Forced draft and factory-built chimneys

I suspect the second.
The chimney company has no control over the amount of pressure...and therefore cannot give a blanket approval.

However, another point is that most of these wood boilers would not, IMHO, be called positive pressure. While they may do that in the appliance itself, the stack requirements are somewhat similar to natural draft units. In other words, if these are positive pressure than an oil burning is VAST positive pressure.

Interesting question and observation, though.....don't remember hearing that before.

Masonry chimneys of the type found in residential construction are extraordinarily incompatible with any sort of forced draft, as that term is literally used (a number of masons, including some who are experienced and do quality work don't even do much to create any serious bond or vaguely pressure-worthy seal between the sections of flue tile, except for the fact that they sit on top of one another and _might_ be given a light smear of some sort of refractory-like goo at the mating surfaces (which, even if it starts out as a seal, may not stay that way with expansion and contraction over time)). I just created a really awful run- on sentence, but hopefully the substance is clear and helpful.

I'm not a big fan of manufactured chimneys when they're surrounded by construction materials, but the good ones, when accessible so that you can observe/ correct anything that goes on, seem to be well designed and made.

I have forced draft setup on my Selkirk stainless chimney (draft inducer). I suspect Selkirk is very similar, if not the same, as the Duravent. I did have to seal every joint, as others have before me. You'll know when you go from normal to "positive" pressure with standard chimney connections. Fly ash will show you where all the leaks are.

Forced draft would be a "draft inducer", not the draft fan that is included with most gassers. The new EKO's come with a draft inducer on the back in addition to the draft fan on the front. I think Econoburn has only a draft fan? That fan, in my opinion, couldn't possibly produce measurable positive pressure in the flue. My EKO's draft fan cannot, this I know for sure...

If the temperature of the air outdoors, in the flue, and coming out of the boiler were all the same then the boiler's draft fan would probably make a positive pressure in the flue and you might then get leakage out of the joints in the flue and into the occupied space of the building.

But the exhaust coming out of the boiler is very hot and once it gets into a vertical flue or chimney it wants to rise and that will pull a partial vacuum (draft,suction) at the bottom of the flue. There are a host of variables involved but it's pretty common for the draft of the chimney to be more than optimum for the operation of the boiler. My tile-lined chimney pulled -.20(inches of water) with my former wood furnace without a draft fan. That's way more than the recommended .03" to .05" (if I am remembering correctly) that Tarms recommends for their Solo boiler.

So what I'm getting at is that probably most chimneys have enough draft to override the positive pressure coming out of the boiler from the draft fan and still result in a negative pressure in the flue. Sometimes so much that a barometric flue damper is required to reduce that draft to make the boiler run as designed.

But that might not necessarily be true for every flue or chimney and so the lawyers don't recommend you use their product.

My understanding is that there are definitely some combustion appliances (that as far as I know are not anything marketed for or used on a residential scale) that really do present a pressurized condition to the flue that is well in excess of anything that is foreseeably likely to be pulled upwards with the natural draft that occurs from the combined warmth of the flue gases and the height of the chimney.

Those true pressurized-flue situations would be a complete and utter no-no with either a masonry chimney of the normal-residential-construction type, or a manufactured chimney of the normal residential construction type.

I don't think that a paddle-wheel-type draft booster/ inducer comes anywhere near giving you the degree of flue pressurization associated with the aforementioned commercial type situations- it's just giving the flue gases a little encouragement along their way. I've not yet become entirely comfortable, personally, with the paddlewheel-type inducer as they often seem to have some spots that could lead to flue gas spill under some conditions. My opinion on that is just my own, not advice.

A masonry chimney with a properly installed and maintained tile liner is fine for the intended purpose, as is a properly installed and maintained manufactured chimney of the metalbestos-type.

I suppose that some of the tankless and other direct vent propane and natural gas appliances actually represent some degree of pressurized flue, but those have specifically designed seamless stainless vent pipe with sealed joints- and it'd be improper to vent those into a regular masonry or metalbestos-type mfg. chimney, or to try to run any other fuel-burning appliance into one of those gas-type vents.

The warnings on the mfg. chimneys and their literature are probably to prevent some fool from trying to put a true pressurized combustion appliance into one

Is this helping make sense of it? I have no credentials on this, but have dug into it for myself to the point of being reasonably confident about the above.

I would feel better if I could give an example of what a “true pressurized combustion appliance” is.

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That might be a Garn, to use a residential sized example. They have a pretty substantial draft fan from what I've read here. Someone who knows might add a little authority to that guess.

Ken Benson said:

pybyr said:

Is this helping make sense of it? I have no credentials on this, but have dug into it for myself to the point of being reasonably confident about the above.

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Yes, thank you. Personally, I'm convinced now that a gasification boiler is not a "forced draft, positive-pressure appliance" as the factory-built chimney manufacturers mean it. Our building inspector here is a real stickler for details, so I want to make sure I have all the bases covered before I submit the permit application.

I would feel better if I could give an example of what a "true pressurized combustion appliance" is.

Ken Benson

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"true pressurized combustion appliance" is such as an oil burner with a gun which blows air into the chamber to atomize the oil. wood burner Has air intake, exhaust, & end of chimney. To pressurize the WB the fan would have to be located at the intake. A draft booster/fan located at the exhaust would depressurize the boiler but would pressurize the chimney & a blower at the top of chimney would depressurize the whole system. A Tarm boiler has a pressurized burn chamber i think.

Ken Benson said:

BLIMP said:

wood burner Has air intake, exhaust, & end of chimney.

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Gasification (which is what we're talking about here) wood boiler has a fan.

Ken Benson

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if the fan blows air into the boiler, its pressurized

Pellet stoves are truly pressurized and that is why a silicone seal is usually required.

To really sort out one from another would require measuring the fan as opposed to the capacity of the chimney with natural draft.

In practice, the reason a lot of gas furnaces and pellets stoves are forced draft is that they don't have chimneys but just vents which go right through the wall. Obviously such a vent does not build up any natural draft.

Here is a hint as to what "positive pressure" might mean in terms of insulated chimneys. There is a standard for such things which states:
"60-inch WC positive-pressure building heating appliance"

Security and other chimney makers actually have a product to meet this standard. However, 60 inch WC is probably 100X or more the pressure that most downdraft residential wood boilers would put into a chimney.

Way back when, our Tarm Solo Plus model had a fan of about 60 watts on it. Most of the power of the fan was used up in the firebox itself, resulting in extremely little "positive pressure" once the flue products hit the stack. It would be my guess that just the standard draft of a reasonable chimney more than offset this.

If you want a more technical or comparative answer, shoot an email to John Gulland, who is an expert on air pressure subjects. He would be able, for instance, to relate the pull of a 6" chimney with .05 WC to the CFM left over once a 60 watt fan pushed air through a convoluted firebox.

http://www.gulland.ca/

I'll shoot him an email and see if he can look at this thread. Also, if you PM our own member metal - he is an engineer at a metal chimney maker.

Our friends at Bioheat and other makers of this equipment probably have some real answers also.

I suspect, as said earlier, that in the case of units with the small fans.....standard chimneys are OK...however, units with extremely powerful fan setups (Woodgun?) might be a different story.

Webmaster said:

Pellet stoves are truly pressurized and that is why a silicone seal is usually required.

To really sort out one from another would require measuring the fan as opposed to the capacity of the chimney with natural draft.

In practice, the reason a lot of gas furnaces and pellets stoves are forced draft is that they don't have chimneys but just vents which go right through the wall. Obviously such a vent does not build up any natural draft.

Here is a hint as to what "positive pressure" might mean in terms of insulated chimneys. There is a standard for such things which states:
"60-inch WC positive-pressure building heating appliance"

Security and other chimney makers actually have a product to meet this standard. However, 60 inch WC is probably 100X or more the pressure that most downdraft residential wood boilers would put into a chimney.

Way back when, our Tarm Solo Plus model had a fan of about 60 watts on it. Most of the power of the fan was used up in the firebox itself, resulting in extremely little "positive pressure" once the flue products hit the stack. It would be my guess that just the standard draft of a reasonable chimney more than offset this.

If you want a more technical or comparative answer, shoot an email to John Gulland, who is an expert on air pressure subjects. He would be able, for instance, to relate the pull of a 6" chimney with .05 WC to the CFM left over once a 60 watt fan pushed air through a convoluted firebox.

http://www.gulland.ca/

I'll shoot him an email and see if he can look at this thread. Also, if you PM our own member metal - he is an engineer at a metal chimney maker.

Our friends at Bioheat and other makers of this equipment probably have some real answers also.

I suspect, as said earlier, that in the case of units with the small fans.....standard chimneys are OK...however, units with extremely powerful fan setups (Woodgun?) might be a different story.

Click to expand...

How does a pstove get pressurized by sucking the air from the exhaust? The exhaustpipe/chimney gets pressurized & leaks but the stove is actually depressurized?

That's what I mean, Blimp - that the VENT is pressurized. That's what we are discussing.

Ken, that spec pretty much says it all. That is a normal chimney. Chimney draft is measured by the suction - is measured in how far it would pull water (water column) up the vent. Chimneys are measured in negative draft, which is why there is a minus sign...in other words, it is pulling. So a chimney with a strong draft of .10 WC would pull water 1/10 of an inch up a tube.

The chimney which is specified for true positive pressure (high pressure) appliances is tested at +60 WC or 600 times as strong in the other direction!

In short, I suspect all the original advice still holds true. Even though any fan forced combustion (oil burner, gasifier) COULD create a tiny positive pressure, in reality this seems to be well less than the standard chimney negative pressure, so everything works out.

Can't comment on the DIY and brand...I think they will all do the job and cannot imagine one being much more difficult than another - after all, they are just sections and fittings.

Here, BTW, is the opinion from Gulland...which seems similar to our conclusions:
"Hi Craig,
Combustion air fans have been used on wood furnaces and
boilers for decades without raising a concern about positive
pressure in the chimney. Note that oil burners use a pretty
strong fan and then use a barometric draft regulator to
reduce draft levels so that the burner fires into roughly
neutral pressure in the combustion chamber. Occasionally
when an oil burner fires you will smell the exhaust in the
house; this suggests momentary positive pressure in the
appliance and vent, but is not considered serious enough to
warrent the use of pressure vent.

I don't think positive pressure in the vent is something
people need to worry about in relation to combustion air
fans.

Regards,
John Gulland
The Wood Heat Organization Inc.
woodheat.org
A non-commercial service in support of responsible home
heating with wood
"