Chimney stack temps

[muncybob]

The Wood Gun info states that the average stack temp of their boiler is less than 300 degrees F. I have never measured the stack temp of my current antique oil boiler but from what I have read the average stack temp of oil boilers is considerably higher than 300*? If that is the case I'm guessing that my current chimney of block and terra cotta liner will be sufficient as long as the liner is in good condition and cleaned? This would save me some $$ and work as compared to installing a SS liner.

My wife and I are leaving early tomorrow morning to visit the Wood Gun people and hopefully make a deposit on a oil/wood boiler. I have a list of questions to ask based on what I have read over the months on this forum and can't thank you people enough for what I have learned.

 

[stee6043]

I think you'd want to have a chimneysweep inspect your chimney no matter what the temps are going to be. You don't want to kill the performance of your new boiler due to a sub-standard chimney setup. An inspection would probably be quite affordable in the grand scheme of things...

 

[maplewood]

The Wood Gun info states that the average stack temp of their boiler is less than 300 degrees F. I have never measured the stack temp of my current antique oil boiler but from what I have read the average stack temp of oil boilers is considerably higher than 300*? If that is the case I'm guessing that my current chimney of block and terra cotta liner will be sufficient as long as the liner is in good condition and cleaned? This would save me some $$ and work as compared to installing a SS liner.

My wife and I are leaving early tomorrow morning to visit the Wood Gun people and hopefully make a deposit on a oil/wood boiler. I have a list of questions to ask based on what I have read over the months on this forum and can't thank you people enough for what I have learned.

I agree with your idea that a lower temp output from the Wood Gun will not aggrevate your chimney as much as the higher temps from the oil boiler.
And, as I'm sure you well know, the Wood Gun will output less creosote-making smoke - again a winning situation.
Did you clear your chimney past the Wood Gun people for design size? If it's okay that way, and it's cleaned and in good condition, then fire up that puppy!
I'm installing my Econoburn 150 in August, and I can't wait to burn up some of my 17 cord stockpile!
Happy burning.

 

[rwh442]

Muncybob,

I used to have an old oil furnace in my detached garage for heating. I had a thermocouple in the stack to see what it ran. Typcially ran around 450 - 550 degrees F. Immediately upon startup the stack temperatures exceeded your 300 degree F worries. The longer I ran it the higher is slowly rose. Stack was 7" adapted up to 8" through the roof.

 

[muncybob]

Interesting as I always thought(don't know why) that even with a gasser I would need to line my existing chimney...and I still will if after a cleaning an inspection reveals any sort of problems(cracks, etc). Since it appears the Wood Gun temps will be below what I am currently experiencing I'm hoping for a good inspection and no liner. Existing terra cotta liner is right size for the E100 and E140 and those are the ones I'm considering. Interesting to note on the AHS web site that the efficiency rating of the oil side is slightly higher than the wood side? They show a "net stack temp". What is that?

 

[Chris Hoskin]

The low stack temperatures from a gasifier make it MORE necessary to have a liner, NOT less. In order to maintain decent draft, you need to maintain temperature in your flue (hotter air rises 'more strongly' than cooler air. A masonry chimney is a big heat sink and will generally promote lower stack temps because the heat is lost to the masonry mass. Second, because the stack temperature is so low to begin with, you need to be concerned about condensation in the chimney. Water vapor is one byproduct of combustion and if you pull heat out of the flue gas by exposing it to a cool-ish chimney you are more likely to get condensate on the inside of your chimney. This condensed water vapor is corrosive and will eventually destroy your clay liner. In extreme cases you can even get condensate running out the bottom of the chimney through the clean out. These problems will be less of a concern if your masonry chimney goes up through the middle of the house (as opposed to up the exterior of the house), but in all cases I would strongly recommend an insulated stainless steel liner be installed with a high efficiency boiler. The insulation keeps stack temperatures up and the stainless is able to handle the corrosive condensate should it develop. Hope this helps,

Chris

 

[heaterman]

The low stack temperatures from a gasifier make it MORE necessary to have a liner, NOT less. In order to maintain decent draft, you need to maintain temperature in your flue (hotter air rises 'more strongly' than cooler air. A masonry chimney is a big heat sink and will generally promote lower stack temps because the heat is lost to the masonry mass. Second, because the stack temperature is so low to begin with, you need to be concerned about condensation in the chimney. Water vapor is one byproduct of combustion and if you pull heat out of the flue gas by exposing it to a cool-ish chimney you are more likely to get condensate on the inside of your chimney. This condensed water vapor is corrosive and will eventually destroy your clay liner. In extreme cases you can even get condensate running out the bottom of the chimney through the clean out. These problems will be less of a concern if your masonry chimney goes up through the middle of the house (as opposed to up the exterior of the house), but in all cases I would strongly recommend an insulated stainless steel liner be installed with a high efficiency boiler. The insulation keeps stack temperatures up and the stainless is able to handle the corrosive condensate should it develop. Hope this helps,

Chris

Yah!! What he said is correct. I have invariably gone back and dropped in a SS liner on all the high efficiency (85-88%) oil fired boilers we have installed, usually after the owner refused it at time of purchase. Then after a couple years their chimney is cracking due to condensation permeating the block or brick, especially with chimneys located on the exterior of the house. Wood can contain even more water vapor in the flue gas so it is very important. Don't do it half way.

 

[muncybob]

This is also what the guys at AHS told me today when we made our visit there. Our chimney is on the exterior so it will get the liner. Since a liner will be going in do I really need to be concerned with cleaning/inspecting it first?

 

[Gooserider]

This is also what the guys at AHS told me today when we made our visit there. Our chimney is on the exterior so it will get the liner. Since a liner will be going in do I really need to be concerned with cleaning/inspecting it first?

Yes... It's a code requirement that the liner go into a clean chimney, as they don't want to take a chance on the stuff in the chimney getting touched off from the liner. Also it is still necessary to be sure that the existing chimney is structurally sound - doesn't sound like that will be a big issue though, and the inspection requirements aren't as stringent when a liner is going in, so mostly you need the cleaning.

Gooserider

 

[muncybob]

Thanks again everybody. Making some calls today....any particular questions I should be asking about the proposed liner product?

 

[Gooserider]

It may be worth doing some homework first...

You will definitely need to know the existing INSIDE size and shape of your existing flue liner, also is your flue straight, or does it have bends in it? If it has bends, where are they? What does the exit from the flue where you will connect to the boiler look like?

What is the minimum / maximum mfgr recommended flue diameter for the boiler you are getting? (Note that code inspectors will be fussy about this, usually the flue must be the same diameter as the boiler outlet (unless the mfgr specifically says it can be smaller) and can't be more than 2x the surface area on an outside chimney)

There are essentially two basic styles of liner construction - flexible, that is a strip of metal wound into a spiral tube, w/ a crimped seam running along the joint; and rigid, which is a series of welded stainless tube sections not unlike an extra tough black connector stovepipe. Obviously you must use flexible in non-straight flues, you can use either in straight ones. The two can sometimes be mixed - a common technique in fireplaces is to use rigid for most of the length, and 6' or so of flex at the bottom to get around the smoke shelf area. If you have a choice, rigid is alleged to be easier to clean, and while I have never heard any complaints about flex liners, I'd feel more comfortable with the fewer / tighter seams in rigid given the positive pressure draft that a boiler uses.

On flex liners, the insulation is usually a big sheet that you wrap around the liner and tape with a special tape, then cover with a stretchy stainless fabric "sock" to protect it as you slide it down the flue. My understanding is that rigid tends to have sections of insulation that you wrap and tape around the pipe as you slide it down the flue, which IMHO sounds like an easier approach.

Note that with either type, you will need to have an original flue that is large enough to fit the OD of both the liner and insulation jacket... If you don't have clearance for the insulation jacket, I have heard of people installing the liner w/o a jacket, putting a block-off plate around the base of the liner (a good idea in any case) and filling the space between the liner and chimney with vermiculite. Not as good as a full insulation jacket, but better than nothing....

Gooserider

 

[muncybob]

I have a person coming in a day or so...but, on the phone he stated that he didn't think I would have enough room in the existing flue to install insulation. He said he thought the SS liner itself would be sufficient to avoid condensation?

 

[Gooserider]

A liner will help, especially if it is properly sealed w/ blockoff plates on the top and bottom to prevent convection cooling flows from setting up in the chimney outside it, but insulation is better than not... However many sellers of liners try to avoid it since it is a pain to install and adds to the cost of the job.

At the same time, there is a minimum amount of space required for the insulation and if you don't have it, you don't have it... In that case you would have to go with a plain liner, and maybe use the poured vermiculite approach that I mentioned earlier...

Gooserider

 

[CrappieKeith]

I'd be wary....stacks temps averaging less than 300 degrees.
Flue gases condense at 250 degree.
I also noted someone said something about 85% effecient with oil.
No way......unless it's a condensing furnace with a drain.
If both fuels operate in the same furnace there will be a draft regulator in the flue.
It will operate both fuels at the same speed which in this case the oil will be the one that determines the draft speed.
In our furnaces for example @ .03" of W.C. that flue gas temps are 400 degrees which when tested the oil burns 80.01%
The wood varies depending on moisture content.

To have a slower draft would say that the gases will turn to liquid. Also condensation especially from oil is very corrosive same with wood gas.

When it comes to liners....Simpson's Duraliner is already blanketed.
We sell it and we give 30% off of the list pricing.