Pressure Versus Vaccum Gasification

The trade terminology is "forced draft" for air pushed into the firebox and "induced draft" for air sucked in from the exhaust side.

A good question, too. The obvious advantage is that when you open the door the fan can suck the smoke into the stove instead of into your face. But I've been wondering lately if there aren't other advantages or design tweaks. I asked a tech over at Bioheat if the induced draft unit could be retrofitted to the forced draft Tarms and he said that there are other changes inside the boiler involving air paths that go along with the different draft systems. He couldn't explain them to me...

Look forward to other folks' opinions and explanations.

I believe that the only advantage (a significant one) to draft induced is the fact that the firebox is not pressurized (no smoke where not desired).
The only exception I can think of is the Woodgun which uses the turbulence to scrape the heat exchanger & eject the ashes.

Forced draft is likely a bit more efficient with respect to electric energy use, pumps/fans are generally more efficient pushing rather than pulling fluid.

Yes indeed easier to blow than suck, that's why ground level deep well pumps work by pushing water down a secondary line to push the water back up the primary line.

I'd like induced draft, it makes more sense to me though I can see it being harder to implement well. It seems safer (no flames licking out of the firebox when the door is open and the sucker (as opposed to blower) is on and I'm personally more comfortable with the concept in a gassifier situation. I don't think it matters one way or another to efficiency though.

A major feature of negative pressure firebox design is the ability to keep the smoke/wood gas/creosote entrained in the airstream and directed to the secondary burn chamber which creates minimal contact with the firebox walls. This helps keep the firebox very clean. If you get a chance, take a look at the inside of a Garn that has been in operation for a while. You'll see some flaky, crusty "stuff" on the firebox walls but very little, if any shiny creosote. In a positive pressure combustion chamber it is much more difficult to keep the gasses from just blowing all over on the way to the nozzle(s). Sounds high tech but it's just easier to make the air follow a desired path when it's being drawn (vacuumed) to a given point instead of pushed (blown).

That along with the no smoke factor is worthy of consideration.

You'll also notice the nearly all of the high tech Euro gasifiers feature negative pressure firebox design. Herlt, Viessmann and Froeling would be prime examples.

After reading a lot about smoky basements, I have been thinking more about induced draft But... It seems leaky gaskets or opening the door when maybe you shouldn't are the main issues? I have also read on here that with an induced draft there can be an issue of buildup on the fan blades causing it to become unbalanced and the motor could be subjected to more heat due to its location. Correct?

Heaterman-good point on the creosote issue. I always find your input extremely valuable.

Noah

Forced draft models do tend to smoke out of the firebox if you try to fill them when there's already a partial load in them. Solution: don't fill them until they have burned to coals, and fill them fast when you do fill them. Another route is to add a draft inducer on the stack. Tjernlund makes a relatively inexpensive paddle wheel style draft inducer that works moderately well. It doesn't reduce the smoke issue totally, but it definitely helps. I generally wire those draft inducers on a SPDT switch with the boiler. One contact for the boiler... one for the draft inducer... that way you can't accidentally leave the draft inducer on and overheat the boiler.

I have looked into some venturi style draft inducers that flow alot more air than the paddle wheel type... but they don't seem to be cost effective at 5 or 6 times the price. I would guess that the venturi style of draft inducer would remove some of the problems inherent with having smoke and creosote build up on the fan blades, but seems like it would be vastly less efficient since you would have to move alot of air to move a little bit... if that makes any sense. They would make more sense if you plumbed outside air into them maybe.

cheers

I've never seen one in person and have only dealt with fans blowing into firebox, how or what kind of fan do the negative pressure boilers use? I guess my question is can you actually purchase replacement parts and is it just the fan blades with the motor outside the firebox or are they the whole unit all together. Where do most of the units place the fan?

CZARCAR said:

called draft inducer or booster. blades inside pipe,motor outside. cut hole in pipe with template bolt gizmo on with seal.

Click to expand...

I've looked at the draft inducers but I figured the higher end boilers would have something better (or cooler looking) that would move more air than these. I don't think the Garn uses one like these. The pictures for Atmos doesn't show allot of details but the fan isn't in the chimney pipe so that's what I'm inquiring about.

Yes, the primary advantage to the user is no smoke roll out during loading (and a bit higher efficiency and easier firelighting). Both the Froling FHG and Tarm Solo Innova (say "inn-nova") have draft induction and it works very well - no bypass damper! Hydronics, I didn't realize that blowing was easier than sucking, but it would help explain why the two draft induced boilers we carry have higher wattage fans than the Solo Plus (which is forced combustion). Just had a customer choose a Solo Plus over the Solo Innova despite that fact that he was going with storage right away - he is off-grid and was counting every watt. Normally if you are going with storage right off then you would choose an induced draft boiler and there is only a modest price difference between the Solo Plus and Solo Innova.

Another 'feature' of the induced draft boilers is that the primary firebox walls are lined with steel aprons that improve primary combustion and direct air to the base of the fuel load for a more consistent burn throughout the burn cycle. Because of the way the air travels through the upper (primary) firebox, because of the steel aprons and because the fire is more limited to the bottom of the upper firebox, the upper part of the primary firebox, and the firebox walls generally, are cooler than in a forced combustion boiler like the Solo Plus. This means that if the boiler were to be used without heat storage, and subjected to the normal on/off cycling of the boiler, that excessive creosote and moisture would condense on the primary firebox walls. Further, this condensation would develop behind the aprons so would not burn off as well once the boiler fired up again. As a result, the induced draft boilers must be used with storage.

So, the primary advantage of the Solo Plus - or any forced draft gasifier - is that it may be installed without heat storage as long as it is limited to use during the coldest part of the year to avoid excessive idling. While any wood boiler will benefit from heat storage, many folks are happy just taking care of the lion's share of their heating needs without storage and using fossil fuel during the Spring and Fall. Considering the cost of adding storage it is no small thing to be able to put in the wood boiler now and be able to add storage in a year or two or three.

How about natural draft vs. pressure vs. vacuum gasification?

Chris Hoskin,
"So, the primary advantage of the Solo Plus - or any forced draft gasifier - is that it may be installed without heat storage as long as it is limited to use during the coldest part of the year to avoid excessive idling. While any wood boiler will benefit from heat storage, many folks are happy just taking care of the lion’s share of their heating needs without storage and using fossil fuel during the Spring and Fall. Considering the cost of adding storage it is no small thing to be able to put in the wood boiler now and be able to add storage in a year or two or three. "

Obviously there are exceptions or embellishments to general case scenarios. For a year and a half I burned 24-7 with my forced draft no storage system EKO40. With the settings on the boiler as received wood consumption was of course high for energy transfer in the summer for simple dhw uses. "Tuning" the boiler for longer run time with less fuel became a pass time endeavor that yielded well and extended that yield into "heating season" benefits but it was not enough so for summer use so I also went to short term daily fires. (I do hope to go to a solar set up for summer use though) The short term fires yielded the dhw I needed and cut idle time and fuel consumption. One of the reasons I chose to run during the summer was the concern of creosote corrosion caused by condensation in the primary chamber and other places during the summer period of non-use. Being able to appreciably cut dependency on fossil fuel was an added bonus but incentive as well. If it had not been for the desire to cut costs and oil dependency the inefficiencies of the "as shipped" status quo gasifier would have remained obscure and that some designs do not allow much for enhanced performance as well. Doubtless storage would be an added bonus for my set up but even as it stands my system does not use an inordinate amount of wood. Given the size and age of my home and the lack of modern insulation techniques the wood I use is less than some with the same sized home would expect. With adequate storage a once a week fire for dhw would render my system as "efficient" by many standards and would reduce or eliminate idle time creosote possibilities as well. With an adjusted usage approach the gasifier does not have to be limited to a specific season and expense wise can be an economically viable system for transferring alternative energy source's in to heat.