What a difference an inch makes?

[drpiotrowski]

Greetings Hearth!

My fiance recently purchased a home in the often sunny but seemingly more often cold and blustery state of Wisconsin.

In hopes of installing a woodstove before winter, I jumped the gun and purchased a used but fully refurbished Jotul 8 woodstove (entirely new baffles, glass, etc). This stove fits our new home perfectly and would really tie the main room together.

The only problem is this: our masonry chimney needs relining. Now, it can fit a 7" liner which is needed for this stove, but if we do someday down the line need to replace it, the chimnney liner would likely be oversized by an inch in diameter.

What difference does an inch make? Will we be kicking ourselves if we install this stove or are we likely OK with a 7" liner should we someday need to install a new stove with a 6" collar?

It's a two story home with a pitched roof, exit at the peak, so the masonry extends about 30'. It is in the center of the house and the new liner would be insulated.

Sorry if this has been answered in some capacity in another thread (and if so, please direct me), but I've been scanning this forum for at least a month and still haven't been able to come up with anything to help me make a decisive decision.

Thank you all so much!

 

[bholler]

With 30' of chimney I wouldn't be scared to run 6"

 

[EatenByLimestone]

I’d go with 6” insulated.

 

[drpiotrowski]

With 30' of chimney I wouldn't be scared to run 6"

Is that allowed? I know others have done it with this stove but I thought it was against code to go down 🧐

Manufacture specs also say use minimum 7" chimney. Are there workarounds for this?

 

[bholler]

Is that allowed? I know others have done it with this stove but I thought it was against code to go down 🧐

Manufacture specs also say use minimum 7" chimney. Are there workarounds for this?

If the manufacturer specifically specs 7" then no it won't meet code. Mechanical code does allow a 2" reduction but manufacturers instructions over ride that.

 

[drpiotrowski]

If the manufacturer specifically specs 7" then no it won't meet code. Mechanical code does allow a 2" reduction but manufacturers instructions over ride that.

Got it. Thanks for the quick responses!

I know it's dependent on a lot of factors, but speaking generally, if we do decide to use our stove and install a 7" liner, would one anticipate any issues if we someday later had to install a newer stove (designed for 6" minimum chimney) into it?

 

[Ashful]

Chimney height, and not diameter, determines static draw. The diameter only dictates ability to maintain that static draw at a given flow rate (CFM), giving you the dynamic value of "draft". So, there's no reason a 7" flue would have an inability to pull proper draft on a future replacement stove spec'd for 6". In fact, with 30 feet of pipe, it's almost definitely going to pull harder than desired, and you may find yourself needing a key damper.

So, why not always oversize, and always go with the largest option? Well, modern stoves are very efficient, and put very little heat into the pipe. A larger pipe will have lower air velocity for a given CFM flow rate, with the flow rate being set mostly by the air inlet control onn the stove, meaning those flue gasses spend more time getting to the top and exhausting. This can increase creosote build-up, given the longer latency and cooling that happens within the pipe.

All said, I'd follow bholler and Limestone's advice, and just install 6", unless the Jotul 8 manual specifically forbids it, by saying "must be 7 inch". There should be enough prior or current Jotul 8 members on this forum to comment on that. Either way, with 30 feet of pipe, it's unlikely you will ever have a "too little draft" at the flow rate of any relatively modern stove. In fact much the opposite, I see a key damper in your future for the purpose of reducing said draft.

 

[stoveliker]

If this is going to be needed to be inspected, you could ask the inspector about going 6", given the tall chimney.

 

[drpiotrowski]

If this is going to be needed to be inspected, you could ask the inspector about going 6", given the tall chimney.

There's an idea... Are you referring to an insurance inspector?

 

[stoveliker]

I was talking about a code inspector.

What slipped my mind is that insurance may not pay if shtf. Regardless of what whomever says when they inspect the system, if they can find something they can use to get out of paying, they will.

That's not a risk I would be willing to take.

Regardless if you go with anything other than what is in the mfg instructions, have approvals in writing - even if that may not be enough, it is the strongest case you can make.

 

[Ashful]

I'll be honest and say that, while I hear a lot of speculation on this forum about the possibility of insurance companies refusing to pay, I've never heard of this actually happening to anyone I know. I'm sure it happens, but at least based on my limited experience in this corner of the world, I'd think there must be some pretty unusual or egregious circumstances connected to said event.

One can infer what they want from that, whether they figure the direct + image costs of fighting these cases is more acceptable than the cost of paying out, I don't know the reason. But based on reading speculation of so many scenarios here and elsewhere, I'm frankly surprised most of the time I hear about some of the claims paid to various direct acquaintances.

 

[EatenByLimestone]

Valid point. It’s only proper to inform people of the possibility.

We also have a limited sample size here. If someone doesn’t tell us it happened, we wouldn’t find out about it.

 

[bholler]

I'll be honest and say that, while I hear a lot of speculation on this forum about the possibility of insurance companies refusing to pay, I've never heard of this actually happening to anyone I know. I'm sure it happens, but at least based on my limited experience in this corner of the world, I'd think there must be some pretty unusual or egregious circumstances connected to said event.

One can infer what they want from that, whether they figure the direct + image costs of fighting these cases is more acceptable than the cost of paying out, I don't know the reason. But based on reading speculation of so many scenarios here and elsewhere, I'm frankly surprised most of the time I hear about some of the claims paid to various direct acquaintances.

As someone involved in lots of insurance claims involving heating appliances I can tell you it's not speculation. It happens all the time. I do reports for both sides fairly often and have been I court as an expert witness multiple times as well sometimes for the homeowners sometimes for the insurance company

 

[bholler]

That being said in this case as long as everything else is right I seriously doubt there would be an issue. I know I wouldn't call it out as a safety risk of I was asked to evaluate it

 

[coaly]

To mathematically determine what difference an inch makes, the formula for square inch area of the round pipe is used. Radius squared X pi.

Radius is 1/2 the diameter. So for a 7 inch pipe, 3.5 x 3.5= 12.25. Pi or 3.14 x 12.25 = 38.465 square inches.

A 6 inch pipe is 3 X 3=9 X 3.14 = 26.26

So 12.2 square inches is the reduction. (under 1/3)

The larger the pipe gets, the greater the difference. An 8 inch pipe is 4 X 4=16 X 3.14=50.24 (almost twice the square inches as 6) This is when cooling from expansion into the larger area starts to become an issue.

Increasing diameter increases the btu capacity the flue can carry. But it takes more btu to do this. Just like a higher displacement engine is capable of more horsepower, it needs more fuel to do it. Heat is the chimneys fuel to produce draft.

Decreasing in size decreases the capacity. Height increases draft, so your higher 6 inch chimney helps make up for the smaller area by increasing the velocity. NET draft, the product of all combined factors is the most important. There are tables and charts used to calculate the net draft with given altitude, internal and external temperature, height, diameter, and resistance of pipe configuration and other resistances in the system. The intake opening normally being the greatest resistance.

Codes do not take this into consideration. They simply state no reduction from stove collar size is allowed to prevent obstructions in the venting system. NFPA 211 Standard allows 1 inch reduction. This is a tested standard codes adopt. Codes can add to it, but not take anything away.

A quick search shows the state of Wisconsin adopted the International Family of Codes which the Mechanical Code allows no reduction. So technically, going by that new building code, no reduction is allowed in a new installation. Many install to the safety Standard and claim it was existing to circumvent the newer adopted codes. The same goes for the ICC requiring all appliances to have a UL Label. Many install and claim it was existing, since it is difficult to prove when the older stove was installed. There wouldn’t be an older stove market if not.

Always think of the chimney as the engine that runs the stove. Hot rising gases in it creates a low pressure area in flue, pipe, and stove. This is measured as draft. This allows atmospheric air pressure to PUSH into the stove intake feeding the fire. The more efficient the chimney is, staying hot inside, the more efficient it makes the stove needing less heat left up to create the same draft.

As others stated, this gives the reasons for reducing to 6 inch that also allows a newer stove with 6 inch outlet to be used to its full efficiency at a later date.

 

[spuldup]

I'll be honest and say that, while I hear a lot of speculation on this forum about the possibility of insurance companies refusing to pay, I've never heard of this actually happening to anyone I know. I'm sure it happens, but at least based on my limited experience in this corner of the world, I'd think there must be some pretty unusual or egregious circumstances connected to said event.

One can infer what they want from that, whether they figure the direct + image costs of fighting these cases is more acceptable than the cost of paying out, I don't know the reason. But based on reading speculation of so many scenarios here and elsewhere, I'm frankly surprised most of the time I hear about some of the claims paid to various direct acquaintances.

For what it is worth, our insurance company required a certified chimney sweep approve the clearances and venting condition of our wood furnace in a home we recently purchased. We still would have to pay the rate of having wood heat but if a fire was a result of said wood heat, no payment would be made without the affidavit from a professional.
It was during this process that we found ourselves having to replace 30' of 6" all-fuel vent with 36' of 8", to the tune of $4700. I did run the 6" for about 2 months until the company could replace it, and the draft was awful. Now with 8" I have installed a key damper because it was too strong for warmer days.

 

[blades]

Why not send a note out to Jotul indicating the length of the flue and what you want to do. I bet that with a 30ft flue and 6" internal diameter flue it is not going to be an issue. The draft on that once things are up to temp will be more than enough for the stove and might still need a damper to slow it down. Codes are guide lines, sometimes in their generic function things do quite work out. That 7" flue is likely based on 12-15ft flue length. In one of my former homes had a very lovely field stone fireplace quit large, it could be fired with wood or natural gas , no glass doors either. The flue was 13" in diameter . When you fired it up there was a huge draft along the floor sucking everything else out of the place It would cause the NG furnace to fire up. Insert with 6" flue about 10 ft long solved that problem. Then it would actually add heat to the home envelope. One other note your new place being two story might want to think about adding a door at the top of the stairs or bottom, reason being hot rise cold sinks and you will get a lot of cold draft coming down the stairs. In days gone by the second story stairwell was always able to be closed off . Back in my youth everything heating wise was gravity systems none of this new fangled forced air stuff. Those home designers kinda forgot the basics. ( also helps with AC in summer keeping the second story cooler for the exact same reason.)

 

[Ashful]

Why not send a note out to Jotul indicating the length of the flue and what you want to do.

I guess I can't speak for the Jotul USA of today, but if they're anything like they were 6 - 10 years ago, you won't get a reply. They won't deal directly with customers, in most cases.

 

[bholler]

I guess I can't speak for the Jotul USA of today, but if they're anything like they were 6 - 10 years ago, you won't get a reply. They won't deal directly with customers, in most cases.

That was only customers with fire lights. They really just wanted to forget about them lol.