and controls being incapable of slowing the burn. A 2/3rds load would be imprudent.

If I were to replace the 12' section with 6" pipe, what would be the probable result?

Click to expand...

GEEZE , lets professorate on a therey or 12, why don't we?? Well, all you guys are right but I actually did it ,so here is my real live experience.

I have the 8 inch stove pipe with both the in line damper and the tee draft dump to outside air & I use both controls for almost 3 years now, on each & every load of wood i burn.

i have the infamous 1970- 1/4 in plate steel ex smoking dragon stove of 12 cubic foot firebox
that I very sucessfully converted DIY style to secondary burn & am enjoying at least a 50 % wood savings because of it as well as a 36% estimated smoke reduction up the chimney.

The stove sits in the basement at the bottom of a 44 ft interior chimney with several tons of excessive draft, on MOST DAYS, although about 5 days a year ,I have poor draft, probably due to weather conditions,prevailing wind,or whatever.

Marc, I hope you realize that when you turn off your primary air , that it is not at all off, but meerly set to the factory stop that still leaves your primary air between 10% to 20 % open.
This is designed into the stove so that there will always be some live flame on the wood & to prevent the stove from a smoldering burn so that it can more easily pass epa requirements.

I think you must already know about this ,as you threatened to file off the factory preset primary air stop ,allowing the primary air to close fully.

But,before you do this, please consider the tee with suction dump to room air. The tee cost 12.00 & the damper cost 9.oo but it has that hole in the middle that you would have to bolt a piece of sheet metal across or buy a manual draft dump (Manual barametric pressure regulator) that Craig mentioned earlier. It is the same piece of equipment,just without the hole in the middle. You will probably have to order it to the tune of $20.oo & ship

So, to kill extra draft you can close off a in line damper, but only so far , as then smoke backs up in the stove & you can get the campfire smell.

Better than this ,is to open the draft dump to room air,letting room air into your stove pipe to kill the suction & lower the draft.

The interaction of the two controls opperating at once gives an amazing degree of control to your stove than can not be achieved in any other way.

On a cold start,I close the draft dump to room air & open the inline damper for max suction & now I am able to put 4in x 4in splits, as well as a few larger ones into the stove & load the last foot by the door with crumpled newspaper & just touch a match to the paper & in 6 to 10 minutes, all my wood is engaged & starting to char with no kindling at all.

The same teqnique will start a load of wood from almost dead coals, with no kindling & often no newspapers although I have to hoe the ashes to expose the embers & liven them up a bit with a electric hairdryer, before & sometimes after,laying the wood.

For calming down the stove, the draft dump to room air is opened fully & the in line damper is closed about 1/8 from fully opened. I could close it 1/4 or 3/8ths from full open , but risk the chance of campfire smell in the basement,so I tend to use the draft dump to outside air more, as it never gives a smell.

Of couse, my stove has fully closable primary air & secondary air controls, so I have pretty good control on all aspects of the burn. The one thing I regret is not having a window in the door to actually see what the flames are doing. I have to opperate my stove by way of a stove pipe thermometer & a stove top thermometer, without any way to see the flames.

The window is inpossible because I have a primary intake air preheat grate just 3/8th in offset from the inside of the loading door. It is actually a thin steel plate with 12 holes drilled in it to let primary air enter the firebox but the air picks up the heat from the steel plate before getting into the firebox.

How good can this stove be controled ? I maintained a 175 deg burn for over two hours
to cure some fireplace masionary cement that I did not want to reach 212 deg so that water would not boil out and the released steam preferate the patch job.

The patch is good & holding, for now, but in april,the stove gets a new 8 inch elbow on the back wall of the stove. acid had corroded some pin holes thru the sheet metal of the elbow
& from then on, the inside of the elbow gets baking soda applied to it ,once a week to neutralize any acid build up. The elbob is only offset from the firebox by 2 inches & ashes collect at the bottom of the elbow causing premature rust thru in only 3 yrs.

So,I recommend for Marc to install a tee in his 8 inch stove pipe & a manual draft dump to room air. If this don't work well enough,then marc will have to modify his primary air so that it can close fully as well as installing close off valves on the secondary air pipes,too.
I used old plumbing water valves for my secondary air adjusting valves & they work great.

But then,my secondary air pipes are old plumbing pipes that I drilled holes in every inch,so they were already threaded to accept the water valves.
No bragging here,just the plain & simple truth.Corrie, from englanderstoves, helped me in the design of the stove & i was so pleased that I bought a new summersheat stove (englander) that I didn't really need, yet, but sooner or later ,it will get installed.

No math needed for me in this matter. My chimney is 20ft and the liner is 5.5". Unless I keep it squeaky clean it doesn`t draw worth a damn! Takes very little creosote to slow down the draft which would seem to indicate that the smaller the dia. of the liner the greater the restriction. Those are my observations over the past 3 seasons.