Suck or Blow that is the question

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
Status
Not open for further replies.
Ok so now that I have every ones attention here is the question, my boiler pushes air into the primary and secondary chambers, and I know some other boilers pull it through with an induction type fan. In my opinion it sound better to pull it through then to push it, less problems with smoke leaking out of the top chamber on the door seal.

What does anyone else think, pros and cons, any opinions

Steve
 
Maine said:
Ok so now that I have every ones attention here is the question, my boiler pushes air into the primary and secondary chambers, and I know some other boilers pull it through with an induction type fan. In my opinion it sound better to pull it through then to push it, less problems with smoke leaking out of the top chamber on the door seal.

What does anyone else think, pros and cons, any opinions

Steve

I would speculate that the most correct is to push, especially if airflow is under microprocessor control. When pushing you can guarantee more precise volumes of air where and when you want. Otherwise primary air is more susceptible to the influence of nozzle restrictions. Also I would think the higher pressure would necessarily improve combustion, perhaps significantly.

But pulling has a lot of compelling advantages in reducing design complexity, improving manufacturability, and avoids a lot of operational problems.

--ewd
 
Although induced draft is simpler, forced draft will mix air and combustion gases together better.

I guess you could make a good case for most EPA rated wood stoves, which use natural draft with no fans.
They carberate pretty well through good design and a lot of well placed air inlets.
 
ewdudley said:
Maine said:
Ok so now that I have every ones attention here is the question, my boiler pushes air into the primary and secondary chambers, and I know some other boilers pull it through with an induction type fan. In my opinion it sound better to pull it through then to push it, less problems with smoke leaking out of the top chamber on the door seal.

What does anyone else think, pros and cons, any opinions

Steve

I would speculate that the most correct is to push, especially if airflow is under microprocessor control. When pushing you can guarantee more precise volumes of air where and when you want. Otherwise primary air is more susceptible to the influence of nozzle restrictions. Also I would think the higher pressure would necessarily improve combustion, perhaps significantly.

But pulling has a lot of compelling advantages in reducing design complexity, improving manufacturability, and avoids a lot of operational problems.

--ewd

I see, however some of the more advanced boilers Froling for one just sucks the air through and use the controls to control dampers and fan speed. I see your point as far as pushing, one thing that I like is the fact that you would not really need a bypass damper if you pull the air through.

STeve
 
Maine said:
ewdudley said:
Maine said:
Ok so now that I have every ones attention here is the question, my boiler pushes air into the primary and secondary chambers, and I know some other boilers pull it through with an induction type fan. In my opinion it sound better to pull it through then to push it, less problems with smoke leaking out of the top chamber on the door seal.

What does anyone else think, pros and cons, any opinions

Steve

I would speculate that the most correct is to push, especially if airflow is under microprocessor control. When pushing you can guarantee more precise volumes of air where and when you want. Otherwise primary air is more susceptible to the influence of nozzle restrictions. Also I would think the higher pressure would necessarily improve combustion, perhaps significantly.

But pulling has a lot of compelling advantages in reducing design complexity, improving manufacturability, and avoids a lot of operational problems.

--ewd

I see, however some of the more advanced boilers Froling for one just sucks the air through and use the controls to control dampers and fan speed. I see your point as far as pushing, one thing that I like is the fact that you would not really need a bypass damper if you pull the air through.

STeve

If Frolings are indeed more advanced, and there is nothing beyond marketing hype that says they are, it's more likely to be despite draft induction than because of it.
 
Maine said:
Ok so now that I have every ones attention here is the question, my boiler pushes air into the primary and secondary chambers, and I know some other boilers pull it through with an induction type fan. In my opinion it sound better to pull it through then to push it, less problems with smoke leaking out of the top chamber on the door seal.

What does anyone else think, pros and cons, any opinions

Steve

My opinion?....you deal with what you have? Works the best...IMO... Improve if you can.
 
ewdudley said:
Maine said:
ewdudley said:
Maine said:
Ok so now that I have every ones attention here is the question, my boiler pushes air into the primary and secondary chambers, and I know some other boilers pull it through with an induction type fan. In my opinion it sound better to pull it through then to push it, less problems with smoke leaking out of the top chamber on the door seal.

What does anyone else think, pros and cons, any opinions

Steve

I would speculate that the most correct is to push, especially if airflow is under microprocessor control. When pushing you can guarantee more precise volumes of air where and when you want. Otherwise primary air is more susceptible to the influence of nozzle restrictions. Also I would think the higher pressure would necessarily improve combustion, perhaps significantly.

But pulling has a lot of compelling advantages in reducing design complexity, improving manufacturability, and avoids a lot of operational problems.

--ewd

I see, however some of the more advanced boilers Froling for one just sucks the air through and use the controls to control dampers and fan speed. I see your point as far as pushing, one thing that I like is the fact that you would not really need a bypass damper if you pull the air through.

STeve

If Frolings are indeed more advanced, and there is nothing beyond marketing hype that says they are, it's more likely to be despite draft induction than because of it.

Well what I meant as far as more advanced, is it has fancy controls that measure residual 02 and stack temp to control the primary and secondary air for max efficiency through the entire burn. My boiler has what I consider fancy controls, candy thermometer on the stack and an aqua-stat on the stack to shut it down when the stack temp goes below 180 works well, I just can't seem to start mine as easy as the Froling, however I am working on it.

STeve
 
Bypass dampers are only needed to aid in starting the fire and warming the chimney. However the bypass usage can be reduced if you build your fire from the top down as many downdraft gasifiers do (with only a 3/4 or so load and kindling on top instead of at the bottom). The problem with my downdraft gasser and the bypass is reloading the primary chamber after the fire has been going. However a smoke damper like I had in an Englander wood furnace (which had to be pushed back and upward to facilitate loading) would eliminate a lot of the smoke that is generated while trying to reload the upper chamber when the backdraft damper is open. The damper acted as a blockage that forced the fire into the chimney exit instead of out the open door. The unfortunate lack of the smoke damper in my gasser seems to me to be nothing but the mfg's idealism (timing the reloading process) being rebuffed by the reality of life (gotta do what ya gotta do) especially since the burning wood in the upper chamber releases too much burning matter for the chimney opening to overcome the unrestricted door opening.
 
OK I may be totally out of place here but aren't most/all of the higher eff gasers pulling air in a directed manner, as opposed to pushing it around the fire box until enough pressure is created to drive into somewhere else, of course the pushing of air is under the influence of whatever load & fire box geometry permit. A side effect of pulling rather than pushing is the lack of smoke in your face/house when loading/lighting as well as the higher eff.
 
Cave2k said:
Bypass dampers are only needed to aid in starting the fire and warming the chimney. However the bypass usage can be reduced if you build your fire from the top down as many downdraft gasifiers do (with only a 3/4 or so load and kindling on top instead of at the bottom). The problem with my downdraft gasser and the bypass is reloading the primary chamber after the fire has been going. However a smoke damper like I had in an Englander wood furnace (which had to be pushed back and upward to facilitate loading) would eliminate a lot of the smoke that is generated while trying to reload the upper chamber when the backdraft damper is open. The damper acted as a blockage that forced the fire into the chimney exit instead of out the open door. The unfortunate lack of the smoke damper in my gasser seems to me to be nothing but the mfg's idealism (timing the reloading process) being rebuffed by the reality of life (gotta do what ya gotta do) especially since the burning wood in the upper chamber releases too much burning matter for the chimney opening to overcome the unrestricted door opening.

I always used to be a fan of top-down fire lighting in my old low-tech wood/ hot-air furnace, but with a downdraft gasifier, I find lighting from below to be immensely faster and more effective at getting rapid and effective gasification. You want as much heat as fast as possible at the refractory/ nozzle. With a fat handful of finely split very dry softwood over the nozzle, then two pieces of pallet slat on either side of that in a V-shape, and split two logs on either side against the firebox walls, with some fine-split wood on top, lit with a propane torch from underneath, I only need to leave the bypass damper open for the couple of brief moments it takes to clean excess ash out of the lower chamber- then pull the bypass shut, and away she goes....

On the original topic of pushing or pulling, I can see the appeal of pulling- simpler overall design (no bypass) and elimination of smoke roll-out. If the internal flows are designed well, I don't see how a lot of pressure should be needed.
 
We have both forced draft and draft induction models... The draft induction is by far more homeowner friendly. Door seals are less of an issue, smoke roll out is basically a non-issue, there are no bypass dampers to stick, and backpuffing or huffing never seem to happen... ever. Air currents through the firebox are smooth which seems to aid in getting the boiler lit easily. Think of it as the difference between pushing a rope through the boiler and pulling one.

The only drawback to draft induction is that they pretty much need to be installed with storage in order to keep the boiler from idling smoke through the induction fan. It wouldn't take much to throw the fan out of balance or completely plug it up if the unit was left idling. Concerning whether the high end units are "advanced" because of or in spite of draft induction is really a moot point since either way the units must be engineered properly for their specific application

Basically, if the homeowner wants to go with thermal storage right out of the gate, I always suggest draft induction. If storage is not an option up front, or if budget demands a staged installation over several years then forced draft is the way to go.

cheers
 
My biomass has a fan on the front and one by the chimminey so I'm assuming it does both ?
 
Piker said:
s... The draft induction is by far more homeowner friendly. Door seals are less of an issue, smoke roll out is basically a non-issue, there are no bypass dampers to stick, and backpuffing or huffing never seem to happen... ever. Air currents through the firebox are smooth which seems to aid in getting the boiler lit easily.

Right, it's hard to dispute the operational advantages of induction.

Think of it as the difference between pushing a rope through the boiler and pulling one.
This is just silly. To build a torch you supply combustion air under pressure and the products of combustion can be managed separately. With induction you are forced to manage air supply and exhaust flow together.

Forced air supply makes the process hotter, more complete, and more robustly controllable by minimizing critical parameters. Consider a waste oil burner, an oxy-acetylene torch, or a jet engine.

Cheers --ewd
 
I am not a boiler designer and I don't own a boiler. In general, I assume we are talking about boilers that are in homes that are occupied 24 - 7. In that environment, the design for failure of individual components would want to be failure to a safe mode. Induced draft might be safer where a seal, gasket, or device left an opening between the combustion products and ambient. My presumption is induced draft would cause leaks to flow toward the flue instead of out of the combustion chambers. So induced draft might be safer if a leak develops.

Induced draft might be more expensive due to the harsher conditions in the exhaust.

All designs fail at some point. Smoke detectors and CO monitors, alarm failure of process that does not default to safe mode.
 
I have seen large boilers with forced draft and both forced/induced draft for very large ones. I don't see any difference whether you "suck or blow". Its just a question of the velocity through the nozzles and the mass rate of flow through the boiler that matters. My only thought is that maybe a suction type fan is not as efficient in sucking as as a forced draft fan is at developing pressure therefore a larger fan/hp would need to be used. Maybe if we could get a few owners here to look at their fans and list whether it "pulls or pushes", amp/watt draw, and size of boiler. Maybe, maybe not. Any HVAC guys know?
 
ewdudley said:
Piker said:
s... The draft induction is by far more homeowner friendly. Door seals are less of an issue, smoke roll out is basically a non-issue, there are no bypass dampers to stick, and backpuffing or huffing never seem to happen... ever. Air currents through the firebox are smooth which seems to aid in getting the boiler lit easily.

Right, it's hard to dispute the operational advantages of induction.

Think of it as the difference between pushing a rope through the boiler and pulling one.
This is just silly. To build a torch you supply combustion air under pressure and the products of combustion can be managed separately. With induction you are forced to manage air supply and exhaust flow together.

Forced air supply makes the process hotter, more complete, and more robustly controllable by minimizing critical parameters. Consider a waste oil burner, an oxy-acetylene torch, or a jet engine.

Cheers --ewd

So then it stands to reason that a unit which can manage primary air, secondary air, and exhaust flow together should work spectacularly if it is engineered properly, just like a forced draft unit needs to be engineered properly to work well.

I think it is also of note that the non lambda solo innova which is draft induction, has a higher efficiency rating than the solo plus which is forced draft.

I think the analogies of the waste oil burner, the acetylene torch, and the jet engine are not good comparisons since none of them use a solid fuel. At their core, they all end up burning a gas, but it's much easier to meter volume and energy content of compressed oxygen, compressed acetylene, oil, and jet fuel than wood and woodgas. We can't measure the amount of wood gas available prior to secondary combustion in a wood gasifier... this changes constantly based on what's going on in firebox. Burning wood gas is an imperfect science that currently needs measured based on post secondary combustion gas parameters. Because of this, draft induction works very well on a solid fuel boiler, and not so much on your jet engine.

The rope analogy was intended to illustrate how the air currents are smoother in the upper chamber - and how with less turbulence and smooth air flow, the fire seems to start easier.

cheers
 
JimboM said:
I am not a boiler designer and I don't own a boiler. In general, I assume we are talking about boilers that are in homes that are occupied 24 - 7. In that environment, the design for failure of individual components would want to be failure to a safe mode. Induced draft might be safer where a seal, gasket, or device left an opening between the combustion products and ambient. My presumption is induced draft would cause leaks to flow toward the flue instead of out of the combustion chambers. So induced draft might be safer if a leak develops.

Induced draft might be more expensive due to the harsher conditions in the exhaust.

All designs fail at some point. Smoke detectors and CO monitors, alarm failure of process that does not default to safe mode.

I have a Power Venter from Field Controls on my oil boiler which is basically an induced draft in lieu of a chimney. The Reillo burner also has a forced draft fan. I Believe FC markets the power venters as "safer" due to the items you mentioned. (I didn't go look it up so I may be wrong here). The point is my venter failed one time and yes we got plenty of smoke in the place. It wasn't pretty. There was more than one failure and not FC's fault. A proper draft on a conventional chimney should be suffice to minimize most problems. Even suction fans fail.

This leads me to another thought as to why you can't direct vent most gassifiers directly out wall a'la Garn? My power venter has a cooling airflow path for the double wall flue. I just don't see runaway fires taken place in a gassifier.
 
The driving force that makes gases flow through the combustion and exhaust path is pressure difference.

Fortunately, gases are pretty dumb and they don't stop and take the time to worry about whether they are being pushed or pulled. All gases know is that there are more molecules on this side than on that side... so move over. NOW.

Compressed gases do burn differently at elevated pressure. Jet engines are good examples.

But step back, folks. We're only talking about tiny fractions of one PSI of compression here, not many, many atmospheres.

The first supersonic wind tunnel I ever saw operated by a huge tank that had the air pumped out of it and when they opened the valve, almost 800 MPH of suction.

For small residential boiler systems, the door seal considerations are more important to the high end designers from the looks of it.

Next topic of contention---- Is it better to PUSH water through a boiler system or PULL it through?
 
Controlled draft induction is operator friendly but requires more hardware to work properly, p/s dampers that by nature are not linear to airflow [ most of the control is within 30% of the 90 degree travel]. Also if the draft inducer only runs at one speed, and 1 damper happens to be closed, the airflow through the open damper becomes greater than if both were open. For draft induction to be controllable the mass air flow/draft inducer needs to be able to modulate, maintaining a constant neg pressure in the firebox throught the burn. By doing this you can still have a small load of wood burning, modulate mass airflow down to a regulated differential pressure, thus allowing the damper air volume proportions to become more linear and controllable for a pid. The end result is the ability to have a low fire and still maintain combustion effiency. Again for this to work you will need a vfd to control a 3 phase motor, or a fan damper on the outlet side of the id fan, these are both do-able but require a lot of planning to properly inplement. With a smaller output boiler such as the froling, smaller draft id motors using pwm control make the technology more convenient for use.
The forced draft side requires 2 modulating draft motors, one for the primary and one for the secondary air control, this system is more easily controlled [linear] as the motor only supplys what is required as opposed to controlling with dampers.
As we are burning wood, o2 feedback is needed for either of there techniques to work accurately.
 
TCaldwell said:
Controlled draft induction is operator friendly but requires more hardware to work properly, p/s dampers that by nature are not linear to airflow [ most of the control is within 30% of the 90 degree travel]. Also if the draft inducer only runs at one speed, and 1 damper happens to be closed, the airflow through the open damper becomes greater than if both were open. For draft induction to be controllable the mass air flow/draft inducer needs to be able to modulate, maintaining a constant neg pressure in the firebox throught the burn. By doing this you can still have a small load of wood burning, modulate mass airflow down to a regulated differential pressure, thus allowing the damper air volume proportions to become more linear and controllable for a pid. The end result is the ability to have a low fire and still maintain combustion effiency. Again for this to work you will need a vfd to control a 3 phase motor, or a fan damper on the outlet side of the id fan, these are both do-able but require a lot of planning to properly inplement. With a smaller output boiler such as the froling, smaller draft id motors using pwm control make the technology more convenient for use.
The forced draft side requires 2 modulating draft motors, one for the primary and one for the secondary air control, this system is more easily controlled [linear] as the motor only supplys what is required as opposed to controlling with dampers.
As we are burning wood, o2 feedback is needed for either of there techniques to work accurately.

I am not sure how much airflow stays linear while modulating airflow whether push or pull. Probably not. I have worked on flow processes that use one actuator to control two valves through a linkage arrangement that adjusted each valve for its stroke relative to the other. Seems old school but it worked. Some of these setups were just brilliant in their simplicity.

Mike
 
Are there any longevity issues with induced draft fans being exposed to more heat?

I think this is a question with a useful answer. Maybe more along the original poster's line of thinking.

Might take a few more years to get a good statistical sample since induced draft residential boilers seem relatively new over here.
 
Piker said:
t stands to reason that a unit which can manage primary air, secondary air, and exhaust flow together should work spectacularly if it is engineered properly, just like a forced draft unit needs to be engineered properly to work well.

Indeed, but in engineering control schemes it is important to avoid inherently unstable design choices in order to arrive at a robust solution. Inducing draft through both the firebox nozzle and the secondary air jets is inherently unstable with respect to the balance between primary air flow, secondary air flow, and fan speed. You say Froling has conquered the problem, which could be true, but to me it seems unlikely.

I think it is also of note that the non lambda solo innova which is draft induction, has a higher efficiency rating than the solo plus which is forced draft.
Non ceteris paribus proves nothing.
We can't measure the amount of wood gas available prior to secondary combustion in a wood gasifier... this changes constantly based on what's going on in firebox. Burning wood gas is an imperfect science that currently needs measured based on post secondary combustion gas parameters. Because of this, draft induction works very well on a solid fuel boiler, and not so much on your jet engine.
No, quite the opposite. As conditions change and the characteristics of the primary combustion take off out of control in one direction, with induced draft the flow of secondary air flow will take off in an aggravating direction, and now the control needs to manage both simultaneously, while controlling the fan speed as well. It's just not a good recipe for a control system that works correctly. But for as much as the Austrians enjoy complexity for its own sake I'm not surprised they've taken up the challenge.


--ewd
 
Status
Not open for further replies.