8" chimney for a 6" stove?

[beanbone]

Hi -

I finally convinced my wife to go with a wood burning stove over a fireplace, but she will only go for a stove that can be viewed with the door open periodically. We installed an 8" double wall chimney system for an old vigilant wood stove. Unfortunately, the stove is warped and not air tight, so I'm in the market for another unit.
I'm looking to buy a hearthstone bennington or phoenix, but both work with 6" chimneys. So, I have two quick questions -

1) Can I use the 8" chimney system for a 6" flue?

2) Can I burn these stoves -- with small fire -- with the doors open?

Thanks!

Brian

 

[PaulRicklefs]

I can't answer the second question as I don't have experience with those stoves but I can help with the venting question.

Only problem with venting a 6" appliance through an 8" chimney is you may have low draft issues and more creosote buildup.

A 6" vent has about 113 sq inches of area and an 8" has about 201 sq inches. Almost double the area. So what will happen is your flue gas will zip up your 6" flue, then hit the 8" chimney and slow down. The slower it moves the cooler it will get as it exits the chimney. It will cause more creosote buildup.

It's not all bad news though. The 8" is still OK to use as long as it is a 2" insulated stainless steel chimney that's installed properly.

Does your chimney go straight up and through the house before exiting through the roof or does it go outside and then up? If it goes straight up then the 8" should be fine, if it goes outside and then up, you may not be happy with it.

 

[beanbone]

Hi -

Thanks for your response. Unfortunately, my pipe does elbow out the wall, and up the side of the house. Do you think buying inexpensive 6" liner is the answer? I say inexpensive because the pipe is already double wall insulated.

 

[slowzuki]

Hmm, its a double whammy with an outside flue, they are cold and don't tend to draft well off the start. I'd say the 8" isn't going to give you as much trouble as the outside flu unless you burn quite regularly. With a fire constantly going, it will keep the chimney warm and drafting well.

2 years ago we used a small stove with a 7" outlet, now we have a larger stove with a 6" outlet. We can't run the new stove with the doors open because the 6" flue won't draft enough to keep smoke from pouring out the front even with a good hot fire. I'd guess you'll be ok with the larger flue, just keep the fire going in the stove all the time.

 

[beanbone]

Well that's good news for us. We are planning to keep the fire going at all times. The Bennington is a real work horse. Do you know if you can periodically leave the door open on these stoves?

 

[PaulRicklefs]

Hi -

Thanks for your response. Unfortunately, my pipe does elbow out the wall, and up the side of the house. Do you think buying inexpensive 6" liner is the answer? I say inexpensive because the pipe is already double wall insulated.

First off I don't like liners. They can and will fail.

Secondly it's too bad it goes out and up but that's what you got.

Thirdly I think you should try running it with an 8" chimney. You will probably find it works OK and will do the job good enough. In a perfect world everything would be perfect but I think your setup will work for you.

 

[beanbone]

Thanks for the tips... My only other option is to buy a new vermont castings like the defiant. I think it's built for 8". I heard they're warranty no longer stands though.

 

[10-cc]

Strange, as for me I installed a P.E. with a 6" flue into an 8" Prefab Security 2100 and I constantly fitting with the overdraft. It sucks like hell so I cannot load the 3cf to the max I only fill it to a half or so, so it will not over fire.

After explaining the situation to the stove shop they suggest to go with a full 6" liner and so I will not have this overdraft problem.

So the question is: For a given stove with a 6" flue connector, which chimney will create more draft: 6" OR AN 8" CHIMNEY ?

Thanks,

 

[PaulRicklefs]

Strange, as for me I installed a P.E. with a 6" flue into an 8" Prefab Security 2100 and I constantly fitting with the overdraft. It sucks like hell so I cannot load the 3cf to the max I only fill it to a half or so, so it will not over fire.

After explaining the situation to the stove shop they suggest to go with a full 6" liner and so I will not have this overdraft problem.

So the question is: For a given stove with a 6" flue connector, which chimney will create more draft: 6" OR AN 8" CHIMNEY ?

Thanks,

6" will create more draft. More velocity.

8" will handle more cfm or volume at same velocity.

Only reason for different sized chimneys is to handle different BTU loads for different appliances.

Basically with your 6" liner you would be only restricting the flow of gas without addressing the draft issue. Why don't you just damper your flue instead? Cheaper and guaranteed to work.

It's likely your 8" chimney is warm all the time and is fairly tall to produce such a strong draft. You have achieved what most people fight to get. Damper it down and enjoy!

 

[slowzuki]

Not so fast Paul. If the chimney is a warm one, ie inside the house, the 8" will always provide more draft. Outside the house it gets complicated. The 8" has less flow restriction but it will run cooler due to more surface area and slower gas velocity inside. Cooler means less buoyancy/draft. So the answer is it depends.

Ken

 

[beanbone]

sounds like I should just take a chance and buy the appliance for 6", hoping it works. My other choice is to buy a vermont casting stove that has a 8" flue (I'm assuming that the vermont casting is probably less efficient though, as it has a larger flue).

 

[PaulRicklefs]

Not so fast Paul. If the chimney is a warm one, ie inside the house, the 8" will always provide more draft. Outside the house it gets complicated. The 8" has less flow restriction but it will run cooler due to more surface area and slower gas velocity inside. Cooler means less buoyancy/draft. So the answer is it depends.

Ken

If you take two chimney systems with the same heat input which are installed identically the smaller diameter will always create more velocity and draft. What we have been talking about is apples and oranges.

Simple answer is his 8" chimney will likely work with a 6" flue but not ideally.

Ideally you want the entire system to be matched but that's not the case. Ideally you want the chimney inside for obvious reasons, but that's not the case. This thread is talking about two very different systems acting is two very different ways. Inside chimneys will always have better draft characteristics than outside chimneys. We can't argue that. But in this case we have an 8" chimney outside so of course the draft will be crap. If he matched it with a 6" it would be better, but it's still outside.

All the chimney does is move products of combustion from high pressure to low pressure through temperature differential. If you are moving air from a tank at 30 psi and put it through a 6" pipe to a tank at 0 psi, the air will be physically moving much faster than if it were a 8" pipe. The volume of air is less say per second, but the speed of it's movement will be more. Simple physics.

Same goes with burning stuff and letting it vent through a 6" or 8" chimney. Faster in the 6", slower in the 8". Same volume of crap going up the chimney. Velocity creates more draft. On and on we go..

 

[10-cc]

Thank Paul for the clear explanation. In my case the chimney is 30' tall running inside the house facing dominant winds without obstructions. The thing is that my previous stove, VC Encore 2550 had an 8' flue and it's draft was excellent but controllable as now with my 6" flue stove, even with the air inlet all the way down, it is overfiring so a have to reduce the wood load into it, also it seems to me that a lot of heat is escaping outdoors.

As for a damper, I think it could be a good solution but I have to be careful using it, as it required adjusting it manually according to the outside weather. I am working on trying first to minimizing the inlet air by sealing every possible leak to the stove, and probably modifying the inlet air shutter so I can close it all the way because by design you cannot close it completely.

Thanks again for all suggestions.

 

[egghead2004]

well, from experience, I have an 8" chimney with two 6" appliances piped into it. My oil fired boiler and my Olso, both 6" flue. The oil boiler always has air pulling from the air intake on the burner as well as the barometric damper set for .04"WC. The Oslo never has a draft issue, in fact I also have to have a 6" cast iron damper in the stove flue to slow things down a bit. The chimney pipe is either 31.5' or 34.5', i forgot how many sections of pipe I put in.

So in my case, it works like a charm with draft to spare. However, not every house has the same air pressure characteristics, so I will not tell you yours will work fine, but it is possible. How's that for a definate maybe!

 

[DAKSY]

I have an 8" chimney with two 6" appliances piped into it. My oil fired boiler and my Olso, both 6" flue.So in my case, it works like a charm with draft to spare.

It may work like a charm but, it's not up to code. Each appliance MUST have it's own vent system. Check your owner's manuals

 

[PaulRicklefs]

I have an 8" chimney with two 6" appliances piped into it. My oil fired boiler and my Olso, both 6" flue.So in my case, it works like a charm with draft to spare.

It may work like a charm but, it's not up to code. Each appliance MUST have it's own vent system. Check your owner's manuals

I certainly don't know the codes for every jurisdiction but the codes in Saskatchewan Canada allow common venting of oil and wood appliances as long as they are interlocked in a way to prevent both from operating at the same time. Oil and wood are the only two that can share a common vent, gas and any other are strictly prohibited.