Minimum temp for chimney outlet?

[larboc]

Lopi Freedom into 15' of insulated flex liner from firesidechinmenysupply (nice stuff btw.) going though a 13" clay square liner on an interior chimney.

I put a thermocouple into the gas stream at the top of the stove in the appliance connector and also put one in the top of the liner at the roof.

I'm thinking the most important temp would be the outlet temp to avoid creosote build up anywhere in the pipe. It's definitely higher than I would think. I think a lot of heat is going up the chimney. Anyone else have a probe up there?
Low fire is about 320F, medium fire 400F, WOT 500F at the roof. Lower thermocouple is appx. 500F, 650F, 800F, respectively.
I have insulation packed around the liner at about the bottom of the attic and a sealed top plate.

 

[Jags]

Those temps seem pretty "normal" in my mind.

 

[larboc]

Just seems like I would only be needing to keep flue outlet temps a little above 212F to keep clean, and anything over that is wasted energy. I figured at 500 degrees I'm wasting ~18,000 btu/hr. It might be worth it to figure out a way to cool the stack temps down a little.

 

[Jags]

It might be worth it to figure out a way to cool the stack temps down a little.

I don't suggest that. 212F isn't high enough unless you are at the coaling stage and that would need to be at the peak of the pipe. I respect your quest to be as efficient as possible, but these EPA stoves were designed to be about as stingy with the waste heat as they can get away with. Trying to pull more heat from the stack can/will often cause some nasty buildup.

 

[CenterTree]

As an old saying goes:
"Heat with your stove, not your chimney"

The stove will do what it can do as far as heat output and grams of emissions out the chimney.
I would think you should have good efficiency AND a decent stack temp ( higher) at the same time with a good EPA stove with an insulated flue.

 

[larboc]

I am measuring at the peak of the pipe, literally a thermocouple stuck in the top of the liner. Whatever system I would have pulling heat from the stack would need to be able to controlled to keep chimney outlet temps in the 250F range.

 

[Jags]

Unless you can find or create a viable market for creosote, I would not suggest doing this.

 

[larboc]

This article and may others like it say ~250F is about where it starts condensing. What makes you think otherwise?

https://www.bae.uky.edu/publications/AEES/AEES-36.pdf

 

[Jags]

I am not disputing if that number is accurate or not.
There has been a "magic heat" add on for exhaust pipes around for years. Its design is pretty much what you are describing as far as reclaiming heat going up the stack. They are horrible add ons to an EPA stove. Notorious for clogging with creosote. Add that to the fact that solid fuel burns in stages (startup, active, coaling or whatever terms you would like to apply) and there is basically no steady state that you can build off of. What can be safely done at 250F degrees during the coaling stage isn't necessarily the same as 250F during the startup.

ETA: And there is a whole 'nuther argument to be had about altering the "system", UL approval, code violations, insurance acceptance, void warranties, passing inspections,etc.

I obviously can't tell you "don't". But on the other hand, I would take a large wager that a very high percentage of engineers that design these things would not be on board with this en-devour.

 

[larboc]

I am 100% agreeing with you. What I am talking about is a controlled device that would only extract heat once the exit of the stack had become hot enough, closing the loop on stack temp. A servo on a bypass valve that passes flue gasses through a heat exchanger for air or DHW preheating. A couple of lines of code in the old arduinio would be all it would take to control. I could adjust the set point based on if I see creosote starting to form or not.

It may seem like a trivial amount of heat, but couple quick calculations translate into about 2 cord of wood per heating season which is why I'm looking at an economizer.

 

[Jags]

Hmmm...I understand but it is still an unapproved product in the system. But if you have no concerns with insurance or inspections or any such thing that is completely up to you.

Whatever you decide to do please be safe. A wise man (and member of this site) once said: (paraphrasing)
"If you are going to invite fire into your home make it as safe as you can."

 

[Highbeam]

I respect your quest to be as efficient as possible, but these EPA stoves were designed to be about as stingy with the waste heat as they can get away with. Trying to pull more heat from the stack can/will often cause some nasty buildup.

I have experienced otherwise. Let's stick to non-cat stoves since that seems to be what the OP has. First, not all EPA stoves are designed with efficiency as their highest priority. Efficiency is NOT a parameter that is regulated. Emissions is regulated so what you will find is that EPA stoves are optimized to meet emissions standards first. Then there are a few other parameters that stove designers look at. Flame action, ease of operation, keeping glass clean, control, strong draft, and yes efficiency.

So what you have is a non-cat that is designed to be as efficient as possible while also meeting the emissions regulations. It's really easy to burn clean when you are being inefficient and dumping vast quantities of energy up the flue. This is one reason that there are 35:1 epa exempt stoves. My old hearthstone was like this, it would shoot vast quantities of very hot exhaust up the stack at all damper settings. Burned clean though. The NC30 seems better about keeping flue temps down which indicates a more optimized design.

There is a lot of heat that can be safely stolen from the stack before creo formation is a problem. So long as you maintain cap temps of 250 or higher then you are not in danger of bulk creo formation.

Powering the draft can be done with much lower temps. The flue on my BK runs 400 internal right over the stove, I hope cap temps are at 250 but don't know.

 

[Jags]

There is a lot of heat that can be safely stolen from the stack before creo formation is a problem. So long as you maintain cap temps of 250 or higher then you are not in danger of bulk creo formation.

I understand what you are saying, but any type of heat transfer must come from a temp differential. Meaning the heat exchanger must be colder than what it is trying to get heat from. The more differential the higher potential for extraction. Just what would go into the system that has any chance of meaningful heat extraction that doesn't also provide a breading ground for creosote.

 

[larboc]

What if the recovery device set on top of the cookplate surface like a pot of water would? I wouldn't think that would constitute an unapproved product.

 

[larboc]

I have experienced otherwise. Let's stick to non-cat stoves since that seems to be what the OP has. First, not all EPA stoves are designed with efficiency as their highest priority. Efficiency is NOT a parameter that is regulated. Emissions is regulated so what you will find is that EPA stoves are optimized to meet emissions standards first. Then there are a few other parameters that stove designers look at. Flame action, ease of operation, keeping glass clean, control, strong draft, and yes efficiency.

So what you have is a non-cat that is designed to be as efficient as possible while also meeting the emissions regulations. It's really easy to burn clean when you are being inefficient and dumping vast quantities of energy up the flue. This is one reason that there are 35:1 epa exempt stoves. My old hearthstone was like this, it would shoot vast quantities of very hot exhaust up the stack at all damper settings. Burned clean though. The NC30 seems better about keeping flue temps down which indicates a more optimized design.

There is a lot of heat that can be safely stolen from the stack before creo formation is a problem. So long as you maintain cap temps of 250 or higher then you are not in danger of bulk creo formation.

Powering the draft can be done with much lower temps. The flue on my BK runs 400 internal right over the stove, I hope cap temps are at 250 but don't know.

Agreed. FYI-fiberglass insulated k-type thermocouple cable isn't that expensive and I just ran it between my SS liner and the old liner. If nothing else it's interesting!

 

[Chimney Smoke]

It may seem like a trivial amount of heat, but couple quick calculations translate into about 2 cord of wood per heating season which is why I'm looking at an economizer.

How much wood do you burn per year? I find it hard to believe that lowering your exhaust temp 150 degrees or so would save 2 cord of wood over the winter.

 

[Highbeam]

I understand what you are saying, but any type of heat transfer must come from a temp differential. Meaning the heat exchanger must be colder than what it is trying to get heat from. The more differential the higher potential for extraction. Just what would go into the system that has any chance of meaningful heat extraction that doesn't also provide a breading ground for creosote.

Blowing on a 10' long single wall stove pipe with a box fan is what I do. We're only talking about lowering the temp a little bit.

 

[larboc]

10 cord in the smoke dragon, we'll see how much this year using the insert more. It for sure uses less.

We'll have a fire going about 240 days out of the year, I figure that would translate to 200 days of burning hard enough to have an extra 200F stack temp.
One BTU will heat about 55 cubic feet of air 1 degree F.
200F*1btu/55cubic feet*55cubic feet/min*60min/hr=200F*60.

At roughly 55 CFM combustion air that's about 12000 btu/hr. that's about 60e6 BTU per heating season
23.2e6 BTU/cord of hard maple.

over 2 full cord "up in smoke".

 

[Jags]

Blowing on a 10' long single wall stove pipe with a box fan is what I do. We're only talking about lowering the temp a little bit.

Staying external is a whole different thing from what I was envisioning. The OP was referencing servos and bypasses. That leads me to think a breach in the pipe which in any form would be considered an unapproved product unless tested.

 

[Highbeam]

It doesn't need to be a magic heat. Some of these EPA stoves are just ridiculously ineffecient and shoot way too much heat up the flue. Way more than needed to power the draft and keep temps up.

 

[Jags]

Well...I guess he could use a thermocouple to turn a fan on and off.

 

[larboc]

Running full tilt, the factory circulation blower can't really do much to the flue exit temperature.

The EPA stoves have high combustion efficiency, and mediocre thermal efficiency at rated but output. They need to have more surface area above the shelf to transfer heat out of the flue gas.

 

[rideau]

10 full cords of wood to heat for one season. That is something that it would be worth your while to deal with, forget all the fancy stuff.
Is your home tight and well insulated?
Maybe think about getting a BK or Woodstock stove?
Three full cords does it easily here in Southern Ontario for a large home with lots of windows in an exposed location.
You are using an awful lot of wood.
To save two cords I'd need to burn less than a cord.....and I am sure there are many others here who are in the same position.
I think you need to deal with something other than your exit temp/`

 

[larboc]

Look at a map, if you're in southern Ontario you are potentially several hundred miles south of me and in a significantly milder climate.
Read where the 10 cord was with the old 70's smoke dragon furnace in the basement with the previous owners firing it. This is my first heating season in this house, but 10 cord is a typical number for a hard winter in a larger house with an inefficient stove up here. Keep in mind that I live farther north than most Canadians.

 

[rideau]

Good luck. I hope for your sake that you find you use a lot less wood.

For the record, I'm officially Zone 4.

Last winter we had extended, and I mean extended, periods that stayed in the -20 range. Plenty of days a lot colder. Ice and snow on the ground into May. It gets pretty cold here.